Alepocephalidae (Alepocephaliformes)

Alepocephalidae (Alepocephaliformes) View in CoL View at ENA ( Fig. 2 View Figure 2 )

Principally, two different arrangements of median caudal cartilages can be found in Alepocephaliformes , each typical for Alepocephalidae or Platytroctidae . Members of Alepocephalidae have two rather small cartilages ( Fig. 2A, B View Figure 2 ): one at the dorsodistal tip of hypural 2 and one at the ventrodistal tip of hypural 3. Each cartilage supports one fin ray of the caudal fin. The hemitrichs of the respective fin rays are widely splayed and bear a peg on their inner side articulating either in lateral grooves of the CMCs or in the area between CMC and neighbouring hypural plate ( Fig. 2C–E View Figure 2 ). The cartilages are concave on their anterior facet ( Fig. 2D, E View Figure 2 ) where they articulate on the edges towards the diastema of hypural 2 and hypural 3, respectively. In posterior view, the cartilages are wider than the hypural plates (see Fig. 1B View Figure 1 for orientation and measurement method). Such pegs are not unique to the fin rays articulating with CMCs, but can also be found in the one to three neighbouring fin rays, depending on the taxon. The situation with two relatively small cartilages each supporting a fin ray was principally found in all members of Alepocephalidae studied, with only slight variations, e.g. direct articulation of fin rays on the CMCs or in-between CMC and hypural plate.