INTRODUCING COASTAL INVERTEBRATES
Naturalists divide living things into distinct groups, the members of which share common characters; this is called classification. The main subdivisions among the invertebrates (animals without backbones) are referred to as phyla (singular, phylum). Some coastal invertebrate phyla are mainly or exclusively marine; others have marine, freshwater and terrestrial representatives; a few are mainly or solely found on land.
SPONGES (PHYLUM PORIFERA)
Sessile, filter-feeding animals whose cells are not differentiated into obvious tissues. There are both vase-shaped and encrusting species. A typical animal has numerous small holes through which sea water enters an inner cavity lined with cells that filter out food particles; filtered sea water then exits via a large pore. Sponges are described.
JELLYFISH, SEA ANEMONES AND ALLIES (PHYLUM CNIDARIA)
Sometimes referred to as coelenterates, these soft-bodied animals show radial symmetry. The phylum is introduced in more detail.
SEA GOOSEBERRIES AND COMB JELLIES (PHYLUM CTENOPHORA)
These are typically translucent or almost transparent animals, whose soft bodies seem to lack any real substance. They are pelagic, moving by means of rows of beating hairs called cilia, and are seasonally abundant. Ctenophores are bioluminescent. The group is described.
SEGMENTED WORMS (PHYLUM ANNELIDA)
Relatives of the familiar earthworm, this diverse group is found in a wide range of habitats, and shows a great range of body forms. The phylum is introduced in more detail.
OTHER MARINE WORMS
A range of other elongated animals bearing the word ‘worm’ in their names are found on the seashore. These include the flatworms (phylum Platyhelminthes), carnivorous creatures that move with a gliding motion; and ribbon worms (phylum Nemertea), unsegmented predators and scavengers, some species of which are extremely long.
CRUSTACEANS (PHYLUM CRUSTACEA)
An arthropod phylum whose representatives are found everywhere in the marine environment. The body form varies, but typically animals have a hard, calcified exoskeleton. The phylum is introduced in more detail.
Conventional wisdom used to place arthropods (invertebrates with jointed legs and a hard exoskeleton) in a phylum with representative sub-groups (called classes) including crustaceans, insects, spiders and allies. Modern thoughts on classification now elevate most of these sub-groups to phylum status.
Acorn worms and sea squirts are the link between invertebrates and vertebrates, respectively animals without and with a backbone. Acorn worms (phylum Hemichordata) are worm-like but have a nerve cord in their collar. As adults, sea squirts (or tunicates) may look primitive but they are more closely related to fish than to, say, sponges, which some resemble. They belong to the phylum Chordata, sub-phylum Urochordata, and their larvae have a notochord and dorsal nerve cord in the tail.
TERRESTRIAL ARTHROPODS
Several arthropod groups are widespread in terrestrial habitats, with a few representatives found on coasts and the upper seashore; none has colonised the marine world. They include the following: insects (phylum Hexapoda), which are introduced in more detail; centipedes and millipedes (phylum Myriapoda), elongated, segmented animals with paired legs on most segments; and spiders (phylum Chelicerata), predators that have bodies divided into two sections (a fused cephalothorax and an abdomen), with four pairs of legs. Terrestrial arthropods are described.
SEA SPIDERS (PHYLUM PYCNOGONIDA)
These are strictly marine arthropod-like animals that superficially resemble true terrestrial spiders. The body is segmented and there are four pairs of segmented legs. Sea spiders are described.
MOLLUSCS (PHYLUM MOLLUSCA)
A large and varied group, molluscs are found in all marine habitats. The body form is tremendously variable, many species having ornate, protective shells. The phylum is introduced in more detail.
STARFISH, SEA URCHINS AND ALLIES (PHYLUM ECHINODERMATA)
Echinoderms are exclusively marine animals whose body shows radial symmetry, this being most obvious in starfishes and sea urchins. Calcified plates protect the body, and spines are characteristic of many species. The phylum is introduced in more detail.
BRYOZOANS (PHYLUM BRYOZOA)
These sessile, colonial animals grow on larger seaweeds, rocks and shells, depending on the species. They feed by means of ciliated tentacles. Many bryozoan species are the favourite food of sea slugs. Bryozoans are described.
MINOR INVERTEBRATE PHYLA
There are a number of other exclusively marine invertebrate phyla with relatively few representatives that are seldom seen on the seashore; their relative obscurity is reflected in the fact that most do not have common names, and they are not dealt with in detail in this book. These phyla include the following: Priapula, resembling fat worms with a spiny proboscis at one end and branched structures at the other; Echiura, plump, cylindrical animals with a proboscis; Sipuncula, unsegmented worm-like animals with a tentacle-fringed mouth, many of which live in burrows; Phoronida (horseshoe worms), plump, worm-like animals whose mouth is surrounded by a horseshoe arrangement of tentacles; Brachiopoda (lamp shells), whose body is attached to substrate by a short stalk, and protected by paired, unequal shells like a bivalve mollusc.
Breadcrumb Sponge Halichondria panicea Diameter to 15cm (sometimes much more)
Breadcrumb Sponge, green form
Ophlitaspongia papilla
Myxilla incrustans
Commonest and most familiar sponge of rocky shores. Found on and under rocks on lower shore, in pools and in sub-tidal zone. BODY forms a spreading mass, with raised bumps and striking pores (oscula). Typically either green or orange. STATUS Widespread and common on suitable shores. SIMILAR SPECIES Myxilla incrustans is also encrusting, typically yellow with obvious oscula; surface has pits and ridges and web-like coat of fine fibres. Slimy and odourless (Halichondria smells strongly). Widespread on lower shore. Ophlitaspongia papilla is blood-red to orange, the smooth surface with regular oscula.
Purse Sponge Grantia compressa Length to 25mm
Distinctive sponge that typically grows attached to larger seaweeds or among short, tufted seaweeds encrusting rocks on lower shore. BODY is sac-like, flattened and buffish white. STATUS Widespread and locally common on suitable coasts.
Estuary Sponge Hymeniacidon perleve Diameter to 15cm
Mound- or cushion-forming sponge that grows on rocks on lower shore, but also on stones, buried or part-buried in silt in estuaries. Tolerates desiccation. BODY is typically orange, surface irregularly corrugated with sparse small pores (oscula). STATUS Widespread and locally common.
Distinctive little sponge. Grows attached to seaweeds, and sometimes rocks, on lower shore and in pools. BODY is irregularly ovoid and buffish white; opening (osculum) has a radiating fringe of spiny hairs. STATUS Widespread and locally common on suitable coasts.
Boring Sponge Cliona celata Diameter to 6cm or more
Encrusting sponge that bores into, and sometimes covers, calcareous rocks and large mollusc shells. Bored shells are washed up on shoreline during gales, once sponge has died. BODY (boring form) comprises spongy projections with terminal pore (osculum) arising from holes bored in substrate; sometimes forms an uneven, spreading mass, with large pores (oscula). STATUS Widespread and common on suitable shores.
Mermaid’s Glove Haliclona oculata Height to 20cm
Much-branched sponge. Grows sub-littorally, attached to stones on sheltered shores and in estuaries. Often washed up bleached, and then not always immediately recognisable as a sponge. BODY is divided in the manner of a leafless tree. ‘Branches’ are round in cross section, orange-yellow with openings in rows. STATUS Widespread and locally common, especially on S and W coasts.
Leucosolenia botryoides Width to 25mm
Distinctive sponge. Grows attached to seaweeds on lower shore. BODY comprises numerous bunches of finger-like white tubes with a terminal pore. STATUS Widespread, commonest on W and NW coasts; almost absent from the SE.
Suberites domuncula
Colourful sponge. Found from lower shore to sub-littoral zone, on rocky coasts. BODY is variable: sometimes a distorted sphere or fig-shaped (this form previously referred to as S. domuncula); also encrusts shells, notably scallops. Typically orange-yellow. STATUS Widespread on S and W coasts; generally absent elsewhere.
Dysidea fragilis Width to 15cm
Encrusting or lobe-forming sponge, found under rocks on lower shores on sheltered coasts. BODY of littoral form typically comprises 2–4cm-long lobes tightly adhering to rocks. Surface is studded with craters, framed by ragged-tipped cones. Usually buffish white. STATUS Widespread and locally common.