Flustra foliacea

Growth form
The newly metamorphosed coronate larvae develops into the first zooid of the new colony, the 'ancestrula'. In its first year of growth, Flustra foliacea forms a flat incrustation on the substratum and commences erect growth during the second year. This is achieved simply by the opposition of actively growing lobes of a colony; on contact two growing edges are deflected vertically (J. Porter, pers. comm.).
The two layers of zooids grow, synchronously 'back to back' forming a bilaminar, erect frond at 90 ° to the original encrusting mat. Branching of the erect fronds varies between branches and colonies (Stebbing, 1971a; Silén, 1981).
Ryland (1976) suggested that erect growth avoids the spatial constraints (availability of substratum and competition) suffered by encrusting forms. Repair of grazing damage, i.e. removal of one bilaminar layer, may result in generation of an new bilateral branch (Stebbing, 1971a).

Growth rates
Stebbing (1971a) reported that growth began in late February/early March but stopped in November in specimens off the Gower Peninsula , with a slight check in growth in August, and no growth occurred over winter. Growth stopped in October in Isle of Man specimens (Eggleston, 1963; cited in Stebbing, 1971a). The winter growth check results in visible annual growth lines, which have been used to age colonies (Stebbing, 1971a; Eggleston, 1972; Menon, 1978). Stebbing (1971a) suggested that the growth line formed a line of weakness, which gave the frond flexibility.

Stebbing (1971a) stated that the length of time spent as an encrusting form was unclear but assumed the first growth line at the base of the frond represented the first winter, 1 years growth. Flustra foliacea colonies regularly reached 6 years of age, although 12 year old specimens were reported off the Gower Peninsula (Stebbing, 1971a; Ryland, 1976). Furthermore, O'Dea & Okamura (2000) demonstrated seasonal fluctuations in zooid size synchronous with temperature regimes, the largest zooid zooids occurring with the lowest temperatures.

Stebbing (1971a) reported that growth rates were reasonably consistent between samples, age classes and years. Stebbing (1971a) reported a mean increment in frond height of 16.8mm/yr, whereas Eggleston (1972) reported that annual lines were usually between 2-3cm apart in Isle of Man specimens, and Menon (1978) reported that Helgoland specimens reached an average of 21.2 mm in height at 2 years old and an average of 79.3 mm after 8 years. Silén (1981) reported that erect fronds grew in zooid number about 10-20 times that of the encrusting base. Menon (1978) reported that growth rates varied in specimens over 5 years old.

At the base of fronds, in the holdfast area, the zooids give rise to layers of non-feeding frontal buds after 3 years of age, which strengthen the base of the frond. The number of layers increases with frond height up to 145mm in height, and up to 20 layers deep (Stebbing, 1971a).

Regeneration and repair
Silén (1981) reported that experimental removal of a notch in the frond was repaired within 5 -10 days. The newly formed margin grew at normal rates (4-5 zooid lengths per month). Removal of one layer of the bilaminar frond, experimentally (Silén, 1981) or by predators (Stebbing, 1971a) was repaired with similar rapidity, the un-damaged layer, halting growth while the damaged area was repaired (Silén, 1981).

Epiphytes
The epizoic fauna of Flustra foliacea was described by Stebbing (1971b) and consisted of 25 species of bryozoan, 5 hydroid species, some sessile polychaetes, barnacles, lamellibranchs and tunicates. The bryozoans Bugula flabellata, Crisia spp. and Scrupocellaria spp. were major epizoites. The stolons of Bugula flabellata penetrate the zooids of Flustra foliacea. Scrupocellaria spp. settled preferentially on the youngest, distal, portions of the frond, possibly to elevate their branches into faster flowing water (Stebbing, 1971b). A small green alga Epicladia flustrae was reported to be a specific epiphyte (Nielsen, 1984). Stebbing (1971a) reported that the growth rates of Flustra foliacea were reduced by ca 50% when encrusted by epizoites. Peters et al, (2003) reported the presence of chemical compounds in Flustra foliacea that demonstrated antagonistic effects against the growth of some associated bacterial species, and electron microscopic examination of the distal end of the zooid revealed no microbial settlement. Dyrynda (1985, cited in Peters et al., 1985) reported the toxicity of extracts of Flustra foliacea on larvae of other modular invertebrates, fish and bacteria.

• Male zooids of Flustra foliacea in the Isle of Man were reported to be full of sperm in September, giving the entire colony a white appearance. Sperm were absent by October. Orange eggs were visible in August and the yellow coloured embryos had entered the ooecia (ovicells) by October (Eggleston, 1970; 1972a). Sperm are shed from pores in the polypide tentacles of male zooids.
• Fertilization in brooding species such as Flustra foliacea is probably internal (Hayward & Ryland, 1998). In bryozoans, released sperm are entrained by the tentacles of feeding polypides and may not disperse far, resulting in self-fertilization. However, genetic cross-fertilization is assumed in oviparous and brooding bryozoans, although there is evidence of self fertilization (Hayward & Ryland, 1998).
• Eggleston (1972a) reported that about one third of zooids produced a single embryo in their first and second years, but that older zooids were infertile. Embryos were brooded over winter and larvae released between February and April.

• Larvae are positively phototactic on release, and swim for only short periods, although in species in which light stimulus is un-important, larvae may delay metamorphosis for 12 hrs of more (Hayward & Ryland, 1998). Daylength is an important cue for larval release in some species of bryozoa, and Flustra foliacea releases larvae in spring (February- April) (Eggleston, 1972a; Hayward & Ryland, 1998), however, at the depths Flustra foliacea can occur light may not be important.
• Larvae are probably sensitive to surface contour, chemistry and the proximity of conspecific colonies. However, Hayward & Ryland (1998) suggested that larval behaviour at settlement is only of prime importance to species occupying ephemeral habitats. Eggleston (1972b) demonstrated that the number and abundance of species of bryozoan increased with increased current strength, primarily due to a resultant increase in the availability of stable, hard substrata (Eggleston, 1972b; Ryland, 1976). Dyrynda (1994) noted that the abundance of Flustra foliacea was greatest in the deepest, and most current scoured, mouth of Poole Harbour due to the presence of circalittoral boulders not commonly found in other parts of the harbour, although reduced salinity probably also restricted its distribution within the harbour. Therefore, recruitment is probably dependant on the availability of suitable substratum.
• The short larval life probably results in good local but poor long-range dispersal. Ryland (1976) reported that significant settlement in bryozoans was only found near a reservoir of breeding colonies. However, the hydrographic regime probably strongly influences potential dispersal, and in the strong currents tolerated by Flustra foliacea, larvae may be transported some distance in a short time, unless constrained within eddies between faunal turf forming species. The sand abrasion tolerated by Flustra foliacea may remove other species, providing Flustra foliacea with space to colonize.