SPHYRNIDAE

FAMILY SPHYRNIDAE View in CoL

Hammerhead sharks have a strong heterocercal caudal fin. A thin plate distal to the haemal arches precedes the caudal fin skeleton (arrow in Figs 12A View Figure 12 , 13A View Figure 13 , 14A View Figure 14 ) in all hammerhead shark species, including the winghead shark ( Eusphyra ). The caudal fin skeleton of examined hammerhead sharks has 84–112 VC, but the winghead shark has 68–70 VC ( Table 1). Only Eusphyra has some vertebrae supporting up to two haemal arches and haemal spines. The basidorsal and interdorsal cartilages of Sphyrna and Eusphyra are rectangular, and the supraneural spines are elongated, slender and taper terminally. The caudal fin skeleton is divided into anterior and posterior diplospondylic caudal regions ( Figs 12–14 View Figure 12 View Figure 13 View Figure 14 , Table 2).

The anterior diplospondylic caudal region of S. lewini , S. zygaena and Eusphyra is well developed (~30% of the entire caudal fin vs. ~11% in S. tiburo and S. tudes ). This region has five VC in S. tiburo , eight VC in S. tudes , 25–29 VC in S. lewini , 27–30 VC in S. zygaena and 16–22 VC in Eusphyra ( Table 1). The basiventrals of S. tudes , S. tiburo and Eusphyra have apophyses similar to the prominence on the anterior margin of the cartilages ( Figs 12B View Figure 12 , 14B View Figure 14 ); the apophysis is more developed in Eusphyra ( Fig. 14B View Figure 14 ). However, in S. lewini and S. zygaena , the anterior apophysis is present only in the most posterior basiventral cartilages, whereas the first basiventrals are rectangular and lack an apophysis ( Fig. 13B View Figure 13 ). The haemal spines of S. tudes and S. tiburo are slightly concave anteriorly, similar to the first haemal spines of S. zygaena and S. lewini ( Figs 12A View Figure 12 , 13A View Figure 13 , 14A View Figure 14 ), and in Eusphyra the first spines are straight. The most posterior haemal spines of S. zygaena , S. lewini and Eusphyra are ‘>’ shaped, with the distal portion more slender than the proximal portion ( Figs 13A View Figure 13 , 14A View Figure 14 ). In the terminal lobe, the haemal elements are reduced and the haemal arches absent in all sphyrnids.

SKELETON CAUDAL FIN TYPE

The carcharhiniform caudal fin skeleton is herein divided into four main morphological types, termed scyliorhinoid, triakoid, carcharhinoid and eusphyrnoid. However, each main morphological type does not represent a monophyletic group. The scyliorhinoid type (family Scyliorhinidae ) is characterized by all haemal spines continuous with the basiventrals (an undivided caudal fin skeleton, without a distinction between anterior and posterior diplospondylic regions). In the triakoid type ( Triakidae , Chaenogaleus and Paragaleus ), the caudal fin skeleton has anterior haemal spines detached from basiventrals (anterior diplospondylic caudal region), and the entire posterior diplospondylic caudal region has slightly anteriorly convex haemal spines. In the carcharhinoid type ( Hemipristis , Hemigaleus , Carcharhinidae , S. tudes and S. tiburo ), the anterior haemal spines are detached from the basiventrals (forming the anterior diplospondylic caudal region), and the posterior diplospondylic caudal region has haemal spines that are ‘>’ shaped in the mid-caudal fin. The eusphyrnoid caudal skeleton type ( S. zygaena , S. lewini and Eusphyra ) has a well-developed anterior diplospondylic caudal region (extending to close to the mid-length of the caudal fin), and most haemal spines are ‘>’ shaped.