Larval settlement behaviour in six gregarious ascidians in relation to adult distribution

Abstract

Settlement influences the distribution and abundance of many marine organisms, although the relative roles of abiotic and biotic factors influencing settlement are poorly understood. Species that aggregate often owe this characteristic to larval behaviour, and we investigated whether this predisposes ascidians to becoming invasive, by increasing their capacity to maintain their populations. We explored the interactive effects of larval phototaxis and geotaxis and conspecific adult extracts on settlement rates of a representative suite of 6 species of ascidians that form aggregations in the field, including 4 aliens with global distributions, and how they relate to adult habitat characteristics. In the laboratory, the larvae were (1) held in light or dark, (2) offered the choice of settling in the light or dark, or (3) held in the presence or absence of adult extract. When confined in either light or dark conditions, all species settled equally in dark and light. Four showed strong geotaxis, 3 settling preferentially on the bottom of experimental chambers, and one on the top. Offered a choice between dark and light, 2 species settled preferentially in the dark with no geotactic preferences and another 2 showed an interaction between light and geotaxis. For 4 of the species, the responses of settlers accorded with, and may contribute to, adult orientation patterns in the field. Adult extracts inhibited settlement of 3 species and failed to influence settlement of the other 3, arguing against conspecific attraction being a cause of aggregation and an explanation of the propensity of ascidians to become invasive.

Habitat formation prevails over predation in influencing fouling communities

“…Our study suggests that under low consumer pressure, mobile (and probably sessile) assemblages on floating pontoons are mainly driven by biotic habitat formation rather than by physical structure through time. This outcome echoes previous results (e.g., Rius, Branch, Griffiths, & Turon, ; Sellheim et al., ; Smith et al., ). The correlations between ecospace and MF categories appeared site‐dependent (Figure ), which may be due to different species pools and relative abundances between sites.…”

Predation contributes to invasion resistance of benthic communities against the non-indigenous tunicate Ciona intestinalis

“…Recruitment panels both in benthic habitats and in the water column were attached in horizontal position, because in a previous study, we observed that recruits of the ascidians C. intestinalis and P. chilensis were more abundant on the underside of artificial structures (e.g., bottom of scallop culture cages) (Dumont et al 2009). Ascidian larvae preferentially settle on shaded substratum (Miller and Etter 2008), which had also been confirmed for C. intestinalis (Rius et al 2010). Panels on rocky bottoms were attached with rigid wire cables (at 5 cm from the bottom) to mesh bags (92 9 59 cm) filled with rocks (moorings) at 4-5 m depth.…”

Coralline algae enhance the settlement of the vermetid gastropod Dendropoma irregulare (d'Orbigny, 1842) in the southwestern Atlantic

“…Many invertebrates are known to settle in response to conspecific attraction, e.g., barnacles (Dreanno et al, 2006;Elbourne and Clare, 2010;Raimondi, 1990), polychaetes (Minchinton, 1997), crabs (Donahue, 2006), and ascidians (Rius et al, 2010). D. irregulare is colonial and highly aggregated as adults, so it is possible that they settle in response to conspecific cues.…”