Developmental styles and larval morphology of hybridized sea cucumbers (Echinodermata: Holothuroidea)

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The sea cucumber Apostichopus japonicus has many colour variations. In order to compare the ability of colour variants of A. japonicus to hybridize and investigate differences in the development of hybrid larvae, we developed a genetic marker of their body colours. Differences in the sequence of the internal transcribed spacer 1 region of ribosomal RNA were correlated with the red variant. Reciprocal crosses of red and green variants produced hybrids that developed to the auricularia, doliolaria and pentactula developmental stages. In crosses between A. japonicus eggs and Parastichopus nigripunctatus sperm, the hybrid embryos developed into auricularia larvae with a post-oral ciliary band. This indicates that the paternal line influences their morphology because this band is not present in A. japonicus. These larvae subsequently metamorphosed into the pentactula and juvenile developmental stages. In crosses between A. japonicus eggs and Cucumaria japonica sperm, the auricularia-like larvae developed into two forms that had different hydrocoel and ciliary band positions. Thus, this larval development type involves a change in the body axes (i.e. dorsal–ventral and right–left). The generation of two larval forms also suggests that the egg fertilized genetically heterogeneous spermatozoa. These results demonstrate that larvae from congeneric crosses have an indirect developmental style (i.e. a long-feeding larva stage). Larvae from interspecific crosses also had indirect developmental styles during ontogenesis but failed to metamorphose to the pentactula and juvenile stages.