Seamount coral reefs are egg case nurseries for deep‐sea skates

Abstract

Egg case nurseries of the boreal skate (Amblyraja hyperborea) and Richardson's skate (Bathyraja richardsoni) were defined and mapped on a bathyal seascape (c. 500–1900 m depths) south of Tasmania, Australia, using 99 towed‐camera transects (157 linear km; N = 50,858 images). In total, 738 skate egg cases were observed (present in 240 images, absent in 50,618); among 113 egg cases examined to identify parent species, 70% were A. hyperborea, 10% B. richardsoni and 20% unidentified Bathyraja species. “Recently laid” egg cases were differentiated from “aged” ones by classifying their colour and condition. The great majority (98%) of egg cases were observed in c. 1100–1400 m depths on seamounts (15 of 36 surveyed), not seamount bases or adjacent continental slope. Egg cases were associated with reefs formed by accumulated skeletal matrix of the stony coral Solenosmilia variabilis, with >90% egg cases (including most of those recently laid) observed on living S. variabilis that characterises a “coral zone” in c. 950–1350 m depths. Water in the coral zone is warmer (+0.66 to 2.37°C) than at the deep distributional limits of adult A. hyperborea and B. richardsoni (2000 and 3000 m, respectively), potentially providing for accelerated embryonic development. Co‐occurrence with living coral infers an energetically favourable local‐scale hydrodynamic environment for egg cases, particularly on seamount peaks, where increased water flow over egg cases would avert smothering by suspended sediment, and compensate for lower oxygen concentration compared to deeper depths occupied by adult skates. Criteria identifying egg case nurseries are strongly met for A. hyperborea at Seamount Z110 (468 egg cases of varied ages, maximum density of 5.47 m−2) and to a lesser extent on five others (Seamounts K1, Z16, Hill U, Z5 and Hill V). An abundance (density) criterion for defining nurseries needs to be flexible because it is a spatially scale‐dependent measure that differs between surveys according to the tools and design employed. Off Australia, coral reef egg case nursery habitat is restricted to a narrow depth range in temperate latitudes where it is scarce and impacted by historical bottom trawl fishing in many locations. There has been effective conservation of nursery habitat, however, because four of the six nursery sites identified here and extensive coral reef areas are protected within marine parks.

First observation of a skate egg case nursery in the Ross Sea

“…(dwarf) based on the known geographic and bathymetric distribution of adults. Other sub‐Antarctic egg case nursery areas have been reported further north, including seamount nurseries for A. hyperborea and Richardson's skate Bathyraja richardsoni (Garrick, 1961) south of Tasmania, Australia, and multispecies nursery areas on Burdwood Bank, east of Isla de los Estados, Argentina (Florencia et al, 2022; Maguire et al, 2023).…”

“…Egg cases were annotated at first appearance on the video and were also identified on still images. Egg cases were classified into one of six categories based on color and condition, following classifications by previous studies (Henry et al, 2016; Maguire et al, 2023; Salinas‐de‐León et al, 2018) (Table 1). These categories give an indication of the age and development of egg cases, with categories (1) and (2) indicating “recently laid” egg cases and categories (3) to (5) signifying “aged” egg cases that have been subject to environmental conditions over an extended period of time on the seafloor.…”

“…It is possible that higher densities of egg cases may be observed elsewhere or at deeper depths. Egg case nurseries for A. hyperborea have been reported from bathyal depths of up to 1900 m (Maguire et al, 2023). However, other video surveys conducted in the Ross Sea region in 2008, 2018, and 2021 have yet to observe skate egg cases.…”

“…Additional records suggest that there may be some overlap in spatial T A B L E 1 Skate egg cases on video and still images by categories and descriptions (as described in Maguire et al, 2023). habitat use between species of the Rajidae and Arhynchobatidae families in some areas (Figure 1a).…”

“…It is possible that higher densities of egg cases may be observed elsewhere or at deeper depths. Egg case nurseries for A. hyperborea have been reported from bathyal depths of up to 1900 m (Maguire et al, 2023). However, Skate egg case nurseries have previously been reported from narrow depth ranges on the continental shelf and slope and in waters with constant bottom temperatures (Amsler et al, 2015;Hoff, 2008;Hoff, 2010).…”