The reproductive season of Acropora in Socotra, Yemen

Observation

Most hermatypic scleractinian corals have an annual gametogenic cycle that culminates in the broadcast spawning of gametes once per year¹. In most reef regions, numerous species spawn in sychrony following full moons when sea surface temperature is either rising or falling^2,3. Determining exactly when spawning takes place has important implications for reef management and clear economic benefits⁴. For example, activities that are likely to limit fertilization success, such as dredging, can be prohibited during these often brief spawning periods⁵. In addition, the diving industry can benefit from public interest in coral spawning.

Here, we document the reproductive condition of Acropora corals on the island of Socotra, Republic of Yemen. The island, located 240 km east of the Horn of Africa and 380 km south of the Arabian Peninsula, supports a diverse scleractinian fauna of over 250 species (including 20 Acropora species)⁶, and includes sites with a high cover of Acropora (Figure 1). Acropora colonies were sampled before the full moon on 15 February 2014 to determine their reproductive condition. Three reproductive conditions were defined based on the appearance of the oocytes as observed with the naked eye in the field⁷ (1) mature - oocytes pigmented and therefore likely to spawn within a month (2) immature - oocytes pale but visible indicating that they are close to maturity and likely to spawn within two to three months (3) empty - oocytes too small to see or absent indicating either that the colony has recently spawned, or is unlikely to do so for at least three months.

Figure 1. The island of Socotra supports diverse assemblages of Acropora species.

A total of 143 Acropora colonies from approximately 14 species were sampled at four sites on the north of Socotra (Samerhur 12°41'40.96"N, 53°29'3.69"E; Qaiso 12°39'58.91"N, 53°24'33.86"E; Dihamri 12°40'20.35"N, 54°11'39.96"E; Hadibo 12°40'0.77"N, 54° 3'7.74"E) between the 31 January and 8 February 2014 (Table 1). Twenty percent of colonies contained mature oocytes (Figure 2), 28% contained immature oocytes and no oocytes were visible in the remaining 52% of colonies (Table 1). Colonies with mature oocytes are highly likely to spawn at some time around the full moon in February 2014, whereas colonies with immature oocytes are likely to spawn in March 2014. The remaining colonies have either spawned recently, or alternatively, there could be a second reproductive season later in the year, similar to Western Australia⁸, Singapore⁹ and some locations in Indonesia¹. Ten species had at least one colony with mature gametes (Table 1) suggesting a multi-species spawning event is likely in February 2014. The Acropora typically reproduce at much the same time as most other broadcast spawning scleractinian corals^10,11 and therefore we predict that many other species will be spawning in February and March in Socotra. Our results confirm previous anecdotal reports of coral spawning on Socotra in February and March⁶.

Figure 2. Mature (colored) oocytes are clearly visible in the branches of Acropora colonies.