Biodiversity & Conservation

Little information was found concerning the community of this biotope.

All boring piddocks begin excavation following settling of the larva and slowly enlarge and deepen the burrow with growth (Pinn et al., 2005). They are forever locked within their burrows, and only the siphons project to the surface opening (Barnes, 1980). A relationship exists between the distribution of piddock species and substratum type. Duval (1963a) examined the penetrability of a variety of substrata by Petricola pholadiformis. It may bore into London Clay, Thanet Sandstone, softer chalk and peaty substrata. It was unable to bore into abnormally hard clays, soft loose mud, shifting sand, gritty and Lower Greensand Gault clay, hard chalk and Blue Lower Lias. Thus its distribution is determined by changes in the substratum of the shore rather than by tidal level (the piddock may flourish from extreme low water to mid-tide level).

Hydroids living in pools in the peat are opportunistic carnivores mainly catching suspended plankton as food.

Filter / suspension feeding organisms such as the piddocks, Barnea candida and Petricola pholadiformis; the peacock worm, Sabella pavonina and sand mason worm, Lanice conchilega, are the dominant trophic group in the biotope, indicating the importance of planktonic inputs to the community. Piddocks probably contribute to the creation of a relatively high silt environment through burrowing activities.

Crabs, such as Carcinus maenas and Cancer pagurus, are the predominant mobile species in the biotope, travelling through as they scavenge for food.

The anemone, Sagartia troglodytes, which may occur in crevices of the peat, uses 'catch tentacles to prey upon small shrimps and crabs. In turn, Sagartia troglodytes is preyed upon by the grey sea slug, Aeolidia papillosa, and attacked by the tompot blenny, Parablennius gattorugine (BMLSS, 2002c) that may frequent the biotope

Algae that grows on the surface of the peat may provide shelter for small crustaceans and possibly a source of food for grazing prosobranchs, such as Littorina littorea, which may occasionally occur in the biotope but is not characteristic.

Species of isopod and amphipod may also feed on detrital matter within the dense algal mat and prey upon each other.

• One of the characteristic species of this biotope, Petricola pholadiformis, has a longevity of up to 10 years (Duval, 1963a) and whose established populations may not exhibit significant seasonal changes, besides spawning in the summer. Variations in the abundance and seaweed species present would be expected to vary between and within locations according to the season. For instance, following storms, the peat may be covered by a layer of sand which could adversely affect the surface of algal species, especially propagules.

Outcrops of fossilized peat in the eulittoral may project above sand level by > 15 cm and form extensive platforms up to 100 m in length across the shore. Fossilized peat tends to be firm and relatively erosion resistant (Murphy, 1981), and occur in localities backed by extensive beach and dune systems, so that the patches of peat exposed varies according to sand movement. The peat is likely to have pits, crevices and undulations in surface level in addition to vacant piddock burrows. Empty piddock burrows can influence the abundance of other species by providing additional habitats and refuges. For instance, Pinn et al. (in press) found a statistically significant increase in species diversity in areas where old piddock burrows were present compared to where they were absent. Pools of water may accumulate in surface depressions which favour hydroids (e.g. Obelia longissima and prawns such as Crangon crangon). The covering of red and ephemeral green algae probably provide cover for cryptic fauna.

Algal species, Ceramium, Ulva, form a characteristic mat over the surface of the peat substratum so primary production is a component of productivity. Many of the characterizing species that are present in the biotope are suspension/filter feeders, so productivity of the biotope would probably be largely dependent on detrital input. However, specific information about the productivity of characterizing species or about the biotope in general was not found.

Most of the characterizing species in the biotope are sessile or sedentary. Consequently, recruitment must occur primarily through dispersive larval or spore stages. Examples of characterizing species are given below.

• Duval (1963a) reviewed the biology of Petricola pholadiformis. The sexes of Petricola pholadiformis are separate. Females are estimated to produce between 3,000,000 and 3,500,000 eggs annually. Gametogenesis takes place between April and early June and a waiting period ensues before spawning occurs towards late July and during August, lasting just over six weeks in total. The juvenile trochophore stage is reached within 28 hours, and the veliger stage in 44 hours. Length of planktonic life was estimated to be in the region of only one and a half to two weeks in duration, after which the young Petricola pholadiformis assume a benthic lifestyle, but remain extremely active. Juveniles of 0.4 cm length possess a very strongly ciliated and mobile foot and large amounts of mucus aid adherence to the substratum. Shell growth may begin in April or during May and continues until after June. Thereafter, growth rings are laid down annually, and annual growth in younger specimens is in the region of 0.7 - 0.9 cm.
  Similarly, the white piddock, Barnea candida, has separate sexes and fertilization occurs externally (Duval, 1963b). Many bivalves spawn during the part of the year when sea temperatures are rising. No information was found concerning length of planktonic life in Barnea candida but El-Maghraby (1955) showed that in southern England Barnea candida spawned in September, being unusual that it started to spawn when the temperature fell at the beginning of the autumn. The maximum age estimated for Barnea candida is 4 years with growth rates ranging from 0.1-6.8 mm per year (Pinn et al., 2005).
• Edwards (1973) reported that the red seaweed, Ceramium virgatum (as Ceramium nodulosum), has a triphasic life history consisting of a sequence of gametophytic, carposporophytic and tetrasporophytic phases in which the first and the third are morphologically similar. Maggs & Hommersand (1993) reported spermatangia in January, March-April, June and August-September; cystocarps in January-February and April-September; tetrasporangia in February-September. Although no information on dispersal has been found directly for Ceramium virgatum, Norton (1992) concluded that dispersal potential is highly variable in seaweeds, but recruitment probably occurs on a local scale, typically within 10m of the parent plant.
• The green seaweed, Ulva is considered to be opportunistic in its colonization of available substrata, its rapid recruitment made feasible by its life cycle, which consists of both sexual and asexual generations. Reproduction can occur throughout the year, but is maximal in summer. The haploid gametophytes (arising from sexual reproduction) of Ulva produce enormous numbers of motile gametes that fuse and germinate to produce sporophytes. Sporophytes also produce large numbers of motile spores that are released in such great numbers that the water can become green (Little & Kitching, 1996). The dispersal potential of such spores is great (> 10 km) so that the species may recruit from distant populations.
• Hydroids, such as Obelia longissima, are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). The hydroid phase of Obelia longissima releases dioecious sexual medusae that swim for up to 21 days (Sommer, 1992) and release sperm or eggs into the sea (fertilization is external). The resultant embryos then develop into planulae larvae that swim for 2-20 days (Sommer, 1992). Therefore, their potential dispersal is much greater than those species that only produce planulae. In addition, few species of hydroids have specific substratum requirements and many are generalists, for example Obelia longissima has been reported from a variety of rock and mud substrata.

Little information was found concerning community development. However, piddocks, Barnea candida and Petricola pholadiformis are likely to settle readily. These piddocks breed annually and produce a large number of gametes. Once established individuals may live for a considerable length of time; Petricola pholadiformis of length 5-6 cm are likely to be between 6-10 years old (Duval, 1963a). Another characteristic component of the biotope is the algal mat of Ceramium and Ulva that caps the peat and development of this algal mat would be expected to be rapid. For instance, panels were colonized by Ceramium virgatum (as Ceramium nodulosum) within a month of being placed in Langstone Harbour (Brown et al., 2001), whilst Ulva spp. Are known to colonize available substrata rapidly. Barnea candida grows rapily to a length of approximately 25-35 mm within 2 to 3 years, living for a maximum of 4 years (Pinn et al., 2005).

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