A scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea.

Echinoderm mass mortality events shape marine ecosystems by altering the dynamics among major benthic groups. The sea urchin Diadema antillarum, virtually extirpated in the Caribbean in the early 1980s by an unknown cause, recently experienced another mass mortality beginning in January 2022. We investigated the cause of this mass mortality event through combined molecular biological and veterinary pathologic approaches comparing grossly normal and abnormal animals collected from 23 sites, representing locations that were either affected or unaffected at the time of sampling. Here, we report that a scuticociliate most similar to Philaster apodigitiformis was consistently associated with abnormal urchins at affected sites but was absent from unaffected sites. Experimentally challenging naïve urchins with a Philaster culture isolated from an abnormal, field-collected specimen resulted in gross signs consistent with those of the mortality event. The same ciliate was recovered from treated specimens postmortem, thus fulfilling Koch's postulates for this microorganism. We term this condition D. antillarum scuticociliatosis.

Elevated pCO2 does not impair performance in autotomised individuals of the intertidal predatory starfish Asterias rubens (Linnaeus, 1758).

The impacts of ocean acidification remain less well-studied in starfish compared to other echinoderm groups. This study examined the combined effects of elevated pCO2 and arm regeneration on the performance of the intertidal predatory starfish Asterias rubens, as both are predicted to come at a cost to the individual. A two-way factorial experiment (~400 µatm vs ~1000 µatm; autotomised vs non-automised individuals) was used to examine growth rates, lipid content (pyloric caeca and gonads), and calcium content (body wall) in both intact and regenerating arms, as well as subsequent effects on rate of arm regeneration, righting time (behaviour) and mortality over 120 days. Autotomised individuals tended to show lower (not significant), survival and growth. Elevated pCO2 had no effect on mortality, body growth, arm regeneration, righting time or arm calcium content. Lipid content was higher in the pyloric caeca, but not in the gonads, in response to elevated pCO2 irrespective of autotomisation. The results of the study suggest that adult A. rubens remain unaffected by increased pCO2 and/or arm autotomy for 120 days, although longer term experiments are necessary as the results indicated that survival, growth and calcification may be impaired with longer-term exposure to elevated pCO2.

The influence of environmental gradients on individual behaviour: Individual plasticity is consistent across risk and temperature gradients.

The expression of individual behaviour as a function of environmental variation (behavioural plasticity) is recognized as a means for animals to modify their phenotypes in response to changing conditions. Plasticity has been studied extensively in recent years, leading to an accumulation of evidence for behavioural plasticity within natural populations. Despite the recent attention given to studying individual variation in behavioural plasticity, there is still a lack of consensus regarding its causes and constraints. One pressing question related to this is whether individual plasticity carries over across temporal and environmental gradients. That is, are some individuals more plastic (responsive) than others in general? Here, we examined the influence of temporal and environmental gradients on individual behavioural responses in a marine gastropod, Littoraria irrorata. We measured individual boldness repeatedly over time and in response to tidal cycle (high vs. low, an index of risk) and daily temperature fluctuations (known to affect metabolism), in a controlled field experiment. On average, boldness increased from high to low tide and with increasing temperature but decreased marginally over time. Individuals also differed in their responses to variation in tide and temperature, but not over time. Those which were relatively bold at high tide (when predation risk is greater) were similarly bold at low tide, whereas shy individuals became much more "bold" at low tide. Most notably, individuals that were more responsive to tide (and thus risk) were also more responsive to temperature changes, indicating that plasticity was correlated across contexts (r = 0.57) and that bolder individuals were least plastic overall. This study provides a rare and possibly first example of consistency of individual behavioural plasticity across contexts, suggesting underlying physiology as a common mechanism, and raises the possibility of correlational selection on plasticity.