Biodiversity & Conservation

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.

Cordylophora caspia shows a clear annual cycle. It dies back in late autumn and over winters as dormant stolons and resting stages (menonts) inside the remnants of the uprights (Roos, 1979; Arndt, 1989; Jormalainen et al. 1994). Arndt (1989) reported that colonies died back in autumn when the temperature fell to about 10 C only to germinate in spring when the temperature exceeded 5 C. Roos (1979) reported that colonies died back in October and new polyps budded again in early spring in the Netherlands. In the Baltic Sea growth was maximal in spring, uprights reaching maximal height at the peak of sexual reproduction in July, with a decline in growth after sexual reproduction (and regression to dormancy in one observation), and subsequent growth in August (Jormalainen et al., 1994).

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.

This community is impoverished and does not exhibit the degree of species diversity and habitat complexity characteristic of other epifaunal communities. In the Tamar estuary, Cordylophora caspia dominated the steep bedrock (ca 100% cover) from +1 to 3m deep. However, from 3-4m the bedrock and small boulders were almost bare except for a few scattered colonies of Cordylophora caspia, frequent Electra crustulenta and rare Balanus crenatus (Moore & Hiscock, 1986). The upper waters in the upper estuary are probably more liable to variations in salinity due to freshwater runoff and riverine input, while the deeper waters may be more saline, allowing Electra crustulenta and Balanus crenatus to survive, albeit at the limit of their range.

• Hydroid branches form a turf that slow water flow within it and may accumulate a modicum of sediment that may itself support some meiofauna, while branches provide substratum for sessile ciliates (Roos, 1979).
• Hydroid turf may also support 'crowds' of the freshwater amphipod Gammarus tigrinus (Roos, 1979).
• Balanus crenatus provides additional surface roughness and creates spatial refuges for other species if present (Standing, 1976; Roos, 1979; Brault & Bourget, 1985).

The majority of productivity within the biotope is secondary through suspension feeding on phytoplankton by bryozoans and passive carnivory by hydroids. Gili & Hughes (1995) suggested that hydroid turfs were important in transferring energy from the plankton to the benthos, however, productivity in this impoverished community is probably low.

Hydroids are often initial colonizing organisms in settlement experiments and fouling communities (Jensen et al., 1994; Gili & Hughes, 1995; Hatcher, 1998). In settlement experiments in the Warnow estuary, Cordylophora caspia was found to colonize artificial substrata within ca one month of deployment, its abundance increasing from June to the end of September with a peak in July (Sandrock et al., 1991). Long term panels at their low salinity station became dominated by Cordylophora caspia, Balanus improvisus and Nais elinguis. Similarly, Roos (1979) reported that Cordylophora caspia recruited to and grew luxuriantly on water lily stalks in summer after early reproduction of nearby colonies in early spring.Cordylophora caspia releases a planula larva, no medusoid phase in formed, although planula may occasionally develop in the parent gonophores being released as juvenile polyps. Planula larvae swim or crawl for short periods (e.g. <24hrs) so that while local recruitment may be good, dispersal away from the parent colony is probably very limited (Gili & Hughes, 1995). Fragmentation may also provide another route for short distance dispersal. However, it has been suggested that rafting on floating debris (or hitch hiking on ships hulls or in ship ballast water) as dormant stages or reproductive adults, together with their potentially long life span, may have allowed hydroids to disperse over a wide area in the long term and explain the near cosmopolitan distributions of many hydroid species, including Cordylophora caspia (Gili & Hughes, 1995; Folino, 1999).

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.

All the species present in the biotope colonize, grow and occupy space rapidly. The community is largely dominated by the hydroid Cordylophora caspia, which while perennial, dies back in the winter months, only to grow back in the spring months. Therefore, the visible cover of Cordylophora caspia probably develops within the first few months of spring, rapidly occupying space. Balanus crenatus grows rapidly in winter (see review) and probably benefits from the lack of competition for food with the hydroid. Overall, the community is species poor and not known to support more than sessile ciliates and mobile amphipods (see habitat complexity) and hence reaches maturity within only a few months in spring.

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