Larvae and juveniles of the deepsea whalefishes Barbourisia and Rondeletia (Stephanoberyciformes: Barbourisiidae, Rondeletiidae), with comments on family relationships
Larvae of the deepsea "whalefishes" Barbourisia rufa (11: 3.7-14.1 mm nl/sl) and Rondeletia spp. (9: 3.5-9.7 mm sl) occur at least in the upper 200 m of the open ocean, with some specimens taken in the upper 20 m. Larvae of both families are highly precocious, with identifiable features in each by 3.7 mm. Larval Barbourisia have an elongate fourth pelvic ray with dark pigment basally, notochord flexion occurs between 6.5 and 7.5 mm sl, and by 7.5 mm sl the body is covered with small, non-imbricate scales with a central spine typical of the adult. In Rondeletia notochord flexion occurs at about 3.5 mm sl and the elongate pelvic rays 2-4 are the most strongly pigmented part of the larvae. Cycloid scales (here reported in the family for the first time) are developing by 7 mm; these scales later migrate to form a layer directly over the muscles underneath the dermis. By 7 mm sl there is a unique organ, here termed Tominaga's organ, separate from and below the nasal rosette, developing anterior to the eye. Larvae of the two species of Rondeletia can be distinguished by the presence or absence of developing spongy bone in the pectoral girdle and sphenotic by at least 9 mm and by the counts of the vertebrae, pelvic-fin rays, and dorsal hypural bones in smaller larvae. The presence of Tominaga's organ in the gibberichthyid Gibberichthys suggests that "the whalefishes", Barbourisiidae, Rondeletiidae, and Cetomimidae, as a group are paraphyletic, and that Rondeletia and Gibberichthys are sister taxa.