Isoechiniscoides sifae

Isoechiniscoides aff. sifae sp. can. N= 1 specimen: female occurrence: ANDEEP I; 60°27.05′S, 56°04.77′W; 3,958 m bsl. Figs 12–14; Supplementary Table 1.13 Description Body. Small Isoechiniscoides of 113 µm body length and 46 µm width. Dorsal cuticle has warts arranged in transverse rows. They are penetrated by epicuticular pillars (Fig. 14F). A pair of mid-dorsal, kidney-shaped cuticular sculptures of 5 × 3 µm and without warts is present on the posterior region (Fig. 14I). Cephalic region. Two mouth plates form the mouth opening region (Figs 12B, 13D, E). Pharyngeal bulb inconspicuous and pharyngeal apparatus not visible. Eyes are absent. Primary clavae ovoid (3.5 µm long) with a small pore on the anterior end. The base of the primary clava has an elongated plate (4.5 µm) that lacks warts and pillars (Fig. 13B, C, 14E, F); the elongated plate of the right primary clava is either hidden on the SEM images due to a backward folding of cirrus A, or it could be completely blown-up due to a preparation artefact (Fig. 14G). However, its contours are visible on the light microscope image (Fig. 13B, C). Secondary clavae (7.5 × 8 µm) dome-shaped (Figs 13D, E, E 2, 14G). External (2.5 µm) and internal cirri (3.5 µm) have a small ring-shaped base (Fig. 13D, 14G). Cirri A are 10.5 µm long and have a thicker and folded cirrophore (Fig. 14F, G). Legs. Leg I sensory organ is an elongated papilla (2.5 µm) with short spiny tip (Fig. 14A). Leg II sensory organ is a 3.3 µm long spine (Fig. 14B). Leg III sensory organ is a spine (5.5 µm) with annulated base (Fig. 14C). Leg IV sensory organ is a spherical papilla (4.0 µm) with a short spiny tip (Fig. 14I). Each leg has six sickle-shaped claws with cuticular folds (Fig. 14H). Caudal region. Cirri E are 14.5 µm long and have a thicker annulated cirrophore (Fig. 14E, I). Rosette-shaped, six-lobed female gonopore is present on the ventral side between legs III and IV (Figs 13F, 14D). The anus is located in the terminal body end and present as an inconspicuous three-lobed unit (Figs 12B, 13F). Differential diagnosis With its body length of 113 µm, Isoechiniscoides aff. sifae sp. can. is a rather tiny deep-sea representative compared to the already described I. higginsi and I. sifae, which measure up to 329 and 188 µm, respectively (Hallas & Kristensen 1982, Møbjerg et al. 2016). In order to exclude the possibility of a shrinkage that might have occurred during desiccation by CPD, we have also measured the specimen when still mounted on a glycerol-mounted slide and there was no shrinkage observable. Therefore, this species can be well separated from I. higginsi by the smaller size, but additionally by the length of the sensory organs of leg I, II, III and IV (4.0, 12.0, 14.0 and 6.5 µm in I. higginsi holotype female to 2.5, 3.3, 5.5 and 4.0 µm in I. aff. sifae sp. can. female), mid-dorsal plates without warts and the unique elongate primary clavae plate. Morphologically, Isoechiniscoides aff. sifae sp. can. is very similar to I. sifae, however, the body size, length of leg sensory organs (3.5, 8.0, 11.6 and 4.9 µm in I. sifae holotype female to 2.5, 3.3, 5.5 and 4.0 µm in I. aff. sifae sp. can. female) and the presence of primary clavae plates are significant differences between both species, which could represent a pair of sister species. We refrain from erecting the new species of Isoechiniscoides due to the rather poor condition of the specimen and the lack of supporting molecular data.