Pelagia noctiluca, a typical jellyfish.
Ranging in size and shape from sea anemones and soft corals to large and impressive jellyfish, Cnidaria is certainly a diverse group of animals. And, armed with stinging cells, some species really pack a punch to the incautious marine biologist.
THE NATURAL HISTORY OF CNIDARIA
Although some cnidarian species are large and superficially complex, in evolutionary terms they are simple animals. They exhibit radial symmetry in their body form. The outer wall of the body comprises two layers, which can be thin, jelly-like or muscular depending on the species. The body layers enclose a body cavity, and this connects with the sea outside via a single opening or mouth. Typically, the mouth is surrounded by rows of tentacles, these coated with stinging cells that, on contact, release a barbed thread armed with toxins. The stinging cells serve both to capture and subdue prey, and as a defence against attack.
In a typical cnidarian, there are two stages in the life cycle. The sexual stage is called a medusa; it is bell-shaped and free-swimming, with a central mouth below and tentacles around the bell margin. The medusa produces sperm and eggs, and the resulting larva eventually settles and forms a sessile polyp stage. Mature polyps produce and bud off small free-swimming medusae, and the cycle repeats itself. In jellyfish, the medusa stage is dominant and the polyp stage is much reduced. In sea anemones, the reverse is the case, the polyp being dominant; fertilisation in this group is internal and there is no medusa stage.
There are many subdivisions within the phylum Cnidaria. The most striking and distinctive representatives are detailed as follows:
JELLYFISH
Pelagia noctiluca, a typical jellyfish.
Although diverse in classification terms, jellyfish can be recognised for what they are by their body form, which comprises an umbrella-like bell with a jelly-like consistency. Depending on the species, the bell can be flattish or domed and almost hemispherical; long tentacles hang around the bell margin in most species. Typical jellyfish are free-swimming and move by means of pulsating contractions of the bell; to a degree, they are also at the mercy of tides and currents.
These intriguing jellyfish live upside down, the stalk attached by a basal sucker to a seaweed frond or eelgrass leaf. The umbrella has radiating arms that end in groups of tentacles. If the need arises, stalked jellyfish can alter their position, using their tentacles to ‘cartwheel’ along.
HYDROIDS
Tubularia indivisa, a typical hydroid.
Typically seen as the polyp stage and often in colonial form, hydroids are a diverse and abundant group of marine animals. However, because many species are relatively small and delicate, and collapse out of water, their true abundance and significance is often overlooked. Most hydroids are sessile polyps, but the Portuguese Man-of-war and By-the-wind-sailor (often thought of as jellyfish) are floating, free-swimming animals.
SOFT CORALS
Red Sea Fingers, a colourful soft coral.
Despite their English name, these are relatively tough animals whose body form is toughened by calcified spines. The coral-like structures are formed of colonies of polyps. The polyps themselves are retractable.
SEA FANS
These colonial relatives of the soft corals form large branched and fan-like structures, the branches arranged more or less in one plane. The individual polyps live along the side branches of the fan.
SEA ANEMONES
Snakelocks Anemone.
Sea anemones are solitary animals that comprise a tough, rather rubbery and cylindrical body with an apical mouth surrounded by tentacles. They attach themselves to a substrate using a basal sucker; many species attach themselves to bare rock, while a few burrow into, or rather are mostly concealed by, soft sediment. Cup corals and jewel anemones are not, strictly speaking, true sea anemones; the former are protected by a calcified cup.
Compass Jellyfish Chrysaora hysoscella Diameter to 30cm
Impressive and distinctive jellyfish, found in inshore waters in summer. BODY comprises a rather flattened brownish bell marked with 32 marginal lobes and radiating ‘V’ markings; has 24 long, trailing tentacles and 4 mouth arms. STATUS Widespread and locally common, mainly Jul–Aug.
Rhizostoma octopus Diameter to 80cm (Not illustrated)
Large and rather solid jellyfish, found in inshore waters in late summer. BODY comprises a smooth, domed bluish umbrella without marginal tentacles; there are 8 trailing arms, these divided terminally. STATUS Locally common, mainly Aug–Oct.
Lion’s Mane Jellyfish Cyanea capitata Diameter to 85cm
Large jellyfish that can deliver a painful sting; found in inshore waters in summer. BODY comprises a rather flattened brown umbrella with slightly undulating margins; there are 65+ slender tentacles, and 4 divided and very frilly arms. STATUS Locally common but mainly in N waters, Jun–Aug.
Blue Jellyfish Cyanea lamarckii Diameter to 30cm
Familiar jellyfish of inshore waters in late spring and summer. BODY comprises a rather translucent purplish-blue umbrella with numerous slender, trailing tentacles and 4 thick, lobed arms. STATUS Widespread and locally common, mainly Jun–Aug.
Pelagia noctiluca Diameter to 10cm
Relatively small jellyfish; mainly offshore but sometimes inshore in late summer. BODY comprises a translucent domed umbrella covered in knobbly warts; there are marginal tentacles and trailing, frilly arms. STATUS Locally common, mainly in the N and W, Jul–Sep.
Moon Jellyfish Aurelia aurita Diameter to 35cm
Generally the commonest jellyfish in inshore waters, and often stranded. BODY comprises a flattened translucent bluish umbrella, through which 4 dark semicircular rings (gonads) can be seen (pink when seen against a dark background); there are small marginal tentacles and 4 frilly arms. STATUS Widespread and locally common, mainly Jun–Aug.
Sessile Jellyfish Haliclystus auricula Length to 35mm
Distinctive jellyfish and generally the commonest stalked species. Found attached to large seaweeds and eelgrasses. BODY is shaped like an inverted umbrella, with 8 radiating ‘arms’ tipped with terminal tentacles; attached to substrate by stalk. STATUS Widespread and locally common.
By-the-wind-sailor Velella velella Length to 6cm
Distinctive pelagic hydroid. Mainly oceanic, but sometimes blown inshore or beached after gales. BODY is bluish purple and comprises a flattened disc-like float, and an upright fin-like ‘sail’; below this hang short tentacles. STATUS Common in the open Atlantic but only occasional inshore, mainly in autumn.
Portuguese Man-of-war Physalia physalis Width to 12cm
Unmistakable colonial hydrozoan. Typically pelagic and oceanic, but sometimes blown ashore after gales. BODY comprises an ovoid pinkish float with a pleated crest, and numerous blue and pink trailing, stinging tentacles. STATUS Common in summer in the Western Approaches; occasional on SW shores.
Hydrozoan medusae
Sexually reproducing stages in the hydrozoan life cycle are pelagic and called medusae (singular, medusa). They are sometimes seen in the surface layers of the sea. Almost transparent, they are easiest to pick out at night with torchlight. They are bell-shaped, resembling miniature jellyfish, and swim in a pulsating manner. Most are tiny, but those of Leuckartiara can be 1cm or so long. STATUS Widespread.
Oaten Pipes Tubularia sp. Length to 15cm
Distinctive hydroids, found in tufted colonies attached to rocks in pools on lower shore and in sub-littoral zone. BODY comprises a long stalk and terminal ‘head’; tube-like mouth is surrounded by oral tentacles, fringed below by sub-oral tentacles. T. indivsa has 40 or so oral tentacles and 30 or so sub-oral tentacles. T. larynx (length to 4cm) is smaller, with branched stems, and head with 20 or so oral tentacles and 20 or so sub-oral tentacles. Both species are widespread and locally common.
Corymorpha nutans Length to 8cm
Distinctive hydroid that recalls a sea anemone. Lives on silty sand and gravel; sub-littoral, very occasionally in pools. BODY comprises a tapering stalk, marked with longitudinal pale lines, and swollen terminally with long tentacles; central mouth is surrounded by shorter tentacles. STATUS Widespread and locally common.
Hydractinia echinata Height to 4mm
Mat-forming colonial hydroid that coats the surface of shells occupied by hermit crabs. Lower shore and sub-littoral zone. BODY comprises a spiny pinkish mat, studded (when immersed) with polyps resembling tiny sea anemones. Out of water, shell looks to have a fuzzy coating. STATUS Widespread and fairly common.
Aglaophenia pluma Length to 7cm
Colonial hydroid. Grows on seaweeds, notably Halidrys sp. BODY comprises a straight yellowish main stem with white side branches, these with tiny cup-like polyps along length. Overall, colony resembles a small feather. STATUS Widespread and locally common, mainly in the S and SW.
Obelia geniculata Length to 5cm
Dynamena pumila
Small but distinctive hydroid. Grows in masses on seaweeds, notably Fucus and Laminaria. BODY comprises a zigzag pinkish-red main stem; short side branches arise from angles, bearing terminal polyps with tentacles. STATUS Widespread and locally common, except in the SE. SIMILAR SPECIES Dynamena pumila (length to 25mm) comprises hair-like chains (sometimes branched) of paired polyps. Epiphytic on Fucus and Laminaria seaweeds. Widespread and common.
Ventromma halecioides Length to 4cm
Branching colonial hydroid found growing on eelgrasses and stones in sheltered lagoons. BODY comprises a straight central main stem with alternate side branches, on which sit 3–5 polyps, each with tentacles. Overall, colony recalls a leaf skeleton or feather. STATUS Widespread but local.
Beadlet Anemone Actinia equina Diameter to 5cm
Beadlet Anemone colony, at low tide in sea cave
Beadlet Anemone giving birth.
Our most familiar sea anemone. Found on a wide range of rocky shores, at mid- to low-tide levels, and in pools and caves. BODY is either uniformly red or green. When expanded, comprises a broad-based column with ring of blue ‘beads’ at top, just below the numerous tentacles. Contracts into a blob when disturbed or out of water. STATUS Widespread and locally common.
Strawberry Anemone Actinia fragacea Diameter to 8cm
Superficially similar to Beadlet Anemone but larger and distinctively marked. Lives on lower shore on shaded rocks and in gullies. BODY is red overall, the column adorned with pale spots. STATUS Local; commonest in SW England.
Snakelocks Anemone Anemonia viridis Diameter to 5cm
Snakelocks Anemone, grey-brown form
Snakelocks Anemone, green and purple form
Familiar and distinctive sea anemone whose tentacles do not retract completely when disturbed or out of water. Lives attached to rocks and in pools on lower shore. BODY has a stout column and numerous long tentacles, sometimes uniformly grey-brown but usually greenish with purple tips. STATUS Widespread and locally common.
Gem Anemone Aulactinia verrucosa Diameter to 5cm
Squat, well-marked sea anemone. Lives attached to rocks on lower shore, and in pools. BODY comprises up to 48 stout tentacles, banded with grey-brown and white; column has longitudinal rows or warts, 6 rows of which are whitish. STATUS Locally common only in SW Britain.
Dahlia Anemone Urticina felina Diameter to 15cm
Impressive and colourful sea anemone. Lives attached to rocks, typically below low-tide mark, but sometimes in pools. BODY comprises short, stout column armed with adhesive warts, to which debris is invariably attached. Tentacles are relatively short and arranged in 10s; range in colour from white to pink or peachy yellow, depending on individual. STATUS Widespread and locally common.
Horseman Anemone Urticina eques Diameter to 30cm
Similar to Dahlia Anemone but larger; column lacks adhesive warts and consequently remains clear of debris. Lives attached to rocks on lower shore; also sub-littoral. BODY comprises a stout column and rather long tentacles; overall colour is yellowish orange, but often with flushes of red. STATUS Widespread and locally common.
Plumose Anemone Metridium senile Length to 25cm
Impressive and distinctive sea anemone. Lives attached to rocks; mainly sub-littoral but also in pools. BODY comprises tall, stout, smooth column with terminal collar, and frothy mass of tentacles. Column is usually either white or orange, tentacles are typically white. STATUS Widespread and locally common.
Daisy Anemone Cereus pedunculatus Diameter to 12cm
Burrowing sea anemone that lives in mud and silty sand. Found from lower shore into deeper water. BODY comprises a long column, attached to buried stone and completely covered by sediment. Only terminal end is visible, with up to 200 shortish tentacles, marbled and mottled blue-grey, buff and white. STATUS Widespread and locally common. SIMILAR SPECIES Peachia cylindrica (diameter to 10cm) lives buried in gravel and sand from low-tide zone into deeper water. Only terminal end is visible, with a broad disc (grey-brown or white) and 12 stout tentacles with ‘V’ markings. Commonest in the W.
Sagartia troglodytes Diameter to 5cm
Burrowing sea anemone that lives in sand, fine gravel and mud. Found on lower shore and sub-littoral zone. BODY comprises a slender column, attached to buried stone and covered in sediment. Only terminal end is visible, with up to 200 rather long tentacles, typically banded and marbled grey-brown and buff. STATUS Widespread and locally common.
Elegant Anemone Sagartia elegans Diameter to 4cm
Attractive but variable sea anemone. Lives on rocks, sometimes in crevices or holes, into which it retreats if disturbed. Found from lower shore into deeper water. BODY comprises a column with non-adhesive pale suckers, and terminal array of up to 200 tentacles. Several colour forms exist, including: pure white; pale tentacles and orange-yellow disc; marbled grey, brown and buff; pinkish tentacles and pale disc. STATUS Widespread and locally common.
Fountain Anemone Sagartiogeton laceratus Height to 6cm
Attractive anemone. Sub-littoral, often part-buried in silt, attached to stones; occasionally on shells of large whelks. In areas where it is common, sometimes hauled in by fisherman, attached to weights and rigs. BODY comprises a smooth orange-buff column, often with pale longitudinal lines. Disc is orange with up to 200 slender tentacles. Reproduces by division, hence it can appear damaged. STATUS Widespread and locally common, mainly in the W.
Sandalled Anemone Actinothoe sphyrodeta Diameter to 5cm
Attractive sea anemone, found on rocks in relatively exposed spots, from lower shore into deeper water. BODY comprises a smooth column, often marked with longitudinal lines but lacking suckers; terminal array of up to 100 slender tentacles. Overall colour can be either pure white, or white with yellow oral disc. STATUS Widespread and locally common.
Calliactis parasitica Length to 10cm
Extraordinary and distinctive sea anemone, invariably found attached to Common Whelk shells occupied by Common Hermit Crab. Relationship is commensal, possibly symbiotic, but not parasitic. BODY comprises a rough-looking orange-buff column and up to 700 rather small buffish tentacles. STATUS Widespread but encountered only occasionally. SIMILAR SPECIES Cloak Anemone Adamsia carciniopados (width to 5cm) lives symbiotically on shells occupied by hermit crab Pagurus prideaux. Body is creamy white with pink spots. Mainly sub-littoral.
Diadumene cincta Length to 6cm
Colourful anemone. Grows on kelp holdfasts and among sponges; lower shore and sub-littoral zone. BODY has smooth column and up to 200 slender tentacles. Usually orange. STATUS Local, S Britain only.
Starlet Sea Anemone Nematostella vectensis Diameter to 1cm
Worm-like, burrowing sea anemone found in silty mud in undisturbed saline lagoons. BODY comprises a slender, unattached column, up to 2cm long and mostly buried in sediment; and 16 slender, translucent tentacles. STATUS Very local, mainly around the Solent and East Anglian coast. Protected by law.
Devonshire Cup-coral Caryophyllia smithii Diameter to 3cm
A hard coral with a calcified external skeleton. Grows attached to rock, usually below low water, sometimes in pools on lower shore. BODY comprises pale pinkish or peachy knob-tipped tentacles atop a hard base; terminal end is oval with radiating ridges, and animal withdraws inside if disturbed. STATUS Widespread and locally common only in the W and N.
Scarlet-and-gold Star-coral Balanophyllia regia Diameter to 2cm
Colourful little hard coral. Grows in rock crevices, usually below low water, sometimes in pools on lower shore. BODY comprises up to 48 blunt-ended tentacles, dotted with pale warts, and a deep orange oral disc. STATUS Local, restricted to SW Britain.
Jewel Anemone Corynactis viridis Length to 4cm
Jewel Anemone, green form
Jewel Anemone, typical form
Attractive and distinctive sea anemone-like soft coral. Grows attached to rock, sometimes in dense groups, usually below low water or occasionally in pools on lower shore. BODY comprises a column and tentacles with pale, knobbed tips. Colour varies, but often pinkish purple or green. STATUS Widespread and locally common in W and N Britain.
Dead-man’s Fingers Alcyonium digitatum Length to 15cm
Fleshy soft coral. Grows on rocks, usually below low water, sometime in pools on lower shore. Dislodged specimens are sometimes washed up. BODY branches into finger-like lobes; usually either orange or white, the surface studded with translucent polyps that can withdraw if disturbed. STATUS Widespread and locally common.
Red Sea Fingers Alcyonium glomeratum Diameter to 20cm
Similar to Dead-man’s Fingers but with more slender, redder ‘fingers’. Grows on rocks, usually below low water. BODY branches into finger-like lobes; red, with contrasting white polyps that can withdraw if disturbed. STATUS Local, mainly SW Britain.
Sea Gooseberry Pleurobrachia pileus Length to 2cm
Generally the commonest sea gooseberry. Pelagic and mainly oceanic, but seasonally abundant inshore, mainly in spring and summer. BODY comprises a transparent ovoid structure with 8 rows of beating and shimmering, iridescent hairs for swimming. Two fishing tentacles trail below. STATUS Occasionally common, mainly in the W and N.
Comb Jelly Beroe cucumis Length to 12cm
Distinctive pelagic animal, occasionally common inshore in summer months. BODY comprises a transparent bag-like structure, open at mouth end and with longitudinal lines of beating hairs. The commensal amphipod crustacean Hyperia galba often lives inside. STATUS Commonest in the W and N.