The Thecosomata are pelagic euopisthobranch pteropods that are important for marine food chains, but threatened by ocean acidification. Members of the suborder Euthecosomata are either torted snails with a coiled, sinistral shell (Limacinidae) or are straight-shelled with a symmetrical body and an unusual ventral mantle cavity (Orthoconcha). The classification and taxonomy of euthecosomes still depends on shells, but is being challenged by initial molecular data. There is a large body of morphoanatomical information dating from the beginning of the last century, and only biological (rather than soft-part anatomical) detail has been added since. For our initial study on pteropod morphoanatomy we have selected the potentially basal orthoconch genus Creseis Rang, 1828. Supplementing Meisenheimer's (1905) monographic work, we redescribe the microanatomy of the Mediterranean C. clava (Rang, 1828) from serial semithin sections and present 3D-models of all major organ systems. In the absence of histological differences we interpret the head to be fused with the foot, forming the wings with reduced labial tentacles and rhinophores. The postpharyngeal nerve ring is strongly condensed, showing fused buccal ganglia and a short visceral loop with two discernable ganglia. The genital system is monaulic and hermaphroditic, with female glands and a potential allosperm receptacle of unclear homology. An open seminal groove connects with the frontal copulatory organ, which shows complex penial infoldings. We confirm the 180° longitudinal rotation of the visceral organs relative to the condition in limacinids. As an alternative to ontogenetic detorsion, progenesis may have skipped torsion. Other obvious paedomorphoses such as single, basally forked tentacular nerves, suggest that heterochrony has been a driving force in thecosome evolution. Retaining the mantle cavity in a ventral position, Creseis has its large mantle gland—necessary for creating a mucus web for feeding—close to the mouth. Further comparative microanatomical data on orthoconchs and limacinids are needed to corroborate our assumptions of homology, and for reconstructing pteropod evolution once reliable molecular phylogenetic hypotheses are available. |
