Competence and discrimination during cyprid settlement in Amphibalanus amphitrite

Amphibalanus (=Balanus) amphitrite is a tropical/sub-tropical barnacle species which is naturally exposed to sea temperatures of 16–27 °C throughout its larval development. An established technique widely employed by investigators of A. amphitrite settlement involves the storage of cyprids at 4–6 °C prior to their use in bioassays. Our study focuses on the effects on ageing temperature on settlement and discrimination of A. amphitrite larvae. Using cyprids aged at 2 temperatures, 6° and 23 °C, we confirmed the general trend in the literature that young d0 cyprids of this species do not appear competent to settle. Performing cyprid settlement assays at 20, 25 or 28 °C, we observed that the proportion of settled cyprids when incubated at 28 °C was greater than that of cyprid incubated at 25 °C and 20 °C. Settlement rates of cyprids aged at 6 °C and 23 °C increased relative to age from d1 to d10 irrespective of temperature. Cyprids lost the ability to undergo attachment and metamorphosis at d14 or d15 when aged at 6 °C or 23 °C respectively. In the choice assays executed in this study, cyprids generally chose to settle on adult extract-treated areas. But localised settlement on adult extract-treated areas decreased with time. Choice settlement assays at set age intervals during the cypris larval stage showed that cyprids were able to discriminate between con- and allospecific adult extracts. The duration and the temperature of cyprids storage influenced selectivity.