Spermatophores of thalassoid gastropods (Paludomidae) in Lake Tanganyika, East Africa, with a survey of their occurrence in Cerithioidea: functional and phylogenetic implications

With few exceptions, spermatozoa-encapsulating packages in molluscs are known mostly from cephalopods and pulmonate gastropods. Among non-stylommatophoran gastropods, the marine Cerithioidea are second only to the Neritimorpha in the number of species that posess a spermatophore, but they have only rarely been found in freshwater taxa of this superfamily. We describe and compare here the sperm packages of 11 paludomid cerithioideans as part of an ongoing study on the evolution and systematics of the thalassoid (i.e., “marine-like”) endemic species flock from Lake Tanganyika. Stereomicroscopic and SEM examination revealed unexpected complexity in shape and structure of spermatophores within paludomids. In addition, we present a survey of other Cerithioidea, which revealed that spermatophores are in general structurally simple and confirmed their presence in 12 marine species (five families) and 15 limnic species (four families), including those of 10 thalassoid species for which spermatophores are described herein. Based on histological sections of the male genital tract, we hypothesize that spermatophores are formed anteriorly, wholly or in part, by the so-called spermatophore-forming organ, and which is considered a synapomorphy of the Paludomidae. In addition, we briefly discuss functional aspects inferred from morphological study of the spermatophore-forming organ, with possible implications for spermatozoan transfer and fertilization. Finally, we place the features of the spermatophores and the spermatophore-forming organ in a phylogenetic framework of cerithioidean and paludomid systematics, which suggests that the spermatophore-forming organ is a synapomorphy of Paludomidae, that a bifurcate spermatophore structure is plesiomorphic, and that the evolution of structurally complex, spiny spermatophores has occurred independently in disparate lineages within the thalassoid species flock.