What came first, the shark or the egg? Discovery of a new species of deepwater shark by investigation of egg case morphology

Abstract

Apristurus ovicorrugatus, a new species of deepwater catshark, is described from northwestern Australia. Unique egg cases belonging to an unknown species of Apristurus prompted a more detailed investigation of Apristurus specimens off northwestern Australia. One specimen previously identified as A. sinensis collected off Dampier Archipelago was found gravid with a single egg case. Removal of this egg case confirmed that this species was responsible for producing the unique egg cases previously recorded. The egg cases of this species have strong T-shaped longitudinal ridges on the dorsal and ventral surfaces which are unique in the genus Apristurus.The ridges most closely resemble those present in Bythaelurus canescens from South America, but are larger and always T-shaped. The holotype is closest morphologically to A. sinensis but differs in having a medium brown buccal cavity (vs. jet black), ridged egg cases (vs. smooth egg cases), fewer intestinal spiral valve turns and larger pectoral fins. The holotype is also similar, and closest on a molecular level, to A. nakayai with which it shares a unique synapomorphic character, the white shiny iris (apomorphic within the genus). A late-term embryo removed from an egg case superficially resembled the holotype except in having two parallel rows of enlarged dermal denticles on the dorsolateral predorsal surface. Recent nomenclatural changes to the genera Apristurus and Pentanchus are discussed and challenged. This study highlights the important contribution that egg case morphology has on oviparous elasmobranch taxonomy.

Dichichthyidae, a New Family of Deepwater Sharks (Carcharhiniformes) from the Indo–West Pacific, with Description of a New Species

“…The 148 mm TL D. melanobranchus recorded by [49] is likely a newly hatched individual based on its small size and the presence of two rows of enlarged denticles running parallel to each other from the posterior head to the first dorsal fin. These enlarged denticle rows have been observed in numerous other late-term embryo and newly hatched scyliorhinoids, e.g., Apristurus ovicorrugatus [53], Bythaelurus bachi [54], and Poroderma africanum [55]. While it has been hypothesized that these double rows of enlarged denticles are used to help the embryo hatch from the egg case, [56] theorized that they act as "initiator" rows which trigger denticle patterning around them, a process like the development of feathers in chicks.…”

“…Some species do share similarities with Dichichthyidae in individual features, such as Chiloscyllium and Hemiscyllium shark egg cases being encapsulated partially or fully with an outer fibrous mat. Several other catsharks' egg cases possess prominent ridges, i.e., Cephaloscyllium laticeps (Duméril, 1853) [62], Apristurus ovicorrugatus (White, O'Neill, Devloo-Delva, Nakaya, and Iglésias, 2023) [53], and Bythaelurus canescens (Günther, 1878) [63]. However, the ridges of these species are made of the same hard, collagenous material of the egg case structure and are not soft and pliable like the ridges of Dichichthyidae egg cases.…”

“…Egg case ridges have been theorized to have a number of functions, including (1) to act as anti-foulant, making it difficult for invertebrates such as sponges and barnacles to attached to the egg; (2) as a means to reduce predation, making it difficult for gastropods to bore through the thick and textured egg casing; (3) by improving the hydrodynamic flow over the egg case which may assist with respiration of the developing embryo; (4) to improve the strength of the egg case making it more resistant to mechanical shock [53]. Although the ridges on dichichthyid egg cases are made up of densely packed fibres, it is possible they serve the same function as in other ridged shark egg cases.…”

Mystery egg revealing secrets from the deep

“…In this issue, through rigorous detective work, White et al (2023) have provided further clarity to our understanding of the deep sea and its fish diversity, shining a light on the deep‐sea catshark genus Apristurus , uncovering a new species called Apristurus ovicorrugatus found off northwestern Australia. Using genetic markers and a gravid female, their work found this new species A. ovicorrugatus , previously misidentified as another species, which was responsible for producing a mystery egg, which displayed morphological characteristics that were unique to the species already classified in the Apristurus genus.…”