Polymixiidae

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 : 622-625

Polymixiidae ( Fig. 22)

In Polymixia there is a small cartilaginous nodule with few, neighbouring, very small, roundish or irregular cartilages on the distal diastemal edge of hypural 3 ( Fig. 22B). Furthermore, there is a minute cartilaginous patch on the distal diastemal edge of hypural 2 ( Fig. 22A). These cartilaginous patches are in very close contact with the hypurals and do not form well-defined entities.

SIZE OF DIASTEMA AND PRESENCE OF MEDIAN CAUDAL CARTILAGES

Some species show highly specialized caudal fins without any diastemal gap. However, most species have a diastema, but not necessarily cartilaginous structures in that area. Thirty-two individuals of 27 species with CMCs and 25 individuals of 25 species without CMCs were measured to examine the size relationships of the diastema and its cartilages, if present ( Fig. 23; see Fig. 1 for measuring method). In specimens with CMCs, the size of the diastema ranged between 2.7 and 23.4% of the total hypural height, with an average of 13.0%. In specimens without CMCs, the diastemal gap ranged from 8.0 to 29.7%, with an average of 15.9%. There is no functional relationship between size of the diastema and presence/absence of CMCs among teleosts.

DEVELOPMENT OF MEDIAL CAUDAL CARTILAGES

Little information on the development of CMCs can be currently presented. Most information is available for salmoniform species ( Fig. 24A–I). Here, the single or two CMCs develop relatively late in comparison to other elements of the caudal fin. All hypurals are at least partly ossified before the first traces of CMCs appear ( Fig. 24B, E, H). This happens in Salmo and Thymallus shortly before the ural centra form. In Coregonus the two ural centra are already present when the first trace of a CMC appears ( Fig. 24H). First appearance of CMCs happens in Coregonus at about 27–30 mm SL, in Thymallus and Salmo between 19 and 25 mm SL. The CMCs form as a new structure in the diastema and do not originate as part of the hypural cartilages. They start as a small cartilage that grows during ontogeny ( Fig. 24B, C, E, F, H, I) until they reach their assumed final size, shape and place in the larger specimens depicted here. In cases where only one CMC develops, it appears in the ventral part of the diastema, close to the edge of hypural 2.

The development of CMCs in Osmerus is similar ( Fig. 24J, K): they appear relatively late in comparison to other caudal fin elements. In Arctozenus risso juveniles of about 30 mm SL, the lower CMC is present in some specimens ( Fig. 24L), but not in all. It is formed as a simple bar embraced by the bases of the left and right hemitrich of the uppermost ray of the lower lobe. In many specimens of about the same size, no CMC is present, whereas in few, the upper CMC is present as a small ball between the bases of the lowest ray of the upper lobe.

In the stomiid Photostomias the lower CMC appears between 21 and 32 mm SL ( Fig. 25A, B) and it develops before the upper CMC ( Fig. 25B). At this stage all hypurals are preformed of cartilage, but show no sign of ossification yet. There is one cartilaginous epural and the ossified uroneural, but there are no ossified vertebral centra. In a 92 mm SL specimen, all hypurals, caudal vertebrae and the single epural (with the tips still cartilaginous) are ossified, and two prominent CMCs are present in the diastema ( Fig. 25C). We lack intermediate developmental stages to determine the exact size at which the upper CMC appears. The same development of the CMCs during early ontogeny was observed in Astronesthes niger and Eustomias sp. (not illustrated here). In the phosichthyid Vinciguerria there is just a single CMC. This cartilage is present in a 9 mm SL specimen ( Fig. 25D). At this stage the hypurals have just started to ossify, but no ural centra are formed yet. In the sternoptychid genus Argyropelecus , the lower CMC develops before the upper CMC. At this stage ( 9.9 mm SL; Fig. 25E) the hypurals, the parhypural and one ural centrum have already partly ossified. In our 13.3 mm SL specimen all caudal vertebrae have formed and the upper CMC is also present ( Fig. 25F). In the sternoptychid genus Sternoptyx there is one single, enlarged CMC. Our smallest specimen ( 8.8 mm SL) already shows this single cartilage ( Fig. 25G). At this stage the urostyle and hypurals already started to ossify. In a 9.5 mm SL specimen the single CMC has elongated in dorsoventral direction ( Fig. 25H).

For the other investigated groups there is almost no information on the development available. In the smallest alepocephaliforms, sternoptychids and bathylagids available in this study, the CMCs are already present. In the smaller specimens of Polyipnus spinosus , the CMCs show a simpler, almost ball-like shape without the typical notch on the diastemal side found in bigger specimens.