The spermatozoon of Carybdea marsupialis (Cubozoa, Cnidaria)

The spermatozoon of Carybdea marsupialis (Cnidaria, Cubozoa, Carybdeidae) is a primitive sperm with a short pear-shaped head and a long tail. The head contains a pyriform nucleus, six mitochondria, two centrioles aligned on the major axis of the sperm, and an anchoring apparatus formed by a complex pericentriolar system and a pyramid-like appendix (spur). The pericentriolar system is formed by three different and concentric orders of pericentriolar processes located around the distal centriole from which they, together with the spur, originate. Anterolaterally to the nucleus and scattered in the midpiece, we have found several pro-acrosomal vesicles with two concentric regions with a different electron density. The tail has a conventional 9+2 axoneme. In the transition zone between the anchoring apparatus and the tail, the central axonemal doublet is not yet present, but there is an electron-dense cylinder internal and adhering to the nine peripheral doublets, and the latter are connected to the plasma membrane by the classical Y-links. The spermatozoa are probably released through ruptures of the endodermal layer of the testes, as in other cnidarian species. The spermatozoon of C. marsupialis is very different from the modified anthozoan sperm, and shares characters with both the Hydrozoa and the Scyphozoa. The shape of the spur, its striations, the shape of the tertiary pericentriolar processes and the presence of the intersecondary processes of the anchoring apparatus are new for the whole phylum and are considered autapomorphic characteristics.