Twinpored eel

Xenomystax atrarius is a deep-water eel of the eastern Pacific, known in English as the twinpored eel, that has been caught over the continental slope ^[1],[2]. It is rare in collections: in 1972 only 19 specimens were known, and its placement in any one eel family was still considered uncertain ^[2]. The IUCN assesses it as Least Concern ^[3].

It is an elongate eel with a long, broad snout and many conical teeth in each jaw; the vomerine teeth form a single row ^[4]. The four Chilean specimens measured by Cervigón and colleagues had a head of 20 % to 22 % of standard length, a maxilla of 47 % to 51 % of head length, and 37 to 40 lateral-line pores in front of the anus ^[5].

The general colour is brownish and paler towards the rear; the gill region, the base of the dorsal fin, the lateral line and the nape are blackish, and the fins are pale ^[5]. The species reaches about 100 cm in total length ^[1]. Peden concluded that Xenomystax rictus is a junior synonym of this species ^[2].

It has been recorded in the eastern Pacific, from southern Canada to Chile, mostly near the continental slope ^[2],[1]. In Chile, Cervigón and colleagues cited it from about the latitude of Valparaíso, and one of their specimens, from the Coquimbo region, was taken by trawl at 350 to 400 m depth ^[5]; the Punta Toro sample, somewhat further south, shows it also occurs in central Chile ^[4]. It is a deep-water bathydemersal fish recorded between 152 and 1050 m ^[1].

A sample of 114 individuals was obtained by trawl off Punta Toro, in central Chile, at 610 m depth ^[4].

In the Punta Toro sample, individuals measured 36 to 66 cm in total length and weighed 19 to 214 g, with the mode at 50 to 52 cm ^[4]. Of 114 stomachs, only 33 held recognisable prey; the rest contained heavily digested organic matter ^[4].

Because the catch was made at about 21 h and most stomachs held only heavily digested organic matter, the author proposed that the species feeds mainly by day ^[4]. A male of 520 mm total length had visible but poorly developed gonads ^[5].

It is a predator that feeds on animals living on the bottom. Among prey that could be identified, crustaceans made up 61 %, elasmobranchs 21 % and bony fishes 18 % ^[4]. The most frequent prey was the shrimp Haliporoides diomedeae, found in 19 of 33 stomach contents (58 %) ^[4].

The shrimp Heterocarpus reedi, the shark Centroscyllium granulosum and the fish Coelorhynchus chilensis also appeared, these three as juveniles ^[4]. The author describes it as a broad-diet carnivorous predator of the second or third order, and notes that in that area it appears to press on commercially important crustaceans ^[4].

The IUCN assesses Xenomystax atrarius globally as Least Concern (LC) ^[3]. Cervigón and colleagues regarded it as a species rare in collections, of which very few specimens were known by the late 1970s ^[5].