Light and Dark Types of Cladonema pacificum (Cnidaria, Hydrozoa): Reproductive Isolation between the Two Types Leading to Speciation.

Two types of Cladonema pacificum (Cnidaria, Hydrozoa) can be distinguished based on whether their medusae spawn after a dark-light transition ("light type") or a light-dark transition ("dark type"). To clarify how these two types are related, we characterized C. pacificum from different sites in the Tohoku region of Japan. Essentially, only dark-type medusae were captured in the Pacific Ocean side, whereas light- and dark-type medusae were found sympatrically at some sampling points in Mutsu Bay and in the Sea of Japan side. We observed that the spawning times of the sympatric light and dark types differed in the field, suggesting that the gametes of the two types rarely encounter each other. A comparison of the DNA sequences of mitochondrial COI and nuclear ITS1 of the analyzed individuals suggests that the two types belong to separate clades. In crossbreeding experiments, offspring of the light and dark types (F1 hybrids) developed into planula larvae and metamorphosed into polyps, which then produced medusae with normal morphology. The F1 hybrid medusae had characteristics that were intermediate between the two types in terms of their spawning in response to both dark-light and light-dark cues, but with less sensitivity than the parent medusae. When F1 hybrids were used as one or both parents, planula larvae failed to metamorphose into normal polyps. These results indicate that the light and dark types of C. pacificum exhibit both prezygotic and postzygotic isolation, accompanied by distinct DNA sequences; thus, they can be considered as undergoing speciation or as already distinct species.