Author: Smith and Radcliffe, 1912
Field Marks:
The only shark with a fin spine on its first dorsal but not its second; very small size, spindleshaped body, long, bulbously conical, pointed snout, second dorsal fin with base about twice as long as that of first, first dorsal fin with origin opposite inner margins or rear tips of pectoral fins, no anal fin, dark colour with conspicouosly light-margined fins.
Diagnostic Features:
Anterior nasal flaps very short, not expanded into barbels; snout very long, bulbously conical but slightly pointed, length about half head length and about equal to distance from mouth to pectoral fins; gill openings very small, uniformly wide; lips thin, not fringed, pleated or suctorial; teeth strongly different in upper and lower jaws, uppers small, with narrow, acute, erect cusps and no cusplets, no bladelike, lowers much larger, bladelike, interlocked, with a high, moderately broad, nearly erect cusp and distal blade, edges not serrated; tooth rows 22 to 23/ 16 to 21. First dorsal fin with a spine, covered by skin or not, but second dorsal without a spine; first dorsal fin well anterior, origin about opposite inner margins or free rear tips of pectoral fins, insertion well anterior to pelvic origins and loser to pectoral bases than pelvics; second dorsal fin much larger than first, base about twice as long as first dorsal base; origin of second dorsal fin over fron half of pelvic bases; pectoral fins with short, narrowly rounded free rear tips and inner margins, not expanded and acute or lobate; caudal fin nearly symmetrical, paddle-shaped, with a short upper and long lower lobe and a strong subterminal notch. No precaudal pits or midventral keels, but with low lateral keels on caudal peduncle. Dermal denticles flat and blocklike, not pedicellate, no posterior cusps on flat, depressed crowns. Cloaca normal, not expanded as a luminous gland. Colour blackish or blackish-brown with conspicuously light-margined fins.
Geographical Distribution:
Oceanic and nearly circumtropical. Western Atlantic: Off Bermuda, southern Brazil and northern Argentina. Eastern Atlantic: Off France and Madeira. Western Indian Ocean: Off Somalia. Western Pacific: off Japan, Taiwan Island and the Philippines.
Habitat and Biology:
A wide-ranging, tropical epipelagic species that occurs near continental and island land masses, sometimes over the shelves, but usually over the slopes at depths from 200 to 500 m. Unlike its relatives Euprotomicrus bispinatus and Isistius brasiliensis, it apparently avoids the surface and has not been caught at night-lights there. It shows vertical migrations on a diel cycle, retreating to the bottom of its depth range during the day and travelling to about 200 m at night. It apparently avoids the central ocean basins far from land, unlike its close relative Euprotomicrus bispinatus, but prefers the edges of land masses in areas of high productivity. These two species, with probably similar feeding capabilities and requirements, are virtually allopatric in distribution, whereas the larger, more powerfully armed, highly specialized and semiparasitic Isistius brasiliensis broadly overlaps both E. bispinatus and Squaliolus laticaudus.
This shark has well-developed photophores, densely covering the ventral surface of the body but sparse on the sides and hardly developed on the dorsal surface. Such photophore patterns in this and other bioluminescent sharks and mesopelagic teleosts has been explained as 'photophore countershading', in which the light-producing underside eliminates the shadow normally formed when the fish is illuminated from above, and hence makes it less conspicuous to potential predators. Development probably ovoviviparous, but embryos have not been obtained. Twelve mature eggs have been found in a single ovary of a mature female, but this does not necessarily imply that large litters are produced.
The spined pygmy shark eats deepwater squid, lanternfish, gonostomatids and idiacanthids, and probably follows its prey on their diel migrations.
Size:
Possibly the smallest living shark, although closely rivaled by Eridacnis radcliffei (Family Prossylliidae). Maximum total length about 25 cm, males maturing at about 15 cm and reaching 22 cm, females maturing between 17 and 20 cm and reaching 25 cm.
Interest to Fisheries: None.
Type material:
Holotype: U.S. National Museum of Natural History, USNM 70259, 150 mm adult male. Type Locality: 13° 42'N, 120°57.3'E, Batangas Bay, Luzon, Philippines, 311 m depth.