Continuous Reproduction Causes Stable Population Structure of Antipatharian-Associated Shrimp Sandyella tricornuta (Decapoda: Palaemonidae)

Sandyella tricornuta (Hendrickx, 1990) is a small (<13 mm total length), obligate and highly specific ectosymbiont of the black coral Antipathes galapagensis Deichmann, 1941 with a remarkable morphology and body coloration that resembles black coral polyps. Sandyella tricornuta population structure was investigated to infer the reproductive periods evidenced with ovigerous females from monthly collection at Espíritu Santo Island (April 2009–March 2010) and along a latitudinal gradient of six islands (24–30° N, July 2009) located along the west coast of the Gulf of California, Mexico. A total of 2,899 S. tricornuta individuals were collected from 70 colonies of A. galapagensis with a mean density of 210 inds./m3 per black coral colony. Sea floor temperature of S. tricornuta/A. galapagensis habitat seasonally ranged between 21 and 30°C. Females constituted > 60% of the entire S. tricornuta population and > 59% of the collected females were ovigerous females, suggesting active reproduction throughout the year and along the latitudinal gradient investigated. Sandyella tricornuta population density and sex ratio was similar throughout the year indicating a seasonally stable population size. Total abundance of S. tricornuta was density independent of black coral colony size and weight indicating that S. tricornuta depends on A. galapagensis, but the black coral colony can grow and develop without S. tricornuta characteristic of a commensalism ectosymbiotic association.