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Summary
Specimens of Antennella sp. were collected by scuba diving during the austral summer on the outer slope of the fringing reefs located on the western coast of La Réunion (55°30′E, 21°50′S) where the species forms microscopic colonies on red algae. A fertile colony was kept alive in a closed system in the laboratory. Like other Antennella species, it exhibits hermaproditism and sexual dimorphism: gonangia of both sexes are on the same stem, with female gonothecae being clearly bigger than male. In both sexes we observed cryptomedusoids with a subumbrellar cavity, an eccentric spadix surrounded by the gametes and a ring of refringent corpuscles on top. Male gonothecae have a compact blue-grey homogeneous mass of spermatozoa while females contain circa five white-opaque oocytes with a clearly visible darker nucleus. When the gametes were mature, one day after collection, medusoid tissues slid towards the base of the gonotheca in both sexes. Female gametes remained free inside the gonotheca while spermatozoa were liberated in the aquarium and fertilized oocytes. Several embryos at different stages of embryonic development (from morula to blastula) were incubated in a single gonotheca. The planula was not observed. This study revealed a previously undescribed pattern of sexual reproduction in the family Halopterididae involving unreleased cryptomedusoids instead of fixed sporosacs. This may be a more advanced regression stage of the medusa than free cryptomedusoids encountered in related families.