Ultrastructure of the spermatozoa of Parenterodrilus taenioides (Protodrilida: 'Polychaeta') and its phylogenetic significance

摘要

The interstitial annelid Parenterodrilus taenioides from coral sands of the island of Moorea (French Polynesia) is a mouthless species characterized by a vestigial non-functioning alimentary canal and a distinctive body shape. It is the only known annelid that has a non-functioning gut and no symbiotic bacteria. Parenterodrilus is thought to represent the sister group of the well-known taxon Protodrilus, generally recognized as Protodrilidae. An ultrastructural study revealed that, as in Protodrilus spp., the males of P. taenioides have lateral organs, i.e. paired epidermal, glandular and ciliated grooves, in the anterior body region where the sperm ducts are located. As in Protodrilus two types of spermatozoa were found, commonly called euspermatozoa and paraspermatozoa. Both types are filiform and the euspermatozoa, like those in Protodrilus spp., have a characteristic midpiece reinforced by symmetrically arranged supporting structures and reduced mitochondria. The ultrastructure of the spermatozoa in P. taenioides and Protodrilus spp. is basically the same but the species can be distinguished from each other on the basis of particular structural details of spermatozoa and the pattern of lateral organs. The two types of spermatozoa and the epidermal lateral organs are unique features of these two taxa; as they are not present in any other potentially related taxon, such as Saccocirrus and Protodriloides, they very likely represent autapomorphies of Protodrilidae. In Protodrilus spp. spermatophores are deposited on the epidermis of the females, whereupon sperm are released and penetrate the body wall. Since in polychaetes the structure of the sperm is correlated with the fertilization biology, the similar structure of sperm suggests that sperm transfer and fertilization are very probably the same in P. taenioides as in Protodrilus.