Crustacea

Large Phylum of mainly marine arthropods (crabd, prawns, lobsters, barnacles), but also containing some terrestrial (e.g. woodlice) and freshwater animals (e.g. freshwater shrimps). The crustacean body is divided into four segments: head, thorax, abdomen, tail (telson). The segments that comprise each region are fused to varying degrees depending on the group. Each segment contains two appendages consisting of a coxa, a basis, an endopodite and an exopodite. In most crustaceans living in the water, one of these branches is converted into a gill. The appendages of the head are modified as antennae, mouthparts and accessory feeding structures. The thoracic appendages are used for walking, swimming, feeding or fighting and show a variety of modifications. These are known as pereopods. The abdominal limbs, known as pleopods, are very variable and used for reproduction.

Crustaceans are charcaterised by possessing an external chitnous shell, strengthened by calcification. This heavy exoskeleton requires periodic moulting (ecdysis). During ecdysis, calcium is resorbed, the old chitinous shell splits and the calcium is redposited onto a new chtin template. Reproduction often occurs just after moulting.

The larval stages of crustaceans are often complex, consisting of a number of instars. The basic stage is the nauplius which is free swimming. In others the embryo passes through the nauplius stage, hatching as the more advanced zoea.

Chthalmus montagui (up to 10 mm diameter) - Chthamalidae Symmetrical conical barnacle with six coarsely ridged plates. Kite-shaped aperture with the terga less than half of the length of the scuta. Found on the upperreaches of moderately exposed rocky shores. Breeds from July to August. Found on the south and west coasts of the UK. Image taken at Prestayn, Denbighshire .

Ligia oceanica (up to 30 mm) Sea slater Ligiidae Oval flattened, greyish body with six pereon segments. Large head with eyes placed laterally. Terminal uropods with stiff peduncles and equal sized rami. Abudnant on all rocky coasts above the high water mark. Particulalrly observed at dusk. Lives for three years, breeding in the third. Common and widespread on all UK coasts with suitable habitat. Image taken at The Spinnies, ??????? .

Talitrus saltator (up to 25 mm) Sandhopper Talitridae Largest sublittoral sandhopper. Flagellum of second antennae is slightly serrated, first antenna is small. Hind margin of the third epimeral plate is straight with numerous small spines. Found above the high water mark or underneath strandline debris on sandy beaches, often abundant. Found on all UK coasts. Image taken at Flint Castle, Flint, Denbighshire .

Barnacles have a modified body termed the prosoma containing six pairs of thoracic appendages (cirri) and three pairs of head appendages (mandibles and maxillae). The cirri are used for filter feeding. In some barnacle (e.g. Acorn barnacles), the body is encased within clacareous plates - the dorsal carina and ventral rostrum with paired lateral plates between. Some barnacles form a cone via two opercular plates, the terga and scuta. Others (e.g. Goose barnacles) are attached to the substrata by a stalk known as the peduncle. The majority of barnacles are hermaphrodites, fertilisation occurring internally and the eggs hatching into planktonic larvae. Eventually the larvae moult into a cyprid stage that contains a bivalved shell and uses its six pairs of legs to find a suitable substratum for settlement.

Corystes cassivelaunus (up to 40 mm carapace) Masked crab Corystidae Sand burrowing crab with an elongated oval carapace and short rostrum. The long stout antennae are covered in bristles and close to form a respiratory tube to the surface. The male has long chilipeds (up to 80 mm), whilst the females are shorter. Found on the lower shoreon fine sandy beaches. Common on all UK coasts. Image taken at Prestatyn, Denbighshire .

Maja squinado (up to 20 cm carapace) Common spider crab Majidae Largest spider crab with chelicerae up to 45 cm long. The rostrum has two thick forward pointing horns. Eye is retractile. Found on corse sand and gravel in deep tide pools. common on the west and southwest UK coasts. Image taken at ??????, Pembrokeshire .

Necora puber (up to 65 mm carapace) Velvet swimming crab Portunidae Flat carapace with five sharp teeth on each side of the eyes. The surface has a velvet texture. Slender chelipeds. Fast swimmer due to modified 'paddles' on the fourth pair of legs. Found rocky coasts. Common on allm suitable UK coasts. Image taken at ???????? .

Carcinus maenas (up to 60 mm carapace) Green shore crab Portunidae Variable coloured, patterned carapace. Five teeth on each side of the eyes. After the females summer moult, breeding commencs with up to 185,000 eggs being laid and attached to the females pleopods. The eggs remain attached to the pleopods until hatching into planktonic larvae. Tolerant of low salinity, permitting its range to extend into estuaries and saltmarshes. Found all UK coasts. Image taken at Prestatyn, Denbighshire . Image is of a gravid female - note egg sac.

Oniscus asellus (10-16 mm) Common shiny woodlouse Oniscidae Smooth shiny grey with light markings and two lines of yellow patches. Yellow and orange forms occur. Purple forms occur due to infection with an iridovirus. Presses itself flat to the substrate when disturbed. Flagellum portion of the antenna has three segments. Common and widespread in a variety of damp habitats including gardens, hedgerows and woodland. Image taken at Durlston Country Park, Swanage, Dorset .

Philoscia muscorum (~11 mm) Common striped woodlouse Philosciidae Yellowish-brown body with a black stripe down the centre. Colour variations occur including red and green forms. Purple forms occur as a result of infection with and iridovirus. Evident junction between the thorax and the narrow pleon. Common and widespread in hedgerows, woodland, gardens and grassland amongst leaf litter and plant debris, rare in Scotland. Image taken at Durlston Country Park, Swanage, Dorset .

Armadillidium vulgare (14-18 mm) Common p ill woodlouse Armadillidiidae May be uniformly slate grey, pink, red or brown. Mottled variations also occur. Rolls into a sphere when disturbed, the sphere is perfect distinguishing it from some similar species - both A. depressum and A. nasatum leave a gap. Tolerant of drier habitats due to its ability to roll up. Common and widespread in gardens, grassland and woodland, particularly with calcareous soils.

Land-living and marine living crustaceans. Possess seven pairs of walking legs and numerous hard plates - divided into the thorax (large broad plates) and the pleon (smaller, narrow plates towards the rear). Require damp habitats for survival, often found under stones and in leaf litter. Scavenge decaying vegetation. Upon release from the pouch of the female, the young woodlice (mancas) have six pairs of legs, gaining the final pair after the initial moult within 24h. The exoskeleton is moulted in two parts as the woodlouse grows. There are ~39 species in the UK.

Porcellio scaber (17 mm) Common rough woodlouse Porcellionidae Slate grey ground colour but can vary with orange and cream forms speckled black, red or brown. Numerous tubercules on the plates. Blunt head. Antennal flagellum has two segments. Two pairs of lungs. Found in a range of habitats under stones, in rotten stumps and under leaves. Favours drier conditions than O. asellus. Common and widespread. Image taken at Salisbury, Wiltshire .

Armadillidium depressum (20 mm) Armadillidiidae Uniformly slate grey with longitudinal rows of yellow blothces in some individuals. Two pairs of lungs. Rolls into an imperfect sphere when disturbed. Found on coastal cliffs and other sites inland. Resistant to drying. Locally abundant in South West England and South Wales.