Featured Elasmobranch – Fine-spined Skate

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Featured Elasmobranch – Fine-spined Skate

December 2009

B. microtrachys

Fine-spined Skate

Bathyraja microtrachys (Osburn & Nichols, 1916)

Family Arhynchobatidae (softnose skates)

Identification: A softnose skate with a disc that is slightly wider than long, broadly rounded pectoral fin apices, a disc surface evenly covered with numerous, small prickles on the dorsal surface, smooth ventral surface, with no enlarged thorns on the dorsal surface except for alar spines on adult males, and a single median row of tail thorns. The tail is slightly longer than the disc, with similar-sized two dorsal fins and minute interdorsal thorns. Coloration is a uniform brown above, being slightly darker along the disc margins, and a white median ventral surface with a strikingly demarcated brown coloration on the disc “wings”.

Size: A relatively moderate sized species with females reaching a maximum of 77 cm and males at least 70 cm total length. Size at birth is about 17 cm.

Distribution: This little known apparently endemic species ranges from about 300 miles southwest of San Diego, California to at least off Washington State.

Habitat: Considered an extremely rare skate, known from only a dozen or more records, recent video analysis of remote operated vehicles by researchers from the PSRC and the Monterey Bay Aquarium Research Institute have found this skate to actually be quite abundant below 2,000 m. Its preferred habitat appears to be on soft bottom substrate in contrast to the Pacific white skate (Bathyraja spinossisima) that occupies a similar depth range, but occurs mostly on deepsea rocky reefs. This is one of the deepest known skate species as it the shallowest record of it comes about 2,000 m, but it is known to occur to at least 3,300 m and possibly more.

Biology: The reproductive mode, as with all skates, is oviparous. Females and males both mature between 60-70 cm total length. The egg cases are small, coarsely striated, with irregular rasp-like denticles, making its texture rough to the touch. What little is known about their diet indicates that they consume deepsea shrimps.

General interest: Occasionally taken as by-catch, the preferred depth range below 2,000 m and habitat, of this species precludes it from being taken in any substantive numbers as by-catch. It may have a wider geographically distribution, but given its very deepsea occurrence and lack of deepsea fisheries and research surveys in the eastern Pacific it range remains poorly known. NOTE: Given the rarity of this species if one should be captured it should be deposited into the fish collection at a major museum such as the California Academy of Sciences (Fish Collection Department) in San Francisco or you can contact us at the PSRC at the below address and we can assist in ensuring that this rare species is properly accessioned.

By David A. Ebert

Pacific Shark Research Center
Moss Landing Marine Laboratories
8272 Moss Landing Road
Moss Landing, CA 95039
debert@mlml.calstate.edu