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External and internal characters, horizontal and vertical distributions, luminescence, and food of the dwarf pelagic shark, Euprotomicrus bispinatus

  • Author(s): Hubbs, Carl L
  • Iwai, Tamotsu
  • Matsubara, Kiyomatsu
  • et al.
Abstract

The object of this paper is to coordinate and expand knowledge of Euprotomicrus bispinatus (Quoy and Caimard, 1824), the pigmy shark : a most remarkable creature, which until recent years had been known for a century and a quarter on the basis of very few, casual observations at sea and of 8 museum specimens. As a result of increasing emphasis on high-seas research we now have data on 37 specimens, and a much richer fund of information on the species.

The pigmy shark is a strange creature, defying most concepts of a shark. In the first place it is tiny: the largest known example spans only 265 mm (10 ½ in.) in total length and weighs less than 70 grams (about 234 ounces) ! Males mature at 170 mm, females at 233 mm or less. Its terete body and wee gill openings remind one of a lamprey. Its essentially diphycercal tail, with large, rounded dorsal and ventral caudal-fin lobes and horizontal termination of the spinal column, little resembles the long-drawn-out, upturned heterocercal tail that is ordinarily associated with a shark. In this, and in some other respects, the appearance of this shark is almost embryonic. Its skeleton is almost devoid of calcification (apparently an adaptation for neutral buoyancy). Its vertebrae are unexpectedly few for a shark, and in radiographs lookr strangely like those of a bony fish. The teeth are strikingly unlike in the two jaws.

This little shark has been taken only at the sea surface, almost exclusively in the vast, relatively sterile central water masses of the world ocean. Its thousands of tiny light organs can combine to produce a bright blue-green glow. We now learn that it undertakes vertical migrations to considerable depth, where it feeds on bathypelagic squids and fishes.

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