First insights into coral recruit and juvenile abundances at remote Aldabra Atoll, Seychelles

Coral recruitment and successive growth are essential for post-disturbance reef recovery. As coral recruit and juvenile abundances vary across locations and under different environmental regimes, their assessment at remote, undisturbed reefs improves our understanding of early life stage dynamics of corals. Here, we first explored changes in coral juvenile abundance across three locations (lagoon, seaward west and east) at remote Aldabra Atoll (Seychelles) between 2015 and 2019, which spanned the 2015/16 global coral bleaching event. Secondly, we measured variation in coral recruit abundance on settlement tiles from two sites (lagoon, seaward reef) during August 2018–August 2019. Juvenile abundance decreased from 14.1 ± 1.2 to 7.4 ± 0.5 colonies m-2 (mean ± SE) during 2015–2016 and increased to 22.4 ± 1.2 colonies m-2 during 2016–2019. Whilst juvenile abundance increased two- to three-fold at the lagoonal and seaward western sites during 2016–2018 (from 7.7–8.3 to 17.3–24.7 colonies m-2), increases at the seaward eastern sites occurred later (2018–2019; from 5.8–6.9 to 16.6–24.1 colonies m-2). The composition of coral recruits on settlement tiles was dominated by Pocilloporidae (64–92% of all recruits), and recruit abundance was 7- to 47-fold higher inside than outside the lagoon. Recruit abundance was highest in October–December 2018 (2164 ± 453 recruits m-2) and lowest in June–August 2019 (240 ± 98 recruits m-2). As Acroporid recruit abundance corresponded to this trend, the results suggest that broadcast spawning occurred during October–December, when water temperature increased from 26 to 29°C. This study provides the first published record on coral recruit abundance in the Seychelles Outer Islands, indicates a rapid (2–3 years) increase of juvenile corals following a bleaching event, and provides crucial baseline data for future research on reef resilience and connectivity within the region.

Genetic Variation in Released Gametes Produces Genetic Diversity in the Offspring of the Broadcast Spawning Coral Acropora Tenuis

All coral species in the genus Acropora are broadcast-spawning hermaphrodites. Fertilization in the ocean requires sufficient numbers of gametes from conspecifics and the contact time for fertilization is thought to be limited by the rapid diffusion of sperm. Many studies have reported a positive correlation between sperm concentration and fertilization success, but it is not clear how released gametes are fertilized in nature (in situ) and how genetic diversity arises at a fertilization event. To elucidate this, we analyzed the changes in sperm concentration in situ after spawning and genotyped sperm and fertilized eggs from seawater using seven microsatellite (MS) markers. This showed that the sperm concentration in situ was suboptimal (<106 sperm/mL), but most of the eggs were fertilized. MS genotyping showed that the alleles of released sperm were diverse and those alleles also appeared in the fertilized eggs. In addition, the MS fragment peak height in released sperm, which presumable reflects the allele frequency of the sperm, was positively correlated with the allele frequencies of the fertilized eggs. Collectively, the genetic diversity of colonies with high fecundity dictates the genetic diversity of their descendants.

Bindin is essential for fertilization in the sea urchin

Species-specific sperm−egg interactions are essential for sexual reproduction. Broadcast spawning of marine organisms is under particularly stringent conditions, since eggs released into the water column can be exposed to multiple different sperm. Bindin isolated from the sperm acrosome results in insoluble particles that cause homospecific eggs to aggregate, whereas no aggregation occurs with heterospecific eggs. Therefore, Bindin is concluded to play a critical role in fertilization, yet its function has never been tested. Here we report that Cas9-mediated inactivation of the bindin gene in a sea urchin results in perfectly normal-looking embryos, larvae, adults, and gametes in both males and females. What differed between the genotypes was that the bindin−/− sperm never fertilized an egg, functionally validating Bindin as an essential gamete interaction protein at the level of sperm–egg cell surface binding.

Synchronized broadcast spawning by six invertebrates (Echinodermata and Mollusca) in the north-western Red Sea

On the evenings of June 11 and 12, 2019, 5 and 6 days before full moon, broadcast spawning by four echinoderm species and two mollusc species was observed on the Marsa Shagra reef, Egypt (25° 14′ 44.2" N, 34° 47′ 49.0" E). Water temperature was 28 °C and the invertebrates were observed at 2–8 m depth. The sightings included a single basket star Astroboa nuda (Lyman 1874), 2 large Tectus dentatus (Forskal 1775) sea snails, 14 individuals of the Leiaster cf. leachi (Gray 1840) sea star and 1 Mithrodia clavigera (Lamarck 1816) sea star, 3 Pearsonothuria graeffei (Semper 1868) sea cucumbers, and 2 giant clams, Tridacna maxima (Röding 1798). The observations presented here provide relevant information on broadcast spawning of non-coral invertebrate taxa in the Red Sea, where spawning is considerably less well documented than in other tropical geographical regions such as the Indo-Pacific and Caribbean.