Akarotaxis nudiceps

4.1 | Early life history of A. nudiceps

To our knowledge, there are four reports of the early life stages of A. nudiceps in the literature. Kellermann (1990) describes two transforming juveniles (37.0 and 39.1 mm L S) that were caught during early March and mid-February in the northeastern Weddell Sea. Fin-ray and vertebrate counts for the 39.1 mm L S specimen were reported as D 26+, A 25+, P 24, V 49. The pigmentation for these transforming specimens was described as being uniformly heavy on the body and lighter on the head. Voskoboinikova (2001) also describes one juvenile (43.1 mm L S) caught in the Weddell Sea during late February. Fin-ray counts for this specimen were D 29, A 25, P 22, and the heavy pigmentation had begun transitioning to a greyish-brown coloration. Flores et al. (2008) found one larva from the Lazarev Sea during April 2004, but the description, length and location were not reported. Finally, Vacchi et al. (1999) collected a small, 14 mm L S specimen that they identified as A. nudiceps in the coastal Ross Sea near Zucchelli Station (74 48 0 75 S, 164 36 0 90 00 E); nonetheless, this specimen was not described or illustrated. This larva was caught with a 5 m 2 Hamburg Plankton Net (500 mm mesh-size) towed to 30 m over a bottom depth of 320 m (Vacchi et al., 1999).

Each of the 14 larval A. nudiceps identified in this study was collected at locations that are relatively nearshore, with most (n = 10) occurring just outside Marguerite Bay (Figure 1). The Palmer LTER sampling grid extends c. 240 km offshore in this area (see Smith et al., 1995), but A. nudiceps larvae have not been found in any other net tows during the more than 30-year time series. This suggests that adult A. nudiceps are likely spawning in neritic areas along the WAP. This is supported by the coastal association of the only other small A. nudiceps larva reported by Vacchi et al. (1999). In addition, histological analysis and nesting behaviour of Parachaenichthys charcoti in the South Shetland Islands region also indicates that spawning occurs in nearshore habitat (Novillo et al., 2018).

In the relatively ice-free austral summer (i.e., December – February), one of the dominant currents of this region, the Antarctic Peninsula Coastal Current (APCC), is flowing in a southwest direction along the coast of the WAP (Moffat et al., 2008). The circulation pattern is less characterized within Marguerite Bay, but the APCC likely creates a cyclonic surface flow within the bay (Moffat & Meredith, 2018). Based on the congregation of larval A. nudiceps across years and APCC flow, we hypothesize that there is a recurring nesting area for A. nudiceps somewhere around the perimeter of Adelaide Island (Figure 1). With the limited data, it is not possible to determine whether the larvae collected farther north (VIMS 22690) and south (VIMS 33107, 24545 and 24518) originated from the Marguerite area or if there are multiple spawning sites along the coast. Yolksac lengths of P. evansii [12.0 – 14.2 mm standard length (L S)], Racovitzia glacialis (12.0 – 13.2 mm L S) and Gymnodraco acuticeps (not reported), captured in similar areas along the WAP in November, suggest that hatching occurs in late spring (October – November) (Kellermann, 1990). The mid-January sampling date for the seven larvae smaller than 14 mm L T (Table 1) in this study suggests that hatching may occur in December. We also did not observe yolk remains on any larvae, indicating A. nudiceps may have a smaller length at hatch than other reported bathydraconids, or absorb their yolk sac more quickly.

Several specimens of A. nudiceps identified in this study were previously misidentified as P. evansii. Although the pigmentation patterns of the two species are similar at early larval stages (Figure 3), a few key differences separate the two species. P. evansii has two parallel rows of ventrolateral spiny scales that run from their hindgut to the caudal peduncle and two parallel rows of dorsolateral spines running from the nape to peduncle (Kellermann, 1990). Nonetheless, at early preflexion stages, or in the wrong light, these spiny scales are easily overlooked. The gut and abdomen are less pigmented in the larval stages of P. evansii when compared to the condition in A. nudiceps (Figure 3), although this difference is difficult to quantify. A second distinguishing characteristic between the two species involves their cranial pigmentation. Each of the 14 larval A. nudiceps examined had dense, uniformly spaced pigmentation on the occipital region of the head (Figure 4a). In contrast, P. evansii is known to have a few large pigment spots on either side of the posterior portion of the head (Kellermann, 1990) (Figure 4b). We have not yet conducted a thorough developmental analysis of P. evansii, but we estimate that most stages have 2 – 6 large pigment spots.