Some are squat and some are squatters: more of the intertidal

About a year ago when my blog was just getting going, I posted photos of some specimens from a Southampton Natural History Society (SNHS) visit to Calshot Beach in southern England, a site with areas of stones, gravels, sands and shingle as well as nearby saltmarsh (the latter not covered here). That visit produced some excellent views of spectacular species such as the Dahlia Anemone Urticina felina, and we were keen to see what could be found this year. This is an annual SNHS event and the findings go towards building up a late summer/early autumn species list for the site - this year, although we picked low tide, it wasn't as low as last year's so we expected a somewhat different range of species.

Some species, or their signs, were familar from last year, such as the shells of Haminoea sea slugs, the Snakelocks Anemone Anemonia viridis, and the introduced North American bivalve, the Quahog Mercenaria mercenaria. Many others were different however - some are presented here and I hope you enjoy them. The first group I want to cover are the molluscs, starting with some primitive armoured species (the chitons) and then a larger (and edible) introduction.

Lepidochitona cinerea (family Ischnochitonidae). Probably the commonest North European chiton, this species is often found on the underside of stones, though this one was on a large bivalve shell. Though it looks smooth, the valves (sections) are rough to the touch, like fine sandpaper. This specimen is about 15mm long - the maximum is about 24mm.

The fairly common Acanthochitona crinitus (family Acanthochitonidae). The valves are less smoothly arranged than in L. cinerea and there are 18 tufts of coarse bristles. This specimen is about 20mm long - the maximum is about 34mm.

Chitons (class Polyplacophora) have a mantle skirt which forms a toughened 'girdle' around the whole edge of the animal and this is where the fringing spines etc are found. The head is small and covered by the girdle and the dorsal surface generally has armoured 'valve plates' as seen here. They graze plant and algal material from hard substrates and, like limpets, are able to withstand wave shocks without being dislodged.

The introduced Mediterranean/Biscay species Crassostrea gigas, the Portuguese Oyster. The shell has several large wrinkles and smaller concentric lines. It can grow to 180mm in length and is attached to the substrate at the hinge end - here it is attached to a dislodged stone.

Sticking with shelled species, but adding legs, a number of crustaceans were also found. As well as the common shore crab Carcinus maenas, some possibly less familiar species were worthy of a photo or two...

OK, this probably is quite familiar - it's the common hermit crab Pagurus bernhardus, using the shell of a Netted Dog-whelk Hinia reticulata. Gotta love hermit crabs! Note the larger right claw which is covered in small knobs or 'tubercles'.

One of my favourites, the Hairy Crab Pilumnus hirtellus. It is covered in hairs which are broader at the tip than the base and help camouflage it among the sediment and detritus of the intertidal zone. The shell is up to 20mm across, which is about the size of this specimen, doing its best to hide in a white tray with a few scraps of seaweed.

Another favourite, and less commonly seen, the squat lobster Galathea squamifera. The rostrum is triangular and pointed with 4 spines on each side, the rear ones also being the smallest. These spines are usually red-tipped (as here) as are those on the outer edge of the claws. The claws are also covered in flat scale-like tubercles. Despite the claws, it filter-feeds on suspended detritus.

The shrimp Palaemon elegans with a straight (rather than clearly up-curved) rostrum and dainty little claws. Note the telson (the flap at the end of the tail) doesn't have any lateral spines. These features separate it from similar species such as P. serratus.

Lastly, I'd like to introduce a couple of soft-bodied species - not the large showy ones, but a couple that are often likely to be overlooked.

The sea anemone Sagartia troglodytes. Similar to S. ornata (some specimens may be extremely difficult to separate to species), but this one clearly shows the pale brown column darkening at the top with speckles. Note the numerous tentacles; there may be up to 200 arranged in a roughly hexagonal pattern (here it is clear that the arrangement is not circular). The attachment to a buried stone is typical.

The Leathery Sea-squirt Styela clava. This is an introduced Pacific species and can be found attached to stones (as here) and pilings around the south-west coasts of Britain.

As with last years post, this is only a snapshot of what can be found on a diverse section of intertidal habitat, but hopefully an interesting one. However, it does illustrate how important volunteers are for recording wildlife, especially less 'popular' groups like many in marine and intertidal habitats - and with knowledge comes at least the potential for conservation.


References

Crothers, J. & Crothers, M. (1988). A key to the crabs and crab-like animals of British inshore waters. Field Studies 5(5): 753-802 (revised reprint).
Gibson, R., Hextall, B. & Rogers, A. (2001). Photographic Guide to the Sea & Shore Life of Britain & North-west Europe. OUP. Oxford.
Hayward, P.J. & Ryland, J.S. (eds) (2000). Handbook of the Marine Fauna of North-West Europe (2000 revision). OUP, Oxford.

Further reading

Crothers, J.H. (1997). A key to the major groups of British marine invertebrates. Field Studies 9(1): 1-177. Very useful if you are new to marine invertebrates.
Hayward, P.J. (1988). Animals on Seaweed. Richmond, Slough. For those interested in generally small intertidal species found attached to seaweeds.
Hayward, P.J. (1994). Animals of Sandy Shores. Richmond, Slough. Not used here although Calshot Beach is sandy in places and supports some of the species in this book.
Hiscock, S. (1979). A field key to the British brown seaweeds (Phaeophyta). Field Studies 5(1): 1-44. Useful alongside Hayward (1988).