Southwestern Atlantic reef-building corals Mussismilia spp. are able to spawn while fully bleached

Abstract

Bleaching events are becoming more frequent and intense worldwide. Southwestern Atlantic corals are considered stresstolerant species that suffer less during bleaching episodes. Therefore, we investigated if the reef-building corals Mussismilia hispida and M. harttii are capable of spawning while fully bleached. We compared spermatozoa concentration and motility, egg diameter and embryo viability between seemingly healthy and bleached colonies for both species. Findings show that both species spawned viable gametes. Concentration and motility of newly-released spermatozoa were similar between healthy and bleached colonies for both species. Unlike M. hispida, a 10% reduction in size was found in egg diameter for M. harttii. Embryo viability for bleached M. hispida colonies did not decrease; for bleached M. harttii there was a reduction from 14.7 to 6.3%, but fertilization and larval development still took place. These findings describe the unprecedented episode of corals spawning viable gametes when entirely bleached, supporting the suggestion of greater resilience for Southwestern Atlantic corals.

Energetic and reproductive costs of coral recovery in divergent bleaching responses

“…Because oocyte size is correlated with maternally provisioned lipids, this response is hypothesized to ensure adequate energy is available for all of the now limited number of oocytes to survive through settlement 25 . Since we observe a similar phenomenon to this study, it is possible that the strategy of producing fewer high quality oocytes over more low quality oocytes is specific to Acropora, as opposed to other coral genera where this strategy has not been observed 21 , 23 , 30 . In A. hyacinthus, bleaching stress appears to induce energetic constraints on reproduction, resulting in hosts provisioning a baseline level of nutrients into oocytes, with the number of oocytes being limited by the energetic costs of recovery.…”

“…Oocyte size is an indicator of maternal condition; individuals with larger energy reserves are able to provision more proteins and lipids to their oocytes and thus produce larger oocyte sizes 43 , 49 . A decrease in oocyte size is often observed as a consequence of severe stresses such as coral bleaching 21 , 23 , 30 , yet we found remarkably similar distributions and averages of oocyte sizes between resistant and recovered colonies, despite clear differences in gamete presence. A study of Acropora millepora demonstrated that under thermal stress, colonies maintained oocyte sizes across bleaching phenotypes, but produced fewer oocytes compared to when they reproduced under non-bleached conditions 25 .…”

“…Colonies that undergo bleaching also display smaller spermary size and abundance and impaired sperm motility, although these negative effects on sperm persist for a shorter duration than those on oocytes 23 , 27 . However, the reproductive costs associated with coral bleaching are species-specific and related to the severity of the heat stress, highlighting the need for further investigation into reproductive output following bleaching across many species and locations 28 – 30 . What is not entirely reconciled is to what degree the bleaching response, as opposed to the heat stress itself, is responsible for these reproductive effects, and whether differential intraspecific bleaching responses have reproductive—and ultimately demographic—implications.…”

A predatory gastrula leads to symbiosis-independent settlement inAiptasia

“…However, in adult polyps the nutritional balance shifts towards the primacy of the symbiosis 19,20 , including our observation of a striking reliance on symbiosis for Aiptasia sexual reproduction. This has been observed in the Hydra/Chlorella symbiosis 21 , and although some aposymbiontic corals can spawn 22 , the breakdown of symbiosis likewise rather severely affects sexual reproductive success in corals (e.g. 23 )…”