CON RELACIÓN A LA EJECUCIÓN:

colony longevity (Cancino, 1983, 1986). This pattern of plant growth was not

obvious at Clachan Seil (fig. 3.5), where growth appeared almost constant, a feature if

of the sublittoral kelps, and declined only in December. However, Cosson (1967) also noted such continuous growth in Laminaria digitata. Settlement of E. pilosa

was minimal on the laminarian at this site, despite the large surface area available.

This may perhaps be explained by the constant dynamic nature of this substratum, f

resultant from the continuous growth observed, acting as a possible anti-fouling mechanism.

Growth of Fucus serratus, mediated by apical meristems in common with the ‘

littoral fucoids and providing a contrast to the perennial Laminaria digitata, took place in the summer months, ceasing completely in winter (fig. 3.3). Cessation of growth has been shown to reflect the low, minimal levels of storage compounds present in the tissues, which are required only for respiration (Barrett & Yonge, 1985). In late autumn, following fruiting, a period of rapid defoliation in consequence of necrosis of reproductive tissue and the abrasion of the spent receptacles was reflected in the negative mean growth rates at both field sites. Concurrently, new frondage produced at the apical meristems (Knight & Parke, 1950) became conspicuous as new vegetative tissue. Aggregated settlement of E. pilosa on

the distal regions of F. serratus occured during the summer months when

uncolonised tissue was readily available. Apical defoliation after fruiting does,

however, present a possible source of substratum loss but this must be less critical to i

E. pilosa than the avoidance of potential competitors.

Electra pilosa, with its potential for exponential indeterminate growth (Hughes & Hughes, 1986; Rubin, 1987; Silén, 1987), rarely realised that potential in the field. Several colonies, however, attained near exponential growth (figs. 3.8a and

3.8b). Although this fact was masked in the mean aSGRs which bear no resemblance ^

Chapter 3 1986; Silén, 1987). At Clachan Seil, growth of individual colonies increased during the summer months before effectively ceasing in November (fig. 3.8b), in a manner similar to that previously described for F lustra foliacea (Stebbing, 1971). A decrease which was closely allied to the decline in surface seawater temperature below lOX (fig. 2.3b). E. pilosa has been shown to respond to differing temperature regimes with a variation in the growth rate (Menon, 1972); 6 X effectively reduced the rate to a minimum, whilst rising seawater temperatures have been reported by Ryland (1970, 1976) as initiating growth rate increases in the Bryozoa. In considering growth rates of the Bryozoa in the field it is perhaps more pertinent then to consider individual colony growth trajectories in relation to laboratory produced optimal growth rates and life histories, rather than the mean aSGR of a field population.

The maintenance of aSGR with increasing colony size at Clachan Seil, both on Fucus serratus and Laminaria digitata, as opposed to the quite rapid decrease in aSGR with size at St. Andrews (figs. 3.9a and 3.9b) was born out by the colony morphologies of Electra pilosa observed at both field sites. The large stellate colonies at Clachan Seil enabled maintenance of a high aSGR (Hughes and Hughes, 1986) both in an environment of severe interspecific competition within the highly diverse epiphytic community of F. serratus, and on the very dynamic unstable substratum of L. digitata, despite the low diversity and complexity of epifauna on laminarian fronds (pers. obs.). Furthermore these extrinsic environmental factors operating at Clachan Seil were compounded by the high incidence of predation by the nudibranchs Adalaria proxima and Onchidoris muricata (pers. obs.). In contrast, the slow growth period of L. digitata at St. Andrews (fig. 3.5) enabled colonies to become established, and the less diverse epiphytic community (pers. obs.) on F. serratus resulted in the more regular, circular form of E. pilosa. Such plasticity of colony morphology is in accordance with the adaptive strategy model of Buss (1979a), but see Okamura (1992), and was viewed as a plastic developmental Page 41

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i

Chapter 3 response to changes in extrinsic environmental factors (McKinney & Jackson, 1991). Indeed, the life-history characteristics of the bryozoan at the two sites and on the two algal species matched the quantitative predictions made by the risk model of McKinney & Jackson (1991). A sheet-like morphology is associated with low levels of disturbance and predictability, whilst a uniserial growth form is indicative of a more unstable ephemeral environment. However, Hayward & Ryland (1975) noted that as the available space on the fronds of F. serratus diminished, growth of

Alcyonidiim hirsutum slowed. Furthermore, the point from which growth began to

slow and the ultimate size of the colonies seemed to depend upon the density of j

settlement rather than on extraneous environmental factors. These factors have not, however, been investigated within the scope of this thesis.

Complete and partial colony mortality have complicated and distorted any

pattern in the mean aSGRs (figs. 3.7a and 3.7b) and have profoundly influenced the

4

dynamics of the bryozoan populations, mediated through the effects of physical and biological disturbance. Disturbance, in providing a means of space renewal, is one of the major structuring forces of the fundamentally space-limited communities on marine hard substrata (Dayton, 1971; Jackson, 1977; Osman, 1977; Sousa, 1979, 1984; Paine & Levin, 1981, Connell & Keough, 1985). Space is not limiting on macroalgae in consequence of the processes of constant regeneration of substrata during the growing season (Seed, 1985), but see Hayward & Ryland (1975). However, disturbance, presented as the constant attrition at the frond apices and the presence of disturbance gradients, is argued by Fletcher & Day (1983) to be a greater force of post settlement mortality than previously credited. Clearly, when the growth rate of a colonial animal which has the potential to realise exponential, indeterminate growth is suppressed, the factor instigating that suppression is of paramount importance, not only to the life history of that animal but also to the whole epiphytic community. The agents of disturbance, colony or frond abrasion and predation, have also been reported as the major forces influencing the growth of the phylactolaemate

Chapter 3

bryozoan Plumatella repens (Bushnell, 1966) and Alcyonidium hirsutum (Hayward

& Harvey, 1974), but consideration of these factors in the role of structuring epifaunal bryozoan assemblages has been limited (but see Seed, 1976; Todd & Havenhand, 1989) and will be discussed in Chapter 4 .

The intimate relationship between the epiphyte and its host may result in costs and consequences for both. This study has not considered the influence of Electra pilosa on the growth rate of either host plant species, whilst, aside from the roles of frond growth, attrition and abrasion, the influences of algal exudates and antifouling mechanisms have likewise not been investigated. There has, however, been recent interest in this area of algal epifaunal ecology. Whilst some believed that the fouling epifauna are inhibitory and deleterious to the plants (Wing & Clendinning, 1971; Woollacott & North, 1971; Bernstein & Jung, 1979; Dixon et al., 1981; Oswald et a i,

1984) yet others considered that the costs to the plant are less severe (Cancino et a i,

1987; Munoz e ta i, 1991).

Chapter 4