Biodiversity & Conservation

Little information regarding the ecology of this community was found. The information that follows is based on survey data (Hiscock & Moore, 1986, Moore et al., 1999), the ecology of hydroid and bryozoans communities (Ryland, 1970, 1976; Gordon, 1972; Boero, 1984; Sebens, 1985, 1986; Gili & Hughes, 1995; Stephanjants, 1998) and the biology of individual species.

This biotope is dominated by suspension feeding bryozoans and hydroids and few macroalgae are found in this biotope due to the high turbidity of the water. 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, a species of Obelia was reported to be an important regulator of local populations of copepods (Gili & Hughes, 1995). Bryozoans such as Conopeum reticulum, Electra crustulenta and Bowerbankia imbricata 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 Hartlaubella gelatinosa, Clava multicornis, Obelia longissima and Obelia dichotoma 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).

Other suspension feeders include the barnacle Balanus crenatus, the sand mason worm Lanice conchilega and ,if present, Mytilus edulis spat and the tube worms Pomatoceros spp.

The crab Carcinus maenas and the shrimp Palaemon serratus are probably scavengers within the biotope, although Carcinus maenas may prey on Balanus crenatus.

Hydroids and bryozoan communities are generally preyed on by sea spiders (pycnogonids) and sea slugs (nudibranchs), however, they are probably excluded from this biotope by the low salinities (see Arndt, 1989). Amphipods and grazing fish such as shannies and wrasse have been reported to take hydroids or bryozoans (e.g. Ryland, 1976; Roos, 1979). Although the reported species may not be present in this biotope, it is likely that estuarine and freshwater amphipods and fish (e.g. sticklebacks) are predators.

Competition for space
Space occupying species such as the hydroids, bryozoans and barnacles probably compete for available space and are early initial colonizers of available hard substratum. Balanus crenatus and Obelia spp. colonized flat substratum, modifying the surface roughness and complexity (see habitat complexity) and attracting the settlement of other species (Standing, 1976; Brault & Bourget, 1985).

Obelia spp. could settle on any surface, including the barnacles, however, the uprights of Obelia spp. physically discouraged settlement of Balanus crenatus cyprids, resulting in increased settlement by ascidians which preferred the reduced water flow conditions between the interstices of the hydroid turf (Standing, 1976).

The hydroid turf provides a potential, filamentous, settlement substratum for Mytilus edulis spat (Standing, 1976; Brault & Bourget, 1985).

The hydroid species found in the biotope probably compete for both space and food, although the upright growth probably maximises their growth, and their relative abundance is probably due to differences in growth rate and their tolerance of variable salinity.

Conopeum reticulum grows rapidly to secure space, and encrusting bryozoans may survive overgrowth by other organisms (Gordon, 1972; Todd & Turner, 1988)

In the strong tidal streams of this biotope Conopeum reticulum may benefit from its proximity to hydroid turf. which results in reduced local water flow and improved feeding efficiency.

Overall, Sebens (1986) suggested that encrusting bryozoans and hydroids were early colonizers but poor competitors that were generally overgrown or out competed by other species. However, their success in this biotope in probably due to the absence of other competitive organisms and predators due to low and variable salinity.

Hydroids and encrusting bryozoans are early colonizers of hard substrata. In settlement experiments in Poole Bay, Dorset, Jensen et al. (1994) noted that hydroids and encrusting bryozoans were most abundant in summer, decreasing in abundance over winter. Brault & Bourget (1985) noted that most settlement of Obelia longissima and Balanus crenatus occurred in spring in the St Lawrence estuary, however, Obelia longissima showed annual variation in settlement intensity, and in one year experienced high mortality in summer only to recover due to new settlement in late autumn. In addition, the length of Obelia longissima branches was maximal in winter and minimal in summer in the St Lawrence estuary. Mortality of Obelia longissima and Balanus crenatus resulted in major changes and loss of species from the community (Brault & Bourget, 1985). In British waters, Balanus crenatus cyprid larvae settle between April and October, while the larvae of Conopeum reticulum are present in the plankton from July to September, Hartlaubella gelatinosa reproduces between May to November, Obelia longissima and Obelia dichotoma reproduce in summer (see species reviews, Gili & Hughes, 1995; Cornelius, 1995b).
Overall, there is likely to be seasonal variation in abundance of the hydroids and encrusting bryozoans and their settlement, probably peaking in the summer months in temperate waters. Seasonal changes in freshwater runoff, in winter months, will probably affect the extent of the biotope into the upper estuary, with species tolerant of reduced salinities that invade the biotope in summer being excluded by lower salinities in winter.

Little information regarding the ecology of this community was found. The information that follows is based on survey data (Hiscock & Moore, 1986, Moore et al., 1999), the ecology of hydroid and bryozoans communities (Ryland, 1970, 1976; Gordon, 1972; Boero, 1984; Sebens, 1985, 1986; Gili & Hughes, 1995; Stephanjants, 1998) and the biology of individual species.

The estuarine epifaunal communities are relatively impoverished and do not exhibit the degree of species diversity and habitat complexity characteristic of other epifaunal communities (e.g. see Gordon, 1972 and Sebens, 1985, 1986).

• 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 ciliates.
• Hydroid turf may also provide suitable settlement substratum for Mytilus edulis spat and refuges for amphipods.
• Balanus crenatus provides additional surface roughness and creates spatial refuges for other species (Standing, 1976; Roos, 1979; Brault & Bourget, 1985).
• Where present Bowerbankia imbricata may cover all available surfaces, including other species.
• The underlying muddy sediments support deposit feeding Arenicola marina in burrows and Lanice conchilega that protrudes from the sediment surface.

The majority of productivity within the biotope is secondary production 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.

The bryozoans Conopeum reticulum and species of Electra produce pelagic cyphonautes larvae with an extended planktonic life of between one to three months in the plankton (Reed, 1991). Colonies of Electra pilosa containing eggs and sperm are present in August and September and cyphonautes larvae are present in the plankton throughout the year, settling between April and the end of November, with peaks in May/June and July to August (Ryland, 1967; Hayward & Ryland, 1998). Electra crustulenta breeds between March and July. Conopeum reticulum breeds between June and early October in the British Isles and larvae were present in the plankton in the same period (July to September) in the River Crouch and River Blackwater (Cook, 1964). Both Conopeum reticulum and Electra spp. are members of fouling communities and probably exhibit good dispersal and potentially very rapid recruitment.

Hydroids are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). Hartlaubella gelatinosa lacks a medusa stage, releasing planula larvae. Planula larvae swim or crawl for short periods (e.g. <24hrs) so that dispersal away from the parent colony is probably very limited (Sommer, 1992; Gili & Hughes, 1995). However, in Obelia longissima and Obelia dichotoma, the hydroid phase releases dioecious sexual medusae, which 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 substrata requirements and many are generalists, for example Hartlaubella gelatinosa, Obelia longissima and Obelia dichotoma were reported from a variety of hard substrata, together with mud and sand in the case of Hartlaubella gelatinosa (Cornelius, 1992; Cornelius, 1995b). Hydroids are also capable of asexual reproduction and many species produce dormant, resting stages, that are very resistant of environmental perturbation (Gili & Hughes, 1995). Rapid growth, budding and the formation of stolons allows hydroids to colonize space rapidly. 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 (Cornelius, 1992; Gili & Hughes, 1995).

Balanus crenatus is an obligate cross-fertilizing hermaphrodite that releases nauplii larvae between February and September, with peaks in April and late summer when phytoplankton levels are highest. Peak settlement occurs in April and declines until October. April-settled individuals may release larvae the same July and reach full size before their first winter. Individuals that settled later reach maximum size by the end of spring the following year, although they only live for 18 months (see review).

The polychaetes Arenicola marina and Lanice conchilega are both probably at the limit of their salinity range in this biotope. In both species external fertilization results in formation of a trochophore larvae, which is pelagic in Lanice conchilega, with potentially wide dispersal, but in Arenicola marina develops within the female burrow, and crawls away as a juvenile (see reviews for detail).

Hydroids are often initial colonizing organisms in settlement experiments and fouling communities (Standing, 1976; Brault & Bourget, 1985; Sebens, 1986; Jensen et al., 1994; Hatcher, 1998). In settlement experiments the hydroids Cordylophora caspia, Obelia dichotoma and Obelia longissima colonized artificial substrata within ca 1-3 months of deployment (Standing, 1976; Brault & Bourget, 1985: Sandrock et al., 1991). Brault & Bourget (1985) reported that Obelia longissima reached a stable abundance within ca 3 months, whereas Jensen et al. (1994) noted that hydroid abundance increased during summer after deployment but increased markedly in the following summer. Once colonized the hydroids ability to grow rapidly and reproduce asexually is likely to allow it to occupy space and sexually reproduce quickly. Conopeum reticulum probably exhibits good dispersal and potentially very rapid recruitment. Hatcher (1998) reported that spring recruitment to an artificial reef in Poole Bay was dominated by tubeworms and encrusting bryozoans including Conopeum reticulum. Conopeum reticulum colonized artificial reef surfaces within 6 months from May to October 1991 (Hatcher, 1998). 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). Mobile fauna and diatoms were reported to occupy the hydroid/ barnacle covered plates within 12 months (Brault & Bourget, 1985). In a detailed study of subtidal epifaunal communities, Sebens (1986) noted that rapid colonizers, including encrusting bryozoans, tube mat forming polychaetes and amphipods and erect hydroids, covered previously cleared (scraped) areas within 1-4 months in spring and autumn. Overall, it appears that the dominant species within the community are likely to establish and grow to maximum abundance rapidly and given the small number of species recorded within the community, reach maturity within 6 months at the most.

None entered.