Calanus finmarchius, a planktonic copepod.
INTRODUCING COASTAL ARTHROPODS
Arthropods are among the most numerous creatures on the planet. A huge diversity of species can be found in the sea and on the seashore, as well as in coastal terrestrial habitats.
THE RANGE OF ARTHROPODS
Until recently, arthropods were, in classification terms, assigned their own phylum. Nowadays, many biologists elevate each of the various arthropod groups to this status and the term ‘arthropod’ has become a more general one relating to animals that have a hard outer skeleton and jointed limbs.
Terrestrial arthropod groups, found on the coast, include insects (phylum Hexapoda), centipedes and millepedes (phylum Myriapoda), and spiders (phylum Chelicerata); these groups are dealt with elsewhere in the book, and insects in particular are introduced. When it comes to the marine environment (both seashore and open sea), by far the most important and diverse arthropod group is the crustaceans (phylum Crustacea).
INTRODUCING COASTAL CRUSTACEANS
Although a few crustaceans are found on land (woodlice, for example) and in fresh water, the vast majority of species occur in the marine environment. Representatives can be found on rocky and sandy shores, among seaweed holdfasts, in open water, and even as parasites. In fact, almost every conceivable marine habitat will harbour a great diversity of crustacean species, often in abundance.
The archetypal crustacean has a body divided into three sections: head, thorax and abdomen. Each section is segmented, although the segments are often fused and can be hard to discern. Each segment bears a pair of appendages, which are typically modified according to their location and function. Those on the head segments are often modified to serve as sensory organs or mouthparts; elsewhere, they can function as gills or legs (either for walking or swimming, or both). The outer skeleton of a crustacean is typically calcified and hard. In order for the animal to grow, the skin must be shed periodically. Prior to moulting, a crustacean typically resorbs the calcium in its skin, making it soft for a short period. In most crustaceans, there are separate sexes and the larvae are planktonic and free-swimming; the larva becomes more adult-like each time it moults.
Crustaceans are divided into four main subdivisions (classes), three of which are easily found on the shore: copepods (class Copepoda); barnacles (class Cirripedia); and class Malacostraca, which includes the bulk of the classic seashore crustaceans. The fourth, ostracods (class Ostracoda), are very small (to 1mm or so) bivalved creatures and hard to detect. Within the Malacostraca are further subdivisions, called orders; these include mysid shrimps (order Mysidacea), isopods (order Isopoda), amphipods (order Amphipoda), and shrimps, prawns, crabs and lobsters (order Decapoda, which literally means ‘ten legs’).
Calanus finmarchius, a planktonic copepod.
Copepods – tiny animals. Free-swimming forms are a significant component of zooplankton; they typically swim in a jerky manner (in short spurts), have a pear-shaped body and often carry a pair of egg sacs. There are several parasitic species of copepod, which usually have unusual body forms compared to those of their free-swimming relatives.
Filter-feeding barnacle Balanus crenatus, showing cirri.
Atypical and sessile crustaceans with highly modified bodies. They live attached to rocks and other hard substrates, either directly or via a long stalk in the case of goose barnacles. The barnacle body is protected by calcified plates. Barnacles feed by filter-feeding; specialised fan-shaped appendages called cirri beat through the water and collect food particles.
Pranus flexuosus, a typical mysid shrimp.
Sometimes called opossum shrimps, these are slender animals with delicate appendages and a mostly transparent body; typically, only the eyes appear dark. Many species are seasonally abundant and important as food for larger marine creatures.
Sea Slater, a typical isopod.
Varied group of small crustaceans, typically with a flattened body that appears distinctly segmented thanks to the dorsal plates. The head is usually obvious, with relatively distinct eyes, and movement is achieved in most species by paired legs on the middle (thoracic) segments of the body. The tail end (telson) is sometimes modified to aid swimming.
Beach-hopper, a typical amphipod.
Small crustaceans with laterally flattened bodies. Typically, the head has obvious paired antennae and eyes; the segmented remainder of the body comprises sections called the pereon, pleon and urosome. Each segment bears specialised appendages; some assist feeding while others are used for walking and swimming.
Common Prawn.
As with all decapods, the head and thorax are fused, and covered with a plate called the carapace. The body is elongated and a prominent rostrum projects forward between the eyes. There are 10 pairs of legs: five pereopods, the first typically bearing pincers; and five pleopods. At the tail end, the telson is modified to form a fan, used for rapid propulsion through the water.
Green Shore Crab.
The head and thorax are fused, and covered with a plate called the carapace. The body is flattened but the shape varies: crabs mostly have a broad carapace, while in lobsters and squat lobsters it is more elongate and slightly cylindrical. There are 10 pairs of legs: five pereopods, the first typically bearing pincers; and five pleopods. At the tail end of lobsters and squat lobsters, the telson is modified to form a fan, used for rapid propulsion through the water. Hermit crabs are highly modified animals; the long, coiled, soft abdomen is protected by the empty mollusc shell in which the crab lives.
Common Shiny Woodlouse Oniscus asellus Length to 14mm
Found under beach debris above high-tide line. ADULT is shiny; marbled grey and brown, with rows of yellow patches. Head has lateral lobes, and jointed end of antennae comprises 3 sections. STATUS Common and widespread.
Common Rough Woodlouse Porcellio scaber Length to 10mm
Found under strandline debris above high-tide line. ADULT is narrower than Common Shiny Woodlouse and greyer, the surface rough not shiny; jointed end of antennae comprises 2 sections. STATUS Common and widespread.
Pill Woodlouse Armadillidium vulgare Length to 11mm
Intriguing woodlouse: can roll into a ball when disturbed. Found under stones and debris above high-tide line. ADULT is slate-grey and rounded in cross section. STATUS Commonest in the S.
Sea Slater Ligia oceanica Length to 30mm
Large, fast-moving woodlouse-like crustacean. Mostly nocturnal but easily found by turning boulders and large stones on rocky shore; also found in crevices in harbour walls. ADULT has flattened grey body with long antennae and conspicuous appendages at tail end. STATUS Widespread and common.
Almost transparent crustacean, found when stones are turned on lower shore, often trapped in surface film of water. ADULT is laterally flattened; head, thorax and anterior part of abdomen are visible through bivalve carapace. Rest of abdomen is slender and ends in a forked appendage. Eyes are red. Male has long 2nd antennae. STATUS Widespread but easily overlooked.
Dynamene bidentata Length to 8mm
Segmented isopod with oval body; uropod has distinctive lateral projections. Rolls into a ball if disturbed. Found in rock crevices and empty barnacle shells on sheltered shores. ADULT is greyish. Male has backward-projecting spines on 6th thoracic segment. STATUS Widespread and fairly common.
Lekanesphaera rugicauda Length to 9mm
Vaguely woodlouse-like isopod. Found in clustered groups under stones in estuaries and brackish water; swims well but rolls into a ball if disturbed. ADULT is oval in outline; yellowish brown, textured with tubercles. Has broad telson and splayed uropods. STATUS Widespread and locally common.
Sphaeroma serratum Length to 10mm
Plump-bodied isopod that curls up if disturbed. Found under rocks on lower shore. ADULT is oval in outline, hemispherical in cross section. Tail end is rounded; outer uropod has serrated margin. STATUS Local, mainly in the SW.
Cymodoce truncata Length to 10mm
Similar to Sphaeroma serratum but separable by studying shape of tail end. Found under rocks and among kelp holdfasts, on lower shore. ADULT is oval in outline, hemispherical in cross section. Telson is often slightly hairy, with posterior projections and lateral uropods. Body is yellowish or pinkish, with granular texture to tail end. STATUS Locally common, mainly in the SW.
Woodlouse-like isopods. Several similar species are common. Found in water; most tolerate brackish or low-salinity conditions. In estuaries or under stones on mid- to upper shore. ADULT is oval with relatively long antennae. STATUS Widespread and locally common.
Gribble Limnoria lignorum Length to 5mm
‘gribbled’ wood
Small isopod whose signs of activity are more easily observed than the animal itself: bores holes into soft, submerged wood and causes decay of pilings of piers and jetties. ADULT has elongate-oval body outline, obvious segments, and broad telson. STATUS Widespread and common, but animal itself is found only by careful searching.
Synisoma acuminatum Length to 25mm
Distinctive elongate isopod. Well camouflaged when resting among Sea Oak fronds. Found on sheltered rocky shores and in pools. ADULT has slender, segmented body with long antennae and relatively long legs. STATUS Widespread and fairly common but easily overlooked.
Astacilla longicornis Length to 25mm
Unusual elongate isopod. Hard to find when resting among seaweeds and hydroids; occurs on sheltered rocky shores and in pools. ADULT is cylindrical with very long pereon segment 4. Second antennae have very long basal segments. STATUS Widespread and locally common in W only.
Idotea linearis Length to 40mm
Idotea chelipes
Elongate isopod with parallel-sided body. Found on sandy shores. ADULT is brown and flattened, with long antennae. Rear margin of telson is smoothly concave. STATUS Locally common. SIMILAR SPECIES I. chelipes is smaller (length to 15mm), with pointed telson tip. Restricted to brackish pools.
Idotea granulosa Length to 20mm
Isopod with narrowly oval outline to body. Found on rocky shores, usually among Fucus and Ascophyllum seaweeds. ADULT has a flattened brown body. Telson is large and tapers; tip is incurved, ending in a narrow, blunt tip. STATUS Widespread and locally common.
Idotea baltica, in floating seaweed raft.
Narrowly oval isopod. Mainly sub-littoral, sometimes in rock pools and among drifting seaweed. ADULT colour is variable but usually either brown or greenish; typically has white spots down centre of dorsal surface. Telson is broad and relatively large, with 3 points at tip. STATUS Widespread and locally common.
Idotea emarginata Length to 30mm
Isopod with narrow, almost parallel-sided outline. Mainly sub-littoral but also in pools and among drifting seaweed. ADULT has flattened brownish body and a dark dorsal stripe. Telson is broad; outer margins curve gently, rear margin is concave. STATUS Widespread and common.
Idotea neglecta Length to 30mm
Isopod with a narrowly oval outline. Mainly sub-littoral but also found in pools and among drifting seaweed. ADULT has a flattened body; usually brown, but sometimes has bold white markings. Telson has curving sides and ends in a distinct point. STATUS Widespread and locally common.
Idotea pelagica Length to 11mm
Small, narrow isopod. Associated with barnacles and mussels on fairly exposed rocky shores. Swims actively. ADULT has a proportionately large head and stout antennae (flagellum shorter than basal segments). Telson has parallel sides but tapers to a point. Body is brown with white marks. STATUS Widespread but local, mainly W.
Speckled Sea Louse Eurydice pulchra Length to 8mm
Eurydice affinis
Compact isopod. Lives buried in intertidal sand when tide is out, but emerges and swims actively with rising tide, particularly after dark. ADULT has narrow, streamlined body, with rounded dorsal surface. Dark chromatophores expand and contract according to ambient light levels. STATUS Widespread and locally common; easiest to find by torchlight after dark. SIMILAR SPECIES E. affinis is smaller and paler. Found on lower shore of sandy beaches, emerging into surf on rising tides. Widespread and locally common on suitable coasts.
Intriguing little amphipod, often found in clustered groups inside empty crab and heart-urchin shells, and the like. ADULT has a compact greyish-white body, through which orange-brown organs show in larger animals; eyes are red and kidney-shaped. First antennae are thick-based and stubby; 2nd antennae are long. STATUS Widespread and locally common, but easily overlooked.
Chaetogammarus marinus Length to 25mm
Laterally flattened amphipod with a curved dorsal outline. Out of water, jerks along on its side. Found on sandy shores, under stones and seaweed at low tide. ADULT has greenish body and elongate eyes. Last 3 segments of body have dorsal clusters of spines. Has 2 pairs of antennae; 1st has a short accessory flagellum. STATUS Widespread and common.
Gammarus locusta Length to 35mm
Gammarus locusta, mating pair
Gammarus locusta, mating pair
Laterally flattened amphipod. Out of water, jerks along on its side. Found on rocky and mixed shores, under stones and seaweed at low tide. ADULT has olive-brown body and kidney-shaped eyes. Last 3 segments of body have dorsal clusters of spines. Has 2 pairs of antennae; 1st has a short accessory flagellum. STATUS Widespread and common.
Gammarus duebeni Length to 17mm
Laterally flattened amphipod with a curved dorsal outline. Out of water, jerks along on its side. Found in brackish water and shores with freshwater run-off (also in river mouths). ADULT is olive-brown with kidney-shaped eyes. Last 3 segments of body have dorsal clusters of spines. Has 2 pairs of antennae; 1st has a short accessory flagellum. STATUS Locally common.
Gammarellus homari Length to 10mm
Well-marked, sculptured amphipod. Mainly sub-littoral but sometimes found in pools or among beached seaweed. ADULT is laterally flattened, with sharp dorsal keels on coxal segments. Pale ground colour is suffused orange-brown and studded with pale spots. Has red kidney-shaped eyes and accessory flagellum on 1st antenna. STATUS Widespread but local.
Dexamine spinosa Length to 12mm
Well-marked amphipod. Lives on lower shore in eelgrass beds and seaweed holdfasts. ADULT is mottled reddish brown and grey, adorned with pale spots; dorsal spines on last 4 segments. Antenna pair 1 is longer than 2, and lacks accessory flagellum. STATUS Widespread but local.
Maera grossimana Length to 10cm
Slender amphipod, found by turning rocks on sheltered shores. ADULT is yellowish, flushed pink; antenna pair 1 is long, with accessory flagellum. Gnathopod 2 is large and subchelate. STATUS Locally common only in the SW.
Apherusa jurinei Length to 8mm
Found in tangled masses of seaweed and laminarian holdfasts. ADULT has curved dorsal outline. Antenna pair 1 lacks accessory flagellum and is slightly longer than antenna pair 2. Eyes are red and kidney-shaped. STATUS Local, mainly in the W.
Parajassa pelagica
Tube-dwelling amphipod. Colonies of tubes often cloak mooring ropes and submerged rocks by late summer. ADULT has well-marked, slender body and long antennae (particularly basal segments). Male has large, articulating 2nd gnathopod with basal spine. STATUS Widespread but rather local. Commonest in the W; most easily found in summer. SIMILAR SPECIES Parajassa pelagica (length to 10mm) has oval gnathopod and whorls of long hairs on antenna pair 2. Lives in tubes, often on kelp holdfasts. Locally common, mainly in the W.
Erichthonius sp. in tube on Eelgrass
Tube-dwelling amphipods. Several superficially similar species make tubes on eelgrass and seaweed stems on sheltered shores. ADULT has slender body and long antennae (particularly basal segments). Male has large, articulating 2nd gnathopod with basal spine. STATUS Widespread but rather local. Commonest in the W; most easily found in summer.
Haustorius arenarius Length to 12mm
Bizarre, unmistakable amphipod whose digging abilities earn it the nickname ‘Bulldozer Shrimp’. Lives buried in clean sand on middle shore, on extensive beaches. If disturbed, swims upside down. ADULT has domed whitish body with large, flattened hind legs used for digging; leg segments are fringed with spiny hairs. Eyes are small, yellowish and hard to discern. STATUS Locally common only on coasts of Wales, and NW and NE England.
Urothoe brevicornis
Burrowing amphipods. Several superficially similar species are found in muddy sand from middle shore to sub-littoral zone. ADULTS have plump, rounded body. Head and eyes are relatively conspicuous; 1st antenna has accessory flagellum. Hind legs are large and adapted for digging. STATUS Widespread and locally common, except in the SE.
Bathyporeia pilosa Length to 7mm
Distinctive plump-bodied amphipod. Lives buried in sand, from middle shore downwards. ADULT has domed body with powerful hind legs for digging; 1st antenna has thickened basal segment. STATUS Widespread and locally common, except in the SE.
Ampithoe rubricata Length to 20mm
Laterally flattened amphipod. Lives in a tube fixed to seaweed or under rocks. ADULT is reddish or greenish with darker speckling. Antennae are relatively long and similar in length; flagellum is absent. STATUS Widespread and common but overlooked.
Sand-hopper Talitrus saltator Length to 25mm
Plump-bodied, slightly laterally flattened amphipod. Out of water, jerks along on its side. Found on sandy beaches. Mostly nocturnal; in daytime, usually buried in sand. ADULT has a marbled grey and whitish body and striking eyes. First antennae are short; 2nd antennae are long, with very long basal segments, and tinged orange. STATUS Widespread and common.
Beach-hopper Orchestia gammarellus Length to 18mm
Laterally flattened amphipod whose outline is curved towards rear end. Hops frantically when disturbed, typically ending up on its side. Found by turning strandline debris on beaches with coarse sand and stones. ADULT has a reddish-brown body, sometimes tinged green. First antennae are short; 2nd antennae are long, with very long basal segments. Eyes are rounded. Male has large 2nd gnathopods. STATUS Widespread and common. SIMILAR SPECIES Hyale sp. (length to 8mm) are hopping amphipods that live intertidally on rocky shores, mainly in the W. Second antennae are longer than 1st but relatively shorter than those of Beach-hopper, with short basal segments.
Atylus swammerdamei Length to 10mm
Pale, laterally flattened amphipod. Lives among sub-littoral seaweeds and on sand; active in surface waters after dark. ADULT is mostly white, stained yellowish brown in patches. Dorsal surface of urosome has 2 sharp teeth with notch between. Eyes are large, kidney-shaped and red. Antennae are long, 1st pair marginally shorter than 2nd and without accessory flagellum. STATUS Widespread and locally common, except in the SE.
Hyperia galba, male
Hyperia galba, female
Plump-bodied pelagic amphipod. Usually lives inside Comb Jelly, or jellyfish Rhizostoma octopus and Pelagia noctiluca. ADULT is usually brownish (sometimes pale), with large head and greenish eyes. STATUS Widespread and fairly common given specific habitat requirements.
Corophium volutator Length to 10mm
Unusual amphipod with dorso-ventrally flattened body. Lives in burrows in estuary mud, but only where consistency allows this, and where diatoms (food) coat the silt surface. ADULT has elongate yellowish body with brown markings, and very long and thickened 2nd antennae. STATUS Widespread and very locally common; absent from many seemingly suitable areas. Important food for Dunlin and other waders. SIMILAR SPECIES C. arenarium is smaller and found in sand rather than silty mud.
Chelura terebrans Length to 6mm
Minute amphipod, found in timber riddled with Gribbles. ADULT is brownish and not markedly flattened. Margins of body segments and legs have bristles; these collect sediment, creating ‘hairy’ or ‘fuzzy’ appearance. Third abdominal segment has backward-projecting spine. Has backward-pointing jointed tail appendage, larger in male than female. STATUS Locally common but associated only with Gribbles.
Caprella linearis Length to 22mm
Caprella acanthifera
Bizarre slow-moving, elongate amphipod. Well camouflaged when resting among seaweeds on sheltered shores. ADULT has slender brownish body with long antennae, large 2nd gnathopod, and long posterior pereopods, used for clinging to seaweeds and other substrates. STATUS Widespread and fairly common, but easily overlooked. SIMILAR SPECIES C. acanthifera is smaller (length to 15mm), with a swollen skull-like head and spiny outgrowths on dorsal surface. Widespread in the W.
Cyathura carinata Length to 13mm
Slender, rather cylindrical isopod. Found mainly in estuaries, often in areas of run-off or streams. ADULT has rather transparent elongate body with relatively short antennae. STATUS Widespread in S Britain.
Edible Crab Cancer pagurus Carapace width to 20cm
Familiar and unmistakable crab. Smaller individuals are found under stones and seaweeds on lower shore; larger individuals are typically in deeper water. ADULT is reddish overall. Carapace is oval with a ‘piecrust’ margin. Pincers are large and black-tipped. STATUS Widespread and common.
Green Shore Crab Carcinus maenas Carapace width to 10cm
Green Shore Crab, infected with Parasitic Barnacle
Green Shore Crab, mating pair
Green Shore Crab, female with eggs
Green Shore Crab, juvenile colour forms
Our most familiar crab, found in a wide range of habitats and salinities. ADULT has an angular-oval carapace with 5 teeth on each side (lateral to the eyes), and 3 equal lobes between the eyes. Carapace colour is variable, but typically marbled green, brown or reddish. STATUS Widespread and common. NOTE Females carry yellowish egg mass on underside of abdomen for several weeks or months. Some individuals are infected with Parasitic Barnacle, which appears as orange ball on underside of abdomen, recalling an egg mass.
Hairy Crab Pilumnus hirtellus Carapace width to 15mm
Small but distinctive crab; extremely hairy, particularly on legs and margins of carapace. Found on rocky and mixed shores. ADULT has angular-oval carapace, with 5 sharp teeth on each side (lateral to eyes); leading edge between eyes is straight with central notch. Legs and upper surface of carapace are marbled dark grey and white, with reddish suffusion in places. STATUS Widespread and locally common.
Furrowed Crab or Montagu’s Crab Xantho incisus Carapace width to 30mm
Furrowed Crab, individual with brown pincers
Furrowed Crab, typical individual
Distinctive crab with sculptured upper surface to carapace and proportionately large pincers. Found on sheltered rocky shores. ADULT is usually reddish overall. Carapace has 5 blunt teeth on each side (lateral to eyes); leading edge, between eyes, is straight. Pincers are usually black but can be brown. Legs are sparsely hairy at most. STATUS Widespread and locally common.
Risso’s Crab Xantho pilipes Carapace width to 30mm
Risso’s Crab, female with eggs
Similar to Furrowed Crab, with sculptured upper surface to carapace, but legs and sides of carapace are fringed with hairs. On lower shore of rocky coasts. ADULT is usually reddish overall. Carapace has 5 pointed teeth on each side (lateral to eyes); leading edge, between eyes, is undulate. Pincers are usually brown but can be black. STATUS Widespread and locally fairly common.
Pirimela denticulata Carapace width to 15mm
Dainty, colourful little crab. Mostly sub-littoral but found under rocks at extreme low water. Quickly scuttles between stones and under rocks when disturbed. ADULT has rather diamond-shaped carapace, very pointed between eyes, with sharp lateral teeth. Colour often matches background, and can be pinkish, or green and brown. STATUS Widespread but local and easily overlooked.
Velvet Swimming Crab Necora puber Carapace width to 60mm
Impressive and distinctive crab with a well-deserved reputation for being aggressive when threatened. Favours sheltered rocky shores. ADULT has an angular carapace with 5 teeth on each side (lateral to eyes) and 10 small teeth between eyes. Upper surface of carapace is brownish with velvety texture. Eyes and joints on front legs are red. Pincers are tinged blue and red. End segments of hind legs are flattened and paddle-like, used for swimming. STATUS Widespread and locally common.
Portumnus latipes Carapace width to 20mm
Dainty little swimming crab, found on sandy substrates. ADULT has rather shield-shaped carapace, usually marbled brown and buff. Hind pereopods have flattened end segment, used for swimming. STATUS Widespread and locally common.
Harbour Crab Liocarcinus depurator Carapace width to 40mm
Distinctive swimming crab. On muddy sand of lower shore and sub-littoral zone. ADULT has angular carapace with 5 teeth on each side (lateral to eyes), and 3 rounded teeth between eyes. Colour is reddish overall, but paddle-like end segment to hind legs is tinged violet. STATUS Widespread and locally common.
Arch-fronted Swimming Crab Liocarcinus arcuatus Carapace width to 35mm
Active swimming crab. On gravelly sand of lower shore and sub-littoral zone. ADULT has carapace with 5 sharp teeth on each side (lateral to eyes) and untoothed, curved margin between eyes. Pincers are relatively small. STATUS Locally common, mainly in the SW.
Marbled Swimming Crab Liocarcinus marmoreus Carapace width to 35mm
Boldly patterned swimming crab. On sand and gravel of lower shore and sub-littoral zone. ADULT has marbled red, brown and greenish carapace. Three blunt teeth present between eyes, middle one smallest. Segment behind pincers does not have tooth on outer edge. STATUS Locally common, mainly in the SW and NW.
Flying Crab Liocarcinus holsatus Carapace width to 40mm
Flying Crab, juvenile
Active swimming crab. Often swims vigorously in surface water if disturbed or placed in bucket. On gravel and coarse sand. ADULT has 3 blunt teeth present between eyes, middle one largest. Segment behind pincers has tooth on outer edge. STATUS Widespread and locally common around coasts of England and Wales.
Masked Crab Corystes cassivelaunus Carapace length to 40mm
Masked Crab, male
Masked Crab, female
Unmistakable crab. Seldom seen alive because it lives buried in sand; dead specimens often washed up on strandline. ADULT is pinkish brown, with an oval carapace and very long antennae; male has very long front legs. STATUS Widespread and locally common.
Sponge Crab Dromia personata Carapace width to 60mm
Distinctive crab that often has sponge on back of carapace, held in place by hind legs. Sub-littoral, sometimes seen by snorkellers or found in crab pots. ADULT is deep-bodied with large front legs and pink pincers. Entire body is covered by dense velvety ‘fur’. STATUS Widespread but local and recorded only occasionally.
Common Hermit Crab Pagurus bernhardus Carapace length to 30mm
Common Hermit Crab, adult with Calliactis parasitica
Common Hermit Crab without shell
Common Hermit Crab protected within whelk shell
Our commonest and most familiar hermit crab. Small individuals occupy empty shells the size of periwinkles; larger animals occupy Common Whelk shells. ADULT is seen as hardened head end, including carapace, legs and pincers; soft-bodied abdomen is hidden inside, and protected by, the mollusc shell it inhabits. Alarmed animal retreats inside shell, sealing entrance with pincer. Right pincer is larger than left. Has sharp-pointed rostrum between eyes. STATUS Widespread and common in most coastal habitats. NOTE Large individuals (usually in whelk shells) sometimes have anemone Calliactis parasitica living on shell. SIMILAR SPECIES P. prideaux (carapace length to 15mm) usually has Cloak Anemone covering shell and has rounded rostrum between eyes. Locally common but mainly sub-littoral.
Pagurus cuanensis Carapace length to 15mm
Small but distinctive hermit crab. Found in pools and sheltered rocky coasts on lower shore and in sub-littoral zone. ADULT is extremely hairy, with colourful eyes and slender eye stalks. Right pincer is larger than left. STATUS Widespread and locally common on S and W coasts.
South-claw Hermit Crab Diogenes pugilator Carapace length to 15mm
South-claw Hermit Crab without shell
Small hermit crab, identified by studying relative sizes of left and right pincers. Lives mainly in Netted Dog Whelk shells on lower shores of sandy beaches. ADULT has left claw larger than right when viewed from above (Pagurus sp. have right claw larger than left). STATUS Widespread and locally common in the SW but easily overlooked.
Chinese Mitten Crab Eriocheir sinensis Carapace length to 5cm
Distinctive crab with specific habitat requirements. Lives in fresh water for most of life, burrowing into muddy banks; adults migrate to estuaries to breed. ADULT has squarish-circular carapace with widely spaced eyes; typically marbled reddish green. In adults, base of pincer segment has diagnostic ‘mitten’ coating of hairs. STATUS Introduced from Far East, now locally common (and an environmental problem) in Thames and Medway.
Pea Crab Pinnotheres pisum Carapace width to 8mm
Pea Crab, in mussel
Delicate, dainty little crab. Lives inside bivalve molluscs, particularly Common Mussel. ADULT has pale legs and almost circular carapace that is pale yellowish with reddish markings. STATUS Fairly common but only inside host molluscs.
Common Spider Crab Maja squinado Carapace length to 20cm
Impressive crab with relatively slender legs. Found on mixed shores; usually in deep water but sometimes in pools and gullies at low tide. ADULT is pinkish orange overall. Carapace is teardrop-shaped in outline and covered with large, angular spines on dorsal surface. Legs have rather long segments and pincers are narrow. STATUS Widespread and locally common.
Great Spider Crab Hyas araneus Carapace length to 10cm
Bulky and impressive spider crab. Carapace is often coated with attached seaweeds, tubeworms and hydroids, making precise shape and texture hard to discern. On gravel and rocky shores, but mostly sub-littoral. ADULT is reddish brown with teardrop-shaped carapace and pointed rostrum divided down middle. Legs and pincers are rather slender. STATUS Widespread and fairly common, least so in the SW and commonest in the NW.
Four-horned Spider Crab Pisa tetraodon Carapace length to 4cm
underside
Pisa armata
Distinctive crab, easily overlooked since it adorns carapace and legs with seaweeds. Found on sheltered rocky and mixed shores; mostly sub-littoral but sometimes in pools. ADULT is reddish buff overall but colour is often obscured by hydroid colonies, seaweeds and encrusting silt. Carapace is sculptured and roughly diamond-shaped, broadest towards rear end; note lateral spines, rear pair of which projects at right angles. Has rostral spines (‘horns’) at front of carapace. Legs are slender and relatively small. STATUS Widespread and fairly common. SIMILAR SPECIES P. armata (carapace length to 4cm) has diamond-shaped carapace with velvety texture. Carapace has fewer spines on dorsal surface, no lateral teeth, 2 teeth in front of eyes, 2 slightly backward-pointing teeth at rear and 1 at far rear. Legs are stout and knobbly. Mainly Dorset to Sussex, and NW Wales.
Eurynome aspera Carapace length to 15mm
Small, short-legged spider crab. Mostly sub-littoral, among algal holdfasts and stones; sometimes in lower-shore pools or on lifted crab pots. ADULT is pinkish buff oval with frontal horns and dorsal tubercles. Colour and texture usually obscured by hydroid growth and detritus. STATUS Widespread but local, in the S and W.
Long-legged Spider Crab Macropodia rostrata Carapace length to 20mm
Long-legged Spider Crab, swimming
Macropodia deflexa
Delicate little crab with relatively long, slender legs. Easily overlooked because it adorns its carapace and legs with seaweeds. ADULT is yellowish brown but colour is usually obscured by seaweeds. Carapace is triangular-oval. STATUS Widespread and locally common on sheltered rocky shores. SIMILAR SPECIES M. tenuirostris has much narrower and longer rostrum. M. deflexa is larger, paler and almost translucent, with a broader, downcurved rostrum.
Scorpion Spider Crab Inachus dorsettensis Carapace length to 15mm
Dainty little crab. Distinguished from Macropodia sp. by more robust front legs and pincers, and presence of spine behind eye. Found on mixed shores, sometimes among seaweeds in pools. ADULT is yellowish buff overall, carapace with tubercles; colour and texture often obscured by coating of sponges and hydroids. STATUS Widespread and locally common.
Broad-clawed Porcelain Crab Porcellana platycheles Carapace length to 15mm
Distinctive little crab, usually found under rocks and boulders; can be overlooked until it moves because its flattened, hairy body (pressed flat to rock surface) is typically coated with silt. ADULT is greyish brown overall above, white and porcelain-like below. Carapace is rounded and fringed with hairs, as are legs. Pincers are broad. STATUS Widespread and locally common.
Long-clawed Porcelain Crab Pisidia longicornis Carapace length to 10mm
Recalls Broad-clawed Porcelain Crab but body is hairless. Found on sheltered rocky shores. ADULT is usually reddish brown overall. Carapace is circular and legs and pincers are relatively slender. STATUS Widespread but local.
Common Squat Lobster Galathea squamifera Carapace length to 35mm
Our commonest squat lobster. If disturbed, shoots backwards by snapping abdomen against thorax. Found under stones on rocky shores, on mid- to lower shore. ADULT is greenish brown overall, with a flattened carapace. Abdomen is curved under carapace, and legs and pincers are slender. STATUS Widespread and locally common.
Spiny Squat Lobster Galathea strigosa Carapace length to 80mm
Colourful squat lobster. Usually sub-littoral but sometimes rock pools. ADULT has red and blue banding. Legs are red and spiny. STATUS Widespread and locally common in deep water; scarce in intertidal zone. SIMILAR SPECIES G. intermedia is smaller (length to 10mm) and reddish overall; slender legs have fine spines. Widespread but mainly sub-littoral.
Rugose Squat Lobster Munida rugosa Carapace length to 30mm
Distinctive squat lobster. Lives on rocky coasts. Mostly sub-littoral but occasionally found at low water. ADULT is reddish with corrugated carapace and very long pincers and pincer legs. STATUS Locally common only in W and N Scotland.
Norway Lobster Nephrops norvegicus Carapace length to 20cm
Slender-bodied lobster-like crustacean. Lives in burrows excavated in soft mud. Commercially exploited and often sold as Scampi or Dublin Bay Prawn. ADULT is reddish pink overall with parallel-sided body. Carapace and pincers bear spines. STATUS Widespread; formerly common but now local and generally scarce due to overfishing.
Common Lobster Homarus vulgaris Length to 40cm
Impressive and unmistakable crustacean. Can grow much larger than quoted size, but commercial exploitation prevents this in most places. Favours rocky and mixed shores; scarce intertidally but sometimes found in pools. Resides in rock crevices and emerges to feed. ADULT is bluish purple overall, fringed on segments and telson with orange. Pincers are massive and unequal. STATUS Widespread and still fairly common.
Hooded Prawn Athanas nitescens Length to 20mm
Small but robust crustacean with a passing resemblance to a miniature Common Lobster. Found under stones on sheltered rocky shores, and in pools with gravel and stones. ADULT is usually bluish or greenish (sometimes reddish), but almost always with a pale dorsal strip. Front legs and pincers are relatively large. STATUS Widespread and locally common, least so in the N.
Crawfish Palinurus elephas Length to 40cm
Distinctive lobster relative that lacks obvious pincers. Heavily exploited commercially, affecting adversely both size and abundance. Found on mixed shores; usually sub-littoral but sometimes in pools and gullies. ADULT is reddish brown overall. Carapace and abdomen are armed with sharp spines, as is swollen base to antennae. STATUS Widespread but generally scarce.
Snapping Prawn Alpheus macrocheles Length to 35mm
Unmistakable prawn with hugely enlarged pincers. ‘Snapping’ sound (like popping popcorn) is audible when emitted by in situ animals under rocks at very low water. ADULT is yellowish orange. One pincer (left or right) is larger than the other, with swollen pinkish-orange articulation that creates snapping sound. Eyes are flush with carapace. STATUS Local and generally scarce in the S, mainly Dorset to Isle of Wight.
Leptomysis sp.
Chameleon Shrimp
Praunus inermis
Sometimes known as opossum shrimps, mysids are small shrimplike crustaceans with slender, almost transparent bodies. They are often abundant and ecologically very important (as food for other animals), but their colourless bodies make them hard to spot and even harder to identify. Often their presence can be detected only by looking for their opaque eyes, which appear disembodied as the animals move through the water. Three fairly typical species are covered here, but there are plenty of others that require an expert eye. Chameleon Shrimp Praunus flexuosus (length to 25mm) is widespread and swarms in sheltered rock pools and estuaries, often hanging motionless in mid-water; it has a ‘hunchback’ profile, divided telson and antennal scale (plate-like structure seen alongside base of antennae) 8 times longer than broad, with a sub-terminal spine on outer margin. P. inermis (length to 15mm) is found on sheltered rocky shores and in pools; has a ‘hunchback’ profile, telson is divided at tip, and antennal scale is 4 times longer than broad, with a sub-terminal spine on outer margin. Leptomysis sp. (length to 15mm) are usually found offshore in mid-water, but come to surface and inshore after dark; body is slender, antennal scale is rather short and rounded, and telson is undivided.
Chameleon Prawn Hippolyte varians Length to 30mm
Hippolyte inermis
Hippolyte longirostris
Small, colourful prawn. Found in pools and under seaweeds on sheltered rocky shores. ADULT is often bright green or yellow (sometimes red or brown). Rostrum has 1 dorsal tooth, just in front of eye. First 2 pairs of legs have minute pincers; these are hard to discern. STATUS Widespread and locally common. SIMILAR SPECIES H. inermis (length to 40mm) is typically green, often with a pale dorsal line at head end. Rostrum is relatively long, slightly downcurved and lacks a dorsal tooth. SW only, often in eelgrass beds. H. longirostris (length to 20mm) is partly transparent and variably marked with pink and brown, creating superb camouflage. Rostrum has 2–4 dorsal teeth. Body has tufts of hairs. Scarce in pools on lower shore, SW only.
Thoralus cranchii Length to 45mm
Robust little prawn. Found in pools and under rocks at low water. ADULT is marbled and mottled brown and reddish. Dark stippling and yellowish bands on legs and tail end enhance camouflage among seaweeds. Rostrum is short, with 3 dorsal and 2 terminal teeth. STATUS Widespread but local, mainly in S Britain.
Eualus occultus Length to 25mm
Compact, partly transparent prawn. Found in pools and under rocks at low water. ADULT is variably colourful, with blue and yellow dots, and sometimes pink patches. Rostrum is rather short, with 2–4 dorsal and 2 terminal teeth. STATUS Widespread but local, SW only.
Philocheras fasciatus Length to 20mm
Well-marked little shrimp. In common with its close relatives, it has no obvious pincers. Favours pools on sandy and mixed shores. ADULT has stout body banded black and white; in good light, patches of bright colours are seen on carapace and telson. Carapace has single dorsal spine and straight-margined projection between eyes. STATUS Widespread and locally common.
Brown Shrimp Crangon crangon Length to 8cm
Widespread and familiar shrimp of sandy shores. Well camouflaged against its favoured substrate; swims well and burrows with ease. Obvious pincers are absent. ADULT has a pale body, suffused buffish brown and stippled with dark dots. STATUS Widespread and locally very common.
Common Prawn Palaemon serratus Length to 70mm
Our commonest prawn. Found in pools and among seaweeds on rocky and mixed shores, from mid-tide level into sub-littoral waters. ADULT is classic prawn shape: elongate body, fan-shaped telson and slender legs, first 2 pairs with slender pincers. Rostrum profile is diagnostic: 6–7 dorsal teeth and elongated, upcurved tip. STATUS Widespread and locally common.
Palaemon elegans Length to 60mm
Similar to Common Prawn but separable using rostrum profile. Found in pools and under seaweed on sheltered rocky shores. ADULT is typical prawn shape. Rostrum is more or less straight along dorsal margin, with typically 8–9 teeth, 2–3 of which lie behind eye socket. STATUS Widespread, commonest in the S.
Palaemon longirostris Length to 75mm
Similar to Common Prawn but separable using rostrum profile. Found in estuaries and brackish water, sometimes penetrating up rivers to limits of tidal influence. ADULT is typical prawn shape. Rostrum is more or less straight along dorsal margin, with 7–8 teeth, 2 of which lie behind eye socket. STATUS Local, in the S and SE only.
Palaemon adspersus Length to 70mm
Similar to Common Prawn but separable using rostrum profile. Found in pools and under seaweeds on sheltered, mixed shores. ADULT is typical prawn shape. Rostrum is more or less straight along dorsal margin, with 5–6 teeth, 1 of which lies behind eye socket; rostrum tip is usually minutely divided. STATUS Locally common, in the S and SE only.
Palaemonetes varians Length to 50mm
Recalls Palaemon adspersus but shape is more compact, pincers are relatively smaller and rostrum tip is usually undivided. Found in estuaries and brackish water, including sites with very low salinity. ADULT is prawn-shaped and body is almost transparent. Dorsal margin of rostrum is almost straight, with 4–6 teeth, 1 of which lies behind eye socket. STATUS Widespread and locally common.
Aesop Prawn Pandalus montagui Length to 45mm (Not illustrated)
Recalls Palaemon sp. (notably Common Prawn) in terms of overall shape, but pincers are minute and rostrum shape is distinctive. Body is flushed with pink, hence alternative common name of Pink Shrimp. Mostly sub-littoral. ADULT has long, slender rostrum, with narrow, upswept tip; upper margin has 10–12 teeth (4 lying behind eye socket), and tip is divided. STATUS Widespread and locally common offshore; rare inshore.
Poli’s Stellate Barnacle Chthamalus stellatus Diameter to 15mm
Broad-based, slightly flattened-conical barnacle, found on exposed rocky shores at mid-tide level and below. ADULT has 6 distinctly ridged greyish-white shell plates. Aperture opening is oval; tergal plates are roughly same length as scutal plates and suture between them is kinked. STATUS Locally common on suitable shores in SW England and S Ireland.
Montagu’s Stellate Barnacle Chthamalus montagui Diameter to 10mm
Broad-based, slightly flattened-conical barnacle, found in large colonies on moderately exposed rocky shores, usually above mid-tide level. ADULT has 6 distinctly ridged grey-brown shell plates. Aperture opening is kite-shaped; plates are bluish, tergal ones shorter than scutal ones and with a kinked suture between. STATUS Locally common on suitable shores in S and W Britain.
Acorn Barnacle Semibalanus balanoides Diameter to 10mm
Shell shape varies according to exposure and crowding, from flattened-conical to columnar. Found on rocky shores: mid-tide level on moderately exposed shores; higher up with increasing exposure. ADULT has 6 shell plates; aperture is diamond-shaped; tergal plates are much shorter than rostral plates, the suture between them straight but stepped. STATUS Widespread and locally common.
Balanus balanus Diameter to 30mm
Large, conical barnacle of rocky coasts; lower shores and sub-littoral. ADULT has 6 deeply ridged greyish-white shell plates. Tergal plates narrow compared to scutal plates; suture between them forms acute angle. STATUS Mainly northern; almost absent from S and SW.
Balanus perforatus Diameter to 30mm
underside, showing calcified base
Volcano-shaped, grey-brown (sometimes purple-tinged) barnacle of rocky shores; lower shore and sub-littoral. ADULT has 6 shell plates (not immediately obvious owing to deeply ridged surface). Tergal and scutal plates are concealed within aperture. STATUS Locally common, in SW only.
Balanus crenatus Diameter to 20mm
Robust barnacle with steep-sided conical profile, and offset opening. Found on sheltered rocky coasts, lower shore and sub-littoral zone. ADULT has 6 smooth whitish shell plates. Tergal and scutal plates are concealed within aperture. STATUS Locally common only in the W.
Balanus improvisus Diameter to 15mm
Broadly conical barnacle. Restricted to brackish water, attached to stones and algae. ADULT has 6 smoothish shell plates, forming a low conical tower. Tergal and scutal plates, speckled white and purple, cover diamond-shaped opening. STATUS Widespread but local, mainly in the S.
Elminius modestus Diameter to 10mm
Flattish barnacle found in sheltered waters, including estuaries. ADULT has 4 shell plates (unusual amongst British barnacles). Aperture is diamond-shaped; suture between the tergal and scutal plates acute. STATUS Introduced from New Zealand, now locally common.
Wart Barnacle Verruca stroemia Diameter to 5mm
Distinctive but unobtrusive. Found on algal holdfasts and under rocks on lower shore. ADULT has flattened, irregular shape, with 4 ridged shell plates; whitish or brownish. STATUS Locally common, except in SE.
Common Goose Barnacle Lepas anatifera Length to 50cm
Lepas pectinata
Distinctive and unusual pelagic barnacle, found in groups (often 100s) attached to flotsam. ADULT comprises a head section, up to 5cm long, with a whitish shell plate embedded in brownish skin, and a long blackish stalk. Feathery cirri protrude between shell plates when immersed and often in dead specimens. STATUS Occasionally beached on shores in W Britain; less frequent in the E. SIMILAR SPECIES L. pectinata (length to 4cm) comprises a stalk and 15mm head section, the plates greyish buff and strongly ribbed. Pelagic, attached to floating objects, often with Common Goose Barnacle. Washed ashore after gales, mainly in the SW.
Pollicipes pollicipes Length to 10cm
Distinctive goose barnacle relative, found attached to rocks at very low water, on exposed shores. ADULT comprises a scaly-looking brown leathery stalk and head with 5 large triangular plates; smaller plates at base of head. STATUS Mainly S species; range extends to SW Britain, where rare.
Buoy Barnacle Dosima fascicularis Length to 3cm
Pelagic barnacle, sometimes washed up after W gales. ADULT has translucent body plates and short stalk attached initially to floating feather or other flotsam, sometimes aided by barnacle’s own secreted white float; also attaches to dislodged mooring floats and buoys. STATUS Occasional on storm beaches in SW Britain, usually in summer and early autumn.
Parasitic Barnacle Sacculina carcini Width to 25mm
Sacculina carcini infecting Green Shore Crab
Unrecognisable as a barnacle relative. Parasite of crabs, notably Green Shore Crab. ADULT comprises a branching network within crab’s body; visible external sign of parasite’s presence is sac-like structure (orange, maturing to brown) containing reproductive organs, located on underside of crab’s abdomen (could be mistaken for egg sac). STATUS Fairly common in Green Shore Crab populations.
Lernaeocera branchialis Length to 50mm
Bizarre and atypical copepod. Juvenile is pear-shaped, parasitic on flatfish. Adult is parasite of Atlantic Cod and related fish, living with 1 end permanently embedded in gill arch. ADULT comprises a swollen, contorted red sausage-shaped mass, with paler egg mass. Attachment end is much divided, branches penetrating host’s blood vessels and body cavity. STATUS Widespread and fairly common.
Lepeophtheirus pectoralis Length to 4mm
Unusual copepod, parasitic on flatfish, notably Plaice. ADULT has a flattened, elongate body and lives adpressed to skin on upperside of fish. STATUS Widespread and generally common.
Caligus elongatus Length to 12mm
Distinctive parasitic copepod of fishes, notably Atlantic Cod, Pollack and various flatfish. ADULT has pale, rather flattish body and is usually attached to gills, or beneath tongue of host. STATUS Widespread and fairly common.
Unusual parasitic copepod of fishes, notably Atlantic Cod and related species. ADULT has rather amorphous-looking buffish-white body, with tube- or handle-like structure attached. Recognisable as a copepod when paired egg sacs are present. STATUS Widespread and fairly common.
Plankton are minute organisms that drift in sea currents. As individuals they can be hard to discern with the naked eye (other than as small dots and specks), but collectively they can transform otherwise crystal-clear sea water into a rich, organic soup. Although plankton is, to a degree, at the mercy of currents and tides, many species possess considerable mobility, including diurnal movements though the water column: some are present in surface waters only after dark.
In general terms, there are 2 types of plankton: phytoplankton, which are microscopic plants; and zooplankton, the term applied to planktonic animals. The importance of plankton in ecological terms cannot be overstated. As photosynthetic organisms, phytoplankton deplete carbon dioxide from the atmosphere and produce oxygen. Together with the zooplankton, they form the basis of the food chain in the marine environment.
Zooplankton is a collective term that embraces a wide range of animal groups, although in inshore British waters crustaceans are often the dominant creatures. Many small copepods are planktonic throughout their lives, but most familiar seashore crustaceans (crabs and squat lobsters, for example) have planktonic larval stages, whose role is to colonise new habitats. Alongside these can be seen larval stages of other groups, including starfish. Most plankton cannot be identified to species level in the field. However, a couple of groups, and a couple of individual species, can be recognised with the aid of a hand lens.
plankton (mostly copepod crustaceans)
Calanus finmarchicus Length to 3mm
By plankton standards, a large and distinctive copepod. Lives in open water; only easily found in surface layers after dark by torchlight. Important food for larval fish, notably Herring. ADULT has narrowly pear-shaped body, whitish with reddish central patch. Note long antennae and ‘tail’ appendages. STATUS Widespread and abundant in summer, but easily overlooked.
Anomalocera patersoni Length to 2mm
Colourful planktonic copepod. Lives in open water but active in surface layers mainly at night. ADULT has vivid green body and long ‘tail’ appendages. Right antenna is modified for reproduction. STATUS Widespread and locally common but easily overlooked.
SEA SPIDERS
Nymphon sp.
Pycnogonum littorale, with egg mass
larval crab
larval squat lobster
larval starfish
Sometimes placed in their own phylum (Pycnogonida), sea spiders superficially resemble their terrestrial cousins; they eat hydroids and sea anemones. Segmented body has 4 pairs of segmented legs. Most are hard to identify but 2 extreme forms can be distinguished, and are regularly found. Nymphon sp. (length to 10mm) are found under stones from mid- to low water, and have a slender body and legs. Move in apparent ‘slow motion’ and curl up when disturbed. Widespread and locally common. Pycnogonum littorale (length to 20mm) is found under stones and among seaweed holdfasts on lower shore, and has a relatively stout body and thick, segmented legs. Widespread but local.