Several morphological studies in the early 20th century revealed that a number of stomiid genera appear unique among teleosts in having 1–10 of the anteriormost vertebrae reduced or absent (Sell, 1916; Regan and Trewavas, 1930; Beebe, 1934; Tchernavin, 1953; Günther and Deckert, 1955, 1959; Morrow, 1964; Morrow and Gibbs, 1964). Only 14 of the currently valid 28 stomiid genera (Nelson, 2006) have previously been studied with regard to vertebral reduction. Furthermore, this reorganisation of the anterior portion of the vertebral column has never been included as a character in phylogenetic analyses, because it has remained poorly understood. Assumptions about the number of reduced vertebrae are mainly based on Regan and Trewavas (1930). They counted the persisting neural arches, parapophyses, and spinal nerves and so determined the number of reduced vertebral centra, however their conclusions were founded on a critical misinterpretation of the occipital innervation. They identified the nerves anterior to the first neural arch as spinal nerves that have lost their respective vertebrae, whereas these nerves are actually spino-occipital nerves that belong to the occiput and have no associated vertebrae. With a new approach, counting the occipital and vertebral myosepta and spino-occipital nerves, we reinvestigate 26 of 28 stomiid genera, revise previous hypotheses that propose one or more lost vertebrae in stomiids, determine the exact number of reduced vertebral centra in the different taxa and discuss their homology.
- 1Occipital myosepta: Myosepta are segmentally arranged sheaths of connective tissue that separate adjacent muscle myomeres. They persist even in the absence of vertebrae, in which case they insert on the notochord. In teleosts there are three myosepta dorsally in the epaxial musculature that are associated with the occipital region (Fig. 1a) as a consequence of an evolutionary incorporation of vertebral segments into the skull at the base of gnathostomes (Allis, 1898; Patterson and Johnson, 1995; Britz and Johnson, 2010; Johnson and Britz, 2010). These occipital myosepta usually insert on the occiput, but in stomiids they may also insert on the notochord between the occiput and the first vertebra (Fig. 1b). The fourth myoseptum attaches to the first vertebra, and the first epineural lies within it. Accordingly, the fourth myoseptum provides a landmark for the first vertebra. Lack of association of the fourth myoseptum with a vertebra is thus evidence that the first vertebra is reduced or absent. The same is valid for the second vertebra, which lies in the fifth myoseptum, the third vertebra within the sixth myoseptum, and so forth.
- 2Spino-occipital nerves: The number of spino-occipital nerves varies between one and three among teleosts, with the lower number of one or two due to either fusion or reduction of these nerves (Sagemehl, 1883; Sewertzoff, 1895; Fürbringer, 1897; Allis, 1898, 1903; Herrick, 1899; Parenti and Song, 1996; Nakae and Sasaki, 2007). They are similar to spinal nerves in giving rise to four main branches: an anterior, a dorsal, a lateral, and a ventral branch. Each anterior branch perforates the myoseptum anterior to the myomere it innervates and sends sensory fibers into the antecedent myomere, where it fuses with the dorsal branch of the antecedent nerve (Allis, 1897, 1903). We provide a description of the spino-occipital nerves and a discussion of their significance in determining the first spinal nerve in stomiids.
- 3First spinal nerve: The spinal nerve that emerges between the fourth and fifth myoseptum and that innervates the fifth myomere is the first spinal nerve according to Allis (1903). Another definition of the first spinal nerve was given by Parenti and Song (1996) as that one emerging from the first free vertebra, with the second emerging from the second vertebra, and so on. We reinvestigated Parenti and Song's Sudan black stained specimens and concluded that their and Allis' (1903) definitions of the first spinal nerve are congruent. Accordingly, the nerve that emerges from the first vertebra (Parenti and Song, 1996) and that lies within the fifth myomere (Allis, 1903) is the first spinal nerve.