Alan J. Butler

Direct observations of larval dispersal in the colonial ascidian Podoclavella moluccensis Sluiter: evidence for closed populations

The colonial ascidian Podoclavella moluccensis Sluiter (Aplousobranchia, Polycitoridae) occurs on discrete patches of substrata, separated by uninhabitable sandy seafloor. The potential for exchange between these patches was determined by continuously observing individual larvae from their release from adult colonies until their settlement. Of 270 larvae, 100 were successfully followed to settlement and 80% settled within 2.5 m of their parent colonies. The average distance moved by larvae was 2.2 m after swimming for < 2 min. The distance over which larvae dispersed was a positive function of the time that they spent swimming, but was influenced by current speed and the behaviour of larvae. We conclude that populations of P. moluccensis have extremely limited potential for gene flow and may be regarded as effectively closed systems.

Modification of Egg Production and Packaging in Response to Food Availability by Nassarius pauperatus

SummaryAdjacent populations of Nassarius pauperatus differ significantly in how hungry their members are. The numbers of eggs and egg capsules produced are positively correlated with how hungry members of a population are, but the average number of eggs per capsule is negatively correlated with hunger. These results were confirmed in a laboratory experiment using two groups from the same population maintained with different levels of food.The significance of this behaviour is discussed and the hypothesis proposed that the flexibility of reproductive patterns is correlated with dispersive ability.

Effect of patchsize on communities of sessile invertebrates in Gulf St Vincent, South Australia

Abstract A qualitative model had earlier been proposed to explained why “good competitors that are poor recruiters” (GC) are more abundant in the sessile assemblages on large substrata than “Poor competitors that are good recruiters” (PC), the PC being relatively more abundant on small isolated patches of substratum. One consequence of this model was that the composition of the fauna, and nature of differences between patch sizes, would vary with larval recruitment rates. In this study, sessile fauna were recorded on shells of the bivalve Pinna and on pier pillings at three sites, and an experiment on colonization of plates of five size was done at two sites, chosen from earlier work to have different recruitment rates. In the compasition of Pinna and pilings, predictions were confirmed for colonial ascidians, bryozoans, the lumped groups PC (bryozoans, tuberworms and barnacle) and “very poor competitors” (tubeworm and barnacles), partially for GC, but not for sponges. In the case of colonial ascidians and GC, the proposed mechanism did not fully explain the results. In the plate experiment, predictions were confirmed for ascidians, PC baranacles and tubeworms, partially for GC but not for sponges or bryozoans. Over the timescale of the experiment, the effect of heavy recruitment of PC was more important than their subsequent dominance by GC. It is suggested that with more time the effect postulated in the model would have become clearer, but also that it would have required very much greater resources to make a more effective test of the model underlying this work, which involves complex population structures and spatial, temporal and spatio-temporal variation. Meanwhile, the study raises questions worthy of study at smaller scale; these concern the population structure and dynamics of particular species, and the responses of major animal groups to variations in food and physical conditions.

‘Stability’ of the fouling communities on the pilings of two piers in South Australia

SummaryThe subtidal communities of sessile organisms on the pilings of two piers were observed by photography of fixed quadrats over a period of two years. Data are presented on species-composition, relative abundance of species, and maximum longevities of individuals and colonies. The fluctuations in various properties of the communities (e.g. measures of species diversity, and percentage vovers of species and phyletic groups) are tested by a priori defined criteria for “narrow stochastic boundedness” and most are found to be narrowly bounded, despite considerable turnover of space. We conclude that most properties of the community vary less through time-that is, the community is in a sense more “stable”-than fouling communities previously studied using small plates immersed for relatively short periods. Long-lived species are relatively more abundant on the pilings than in plate-studies. These differences are attributed to the large size and long period of submersion of the pilings. Our results appear similar to those of other studies on large, old substrata.

Development and long term dynamics of a fouling assemblage of sessile marine invertebrates

The construction of a new pier in upper Spencer Gulf, South Australia, was used as an opportunity to test theories about recruitment and dynamics of subtidal assemblages of sessile invertebrates. The fouling fauna was monitored for ca six years after initial immersion of piles using photographs of fixed positions and direct observation by divers. Assemblages were traced through time using non‐metric multidimensional scaling (MDS). Faunal composition differed at sites along the pier throughout the study, but the composition at all sites tended to change in a similar way over time, and seemed to be changing more slowly near the end of the study. Abundances of key taxa fluctuated markedly from site to site along the pier, but for some taxa a trend was discernible over and above the variability. Predictions based on experiments on piers in more sheltered waters in an adjacent gulf were not fulfilled; although over 50% of pile surface area was covered by encrusting or mound‐forming colonial animals such as spo...

Recruitment of sessile invertebrates at five sites in Gulf St. Vincent, South Australia

Abstract Recruitment of sessile invertebrates to small asbestos plates was measured at five piers in Gulf St. Vincent and Investigator Strait, South Australia. The established faunas on the piles of the piers differed at the level of higher taxa (sponges, colonial ascidians, bryozoans, cnidarians). Recruitment differed at that level and also with respect to certain lower taxa. The piers were not ranked in the same way by the recruitment of different higher taxa, but within one (Ectoprocta) the recruitment of different species tended to rank piers in a similar way. One site stands out as having high recruitment rates of most, but not all, animal groups. In further testing of hypotheses concerning the dynamics of sessile communities, an approach at the level of higher taxa and growth forms appears worth pursuing.

The measurement of hunger as a relative estimate of food available to populations of Nassarius pauperatus

SummaryIt is often difficult to estimate the amount of food available to an animal population, especially for species having unspecialised diets. The intertidal snail Nassarius pauperatus is omnivorous, feeding on both algae and animal carrion. Sampling the standing crops of carrion and algae was not thought to be a valid measure of the amount of food available to N. pauperatus. Instead, food availability was estimated indirectly, by measuring how hungry snails were.The percentage of snails feeding within 15 min of having been offered a meal of the bivalve Katelysia scalarina (Lamarck) was found to be a realiable and comparative estimate of food availability, and was measured in the field at 17 locations. The mean length of adult Nassarius pauperatus was negatively correlated with hunger, and at locations having high indices of hunger there was evidence of intraspecific competition for space to feed around items of carrion.These results are discussed and it is proposed that Nassarius pauperatus are surrounded by algal food of poor quality, with carrion being an important part of their diet for survival, growth and reproduction.

A shortage of food for the terrestrial snail Helicella virgata in South Australia

SummaryTwo field experiments were conducted to investigate the likelihood of snails experiencing a shortage of food. In the first experiment and in one by a previous worker, various measures of growth, reproduction, activity or survival decreased with increasing density of snails. These results appear inconsistent with the hypothesis of an absolute shortage of completely-accessible food. They could be explained by a modification of that hypothesis or by the hypothesis that snails at high density interfere with each other by means other than competition for food.The results of the second experiment suggest that the food naturally available on the study-area is of poor quality; the snails may experience a relative shortage of food, of the kind where food is abundant but much of it is low in nutrients.

Relationships between height on the shore and size distributions of Thais SPP. (Gastropoda: Muricidae)

Abstract Average body sizes in a variety of marine animals have been reported to be correlated with height in the intertidal zone. This paper reports field observations on four species of Thais showing that the relationship between size distribution and intertidal height or subtidal depth is highly variable. Intertidal size gradients in Thais have been explained by a size dependent set of responses to light and gravity, but experiments reported here do not indicate that snails of different sizes behave differently with respect to the intertidal gradient per se and it is argued that, when there is a size gradient, it is produced by the direct responses of snails to factors such as shelter, temperature, desiccation, and food. It is suggested that a response to token stimuli is unlikely to evolve when the token stimuli are poorly correlated with the relevant environmental factors, especially if the latter can be monitored directly by the animal.

Antifouling activity oflyso-platelet-activating factor extracted from australian spongeCrella incrustans

Larvae of the ascidianClavelina moluccensis do not settle in the field on the spongeCrella incrustans and are deterred from settling by crude chemical extracts of the sponge. Progressive refinement of extracts, guided by bioassays, led to the identification oflyso-platelet-activating factor (lyso-PAF) from the sponge.lyso-PAF is inhibitory in assays using ascidian, bryozoan, barnacle, and algal settlement and mussel byssal attachment. Its role in the field has not yet been demonstrated.