RE-DESCRIPTION OF THE EXTERNAL MORPHOLOGY OF SOMNIOSUS ROSTRATUS (RISSO, 1826), WITH SPECIAL REFERENCE TO ITS SQUAMATION AND CUTANEOUS SENSORY ORGANS, AND ASPECTS OF THEIR FUNCTIONAL MORPHOLOGY (PISCES SELACHII SQUALIDAE)

SUMMARY The availability of a new S. rostratus (Risso, 1826) specimen (Pisces Selachii Squalidae), the re-examination of two museum collections and a survey of literature on the subject has provided the opportunity to clarify some problems concerning this shark and other species of the genus Somniosus. The main topics investigated were morphometric characters, squamation morphogenesis, distribution of cutaneous sensory organs and modified scales. Morpho-functional problems are discussed, with special reference to hydrodynamic aspects. The little sleeper, S. rostratus, has a typical squaloid morphology. In all ontogenetic stages, a peculiar short keel occurs in the caudal peduncle, above the ventral hypochordal lobe. The function of the keel could be to improve turning ability and/or to act as a stabilizer. The caudal morphology belongs to the squaloid type (THOMSON & SIMANEK, 1977) and differs from that of the strictly benthic species. On this basis we therefore believe that S. rostratus is a benthopelagi...