Stomiidae

Moritz, Timo, Buchert, Jan & Schnell, Nalani K, 2019, Unexpected diversity of median caudal cartilages in teleosts, Zoological Journal of the Linnean Society 186 (3), pp. 599-632 : 616-618

Stomiidae ( Stomiiformes ) ( Fig. 14)

In Astronesthes lucifer there are two prominent CMCs ( Fig. 14A–D). They are like massive caps on the distal diastemal edges of hypural 2 and hypural 3 with laterally extending arms bending in the hypural direction. Thus, articulation on the margins of hypural plate 2 and 3 is facilitated by a notch in the anterior– posterior direction on the mid-parts of the cartilages, while at the same time the arms encompass the hypural plates laterally. Each CMC supports one ray that articulates with the dorsalmost part of the upper CMC and the ventralmost part of the lower CMC, respectively. In the largest specimen, 73.9 mm SL, there are two additional very small ball-like cartilages anterior to the CMCs on the diastemal margin of the hypurals ( Fig. 14B).

In Aristostomias xenostoma ( Fig. 14E, F) and Bathophilus vaillanti ( Fig. 14G, H) there are two prominent CMCs in the diastema. In both species the upper CMC is slightly smaller than the lower one and they are both roundish in lateral view. In latero-posterior view the upper CMC in A. xenostoma resembles a bean with the slightly concave side facing hypural 3 ( Fig. 14F). The lower CMC in A. xenostoma is roughly boomerang-shaped with blunt tips facing ventrally, therewith embracing the sides of hypural 2 with its lateral arms ( Fig. 14F). In B. vaillanti the CMCs are slightly bean-shaped in posterior view, embracing the edge of hypural 2 and hypural 3 with their concave side facing anteriorly ( Fig. 14H). The CMCs in A. xenostoma and B. vaillanti are connected by strong connective tissue ( Fig. 14H). In A. xenostoma there is no direct articulation between any caudal fin ray and the CMCs. The lowermost caudal ray of the upper lobe articulates with the ventral edge of hypural 3, whereas the uppermost caudal ray of the lower lobe articulates with hypural 2. In B. vaillanti ( Fig. 14G) the lowermost caudal ray of the upper lobe articulates with hypural 3 and the upper CMC, its proximal tip points ventrally, therewith embracing the upper CMC. The uppermost caudal ray of the lower lobe articulates with hypural 2 and the lower CMC, its proximal tip points dorsally, therewith embracing the lower CMC. There are no medial pegs on the caudal rays in A. xenostoma and B. vaillanti . The CMCs are only slightly overlapping the edges of the hypurals in width. Hypural 3 gains in width more dorsally; hypural 2 more ventrally and at that point the width of the CMCs and the width of the hypurals are about the same ( Fig. 14D, H).

There are also two CMCs present in the diastema of Chauliodus sloani ( Fig. 14I–K). They are of similar size with the upper one being slightly larger. In lateral view the lower CMC is roundish, and when seen in posterior view its shape resembles a bar or handle about double the width of a hypural plate. It seems somehow related to the upper ray of the lower lobe, but there is little connective tissue. In lateral view the upper CMC has an upright ovoid outline, but when seen from posterior side, its strongly curved, bean-like shape becomes visible. The concave part of the ‘bean’ is facing ventrally towards the diastema. Two rays are articulating on this cartilage: in its lower half the lowest ray of the upper lobe and at its dorsal tip the penultimate ray of the upper lobe. None of the fin-ray bases show medially directed pegs.

In Grammatostomias dentatus there are two small CMCs ( Fig. 14L). The upper CMC is bigger than the lower one, cordate in posterior view and with the tip pointing ventrally. The lower CMC is bean-shaped in posterior view, with its concave facet directed towards hypural 2. Neither cartilage has a direct articulation with caudal fin rays.

In Eustomias obscurus the upper and lower CMCs are roundish in lateral view and V-shaped in posterior view ( Fig. 14M). In the upper CMC, which is smaller than the lower one, it becomes evident in posterior view that the left and right parts of the V are separate and not fused, thus actually representing two, bilateral cartilages. The CMCs are connected by a strong ligament ( Fig. 14M) and have no direct articulation with caudal fin rays.

In Photonectes albipennis there is only a single CMC ( Fig. 14N), resembling the lower CMC in Chauliodus . It is positioned like a cap on the inner distal edge of hypural 2. An association with the uppermost ray of the lower lobe seems not very tight, if present at all.

CMCs are absent in Malacosteus australis ( Fig. 14O) and Stomias boa ( Fig. 14P). Here the interior most rays of the caudal lobes articulate with hypural 2 and hypural 3, respectively.