|Basic Information||Biotope classification||Ecology||Habitat preferences and distribution||Species composition||Sensitivity||Importance|
Image Keith Hiscock - Cordylophora caspia and Electra crustulenta on reduced salinity infralittoral rock. Image width ca 6 cm.
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IR.SIR.EstFa.CorEle recorded () and expected () distribution in Britain and Ireland (see below)
Hydroids may be important in transferring energy from the plankton to the benthos (bentho-pelagic coupling), due to their high feeding rates (Gili & Hughes, 1995), and bryozoans may be equally important in this community. For example, Obelia was reported to be an important regulator of local populations of copepods (Gili & Hughes, 1995). Bryozoans such as Electra crustulenta are active suspension feeders on bacteria, small flagellate phytoplankton, algal spores and small pieces of abraded macroalgae or detritus, although they are probably dependant on currents to bring adequate food within reach (Winston, 1977; McKinney, 1986; Best & Thorpe, 1994; Hayward & Ryland, 1998). Hydroids such as Cordylophora caspia are passive carnivores that capture prey that swim into, or are brought into contact with their tentacles by currents. Prey are then killed or stunned by the nematocysts born on the tentacles and swallowed. Diet varies but is likely to include small zooplankton (e.g. nauplii, copepods), small crustaceans, chironomid larvae, detritus and oligochaetes, but may include a wide variety of other organisms such as the larvae or small adults of numerous groups (see Gili & Hughes, 1995). The barnacle Balanus crenatus is also a suspension feeder on phytoplankton, zooplankton and detritus.
The three species recorded in the biotope probably compete for space when they occupy the same hard substrata and all grow rapidly. However, Cordylophora caspia can probably grow on the shells of Balanus crenatus and encrusting bryozoans may survive overgrowth by other species (Gordon, 1972; Todd & Turner, 1988). However, in the Tamar estuary Cordylophora caspia dominated the shallower areas of the biotope, while Electra crustulenta and Balanus crenatus occurred in deeper water, presumably removed from the lowest salinities and freshwater influence at nearer the surface.
Few of the typical predators of hydroids and bryozoans (Ryland, 1976; Gili & Hughes, 1995) are present in the low, variable salinities characteristic of this biotope. Roos (1979) reported that the freshwater amphipod Gammarus tigrinus ate the polyps of Cordylophora caspia in the low and variable salinity river system of western Holland. It is likely that estuarine and freshwater amphipods and fish (e.g. sticklebacks) are potential predators on the hydroid in this biotope. The lagoonal sea slug Tenellia adspersa feeds on Cordylophora caspia in lagoons and brackish waters (Gaulin et al., 1986; Chester et al., 2000) and tolerates salinities as low as 3 psu (see MarLIN, review). Arndt (1989) suggested that the marine distribution of the brackish water hydroid Cordylophora caspia was probably limited by food availability, competition from Clava spp. or Laomedea spp. and predation e.g. from the nudibranch Tenellia adspersa (as Embletonia pallida). However, Tenellia adspersa, Clava spp or Laomedea spp. were not recorded in this biotope.
Electra crustulenta breeds between March and July in British waters (Hayward & Ryland, 1998). Electra crustulenta probably has a similar life history to Electra pilosa, and is probably adapted to ephemeral habitats, growing and reproducing rapidly, although the colony may potentially survive for many years.The barnacle Balanus crenatus reproduces between February and September, larvae settling in a peak in April from October. Balanus crenatus has a life span of only 18 months, and unless recruitment is continuous, the population probably fluctuates but no evidence was found.
The biotope probably experiences seasonal changes in physical conditions, with increased riverine input and hence suspended sediment, nutrients, and reduced salinity in winter months, followed by reduced riverine input, water levels and water flow rates in the summer months.
Balanus crenatus releases planktonic nauplii that develop into a specialized settlement phase, the cyprid (see review). The nauplii may spend >30 days in the plankton, and cyprids settle between April and October with a peak in April. Therefore, dispersal potential is high, depending on the local hydrographic regime. Balanus crenatus also colonized settlement plates or artificial reefs within 1-3 months of deployment in summer, (Brault & Bourget, 1985; Hatcher, 1998), and became abundant on settlement plates shortly afterwards (Standing, 1976; Brault & Bourget, 1985). In this biotope most recruits probably come from other populations within the Tamar and Plymouth Sound.Electra crustulenta probably has a similar life history to that of Electra pilosa, which has a planktonic larvae with a protracted life in the plankton and potentially extended dispersal, and can colonize a wide variety of substrata. It is probably adapted to rapid growth and reproduction (r-selected), capable of colonizing ephemeral habitats, but may also be long lived in ideal conditions (Hayward & Ryland, 1998). In settlement studies, Electra crustulenta recruited to plates within 5 -6months of deployment, although it did not recruit to the low salinity panels occupied by Cordylophora caspia in their study (Sandrock et al, 1991). Standing (1976) noted that the branches of Obelia longissima physically interfered with recruitment in Balanus crenatus and dense Cordylophora caspia branches may have a similar effect as well as potentially consuming larvae of other species such as Electra crustulenta. However, in the riverine/estuarine transition occupied by this biotope, Balanus crenatus and Electra crustulenta are probably at their limit of salinity tolerance and recruitment is probably low.
This review can be cited as follows:
Tyler-Walters, H. 2002. Cordylophora caspia and Electra crustulenta on reduced salinity infralittoral rock. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 30/11/2015]. Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=27&code=1997>