Selection of predator-free settlement sites by larval ascidians

Behavioural Responses of the Colonial Sea Squirt Botrylloides violaceus Oka to Suspended Food Micro-Particles in Laboratory Cultures

Violet sea squirts are noteworthy model organisms, because they provide insights into various physiologic processes, including cell senescence, ageing, apoptosis and allorecognition. Consequently, their culture is critical to permit experimental studies. Most papers refer to short periods of rearing using various feeds, both living and conserved, missing a formal justification for their use or indications of their actual nutritional value. Here, we use two behavioural responses—the percentage of open siphons and the frequency of zooid contractions—as compared to the abundance of suspended microparticles during feeding tests, to identify feeds able to promote filter-feeding. The results will enable to formulate compound diets that maximise positive physiological responses. Our tests demonstrated that plant items, such as dry microalgae and cyanobacteria (Arthrospira platensis, commercially known as Spirulina), along with living planktonic Haptophyta (Isochrysis galbana), trigger clear positive reactions, represented by a higher frequency of zooid contractions and larger proportions of open siphons. These responses correspond to decreases in the concentrations of suspended microparticles during the experiment and indicate higher filter-feeding activity. In contrast, feeds commonly administered to colonies, such as milk powder, dried eggs and artificial plankton, triggered negative behavioural responses, and their intake was lower during the feeding trials.

Living on the edge: Early life history phases as determinants of distribution in Pyura praeputialis (Heller, 1878), a rocky shore ecosystem engineer

The distribution of intertidal organisms can depend on processes operating early in their life history. The ascidian Pyura praeputialis, a mid- to low-intertidal habitat-forming ecosystem engineer, was strongly associated with specific types of habitat (biogenic vs. bare rock). We examined field patterns and performed laboratory and field experiments to assess the nature of this association. Recruits were frequently found on the tunics of conspecifics and clumps of turfing coralline algae. Larvae preferred these same habitats in a series of laboratory settlement assays. Laboratory-reared juveniles (20- & 50-days-old) survived poorly on bare rock in the laboratory, while those on rugose surfaces - the tunic of adults and turfing corallines - showed high survivorship. Field-collected juveniles (<2 cm) affixed to these rugose habitats also exhibited high survivorship in the field. We conclude that both pre and post-settlement processes determine spatial pattern in this important habitat-forming taxon. The acute sensitivity of juveniles to desiccating conditions was unexpected in an intertidal organism.

A long-term mesocosm study on the settlement and survival of juvenile European lobster Homarus gammarus L. in four natural substrata

To date, the natural substratum preferences of early benthic phase (EBP) European lobsters (Homarus gammarus) remain largely unknown. This study utilised a large scale mesocosm experiment to determine if the animal favours cobble ground, similar to its American counterpart (Homarus americanus), or has other substratum preferences. Postlarvae were provided with the choice of settling on four natural substrata: sand, coralline algae, mussel shell and cobble. Over a nine month period, the number and size of juveniles on each substratum was recorded, with loss of chelipeds used as an indication of social interaction. After a 30 day period, a non-random distribution of lobsters was observed on the four substrata. Juveniles were more abundant in substrata which provided pre-existing shelter in the form of interstitial spaces, i.e. cobble and mussel shell, than in sand or coralline algae. The survival of individuals from postlarvae to 30 day old juveniles ranged from 5 to 14% with surviving benthic recruits showing a clear mode at 6-8 mm carapace length (CL) in size distribution. The density of lobsters per m(2) of cobble remained relatively constant (18/m(2)) throughout the study period while the density of juveniles on mussel shell decreased significantly (35 to 5/m(2)). The size distribution of lobsters on each substratum also varied with time. By the conclusion of the trial, lobsters found in mussel shell had a mode of 8-10 mm CL within a range of 6-14 mm CL while those in cobble had a mode of 10-12 mm CL within a range of 8-24 mm CL. Overall, the results underline the importance of shelter-providing habitat such as cobble or crevice-type substrata to EBP European lobsters. They also confirm that for a shelter-dwelling animal such as a lobster, the physical structure of the habitat is a key factor in determining both the size and number of its inhabitants.

Seasonal variation in the effects of hard substratum biofilming on settlement of marine invertebrate larvae

The effect of season on "biofilming";, as a cue for the settlement of marine invertebrate larvae, was investigated in a long-term field study during the years 1992-1994. The series of settlement experiments was conducted in a tidal rapid on the west coast of Scotland, and involved manipulations of artificial panels. Biofilming of substrata, whilst excluding larval settlement, was achieved by the enclosure of panels within tight-fitting (but removable) mesh screens so that the number of settlers on filmed and unfilmed substrata were counted in the initial absence of other incumbent post-larvae. Depending on larval species, the effects of biofilming were found to be either facilitatory or inhibitory. Significant within- and between-species seasonal differences in the settlement responses were detected, and a reversal of the effect of biofilming on larval settlement response, from inhibitory to facilitatory and vice versa, was noted with season in the case of some taxonomic groups and species (e.g. Tubulipora sp., Plagioecia sp., Electra pilosa (L.)). The present study emphasizes the need for extended field studies of larval responses to environmental cues, when the focus of interest is in drawing general inferences about naturally occurring behavioural patterns at settlement.