Development of Early Stages of Pelagic Armorhead Pseudopentaceros Wheeleri with Notes on Juvenile Ps. Richardsoni and Larval Histiopterus Typus (Pisces, Percoidei, Pentacerotidae)

Two larvae (4.2 and 7.2 mm notochord length (NL)) and 28 juveniles (8.3-15.3 mm standard length (SL)) of the pelagic armorhead, Pseudopentaceros wheeleri, are described from surface tows taken near seamounts in the central North Pacific Ocean (29.5-30.5° N, 178-180° E) during February 1985. Diagnostic features of the larvae include dark pigment on all but the posterior 9-20% of the body, a thick, long intestine (preanal length 76.4-76.9% NL), a supraoccipital crest, spiny supraorbital ridges, and preopercular spines. Juveniles are darkly pigmented over the head and body, with a blotched pattern on specimens >11 mm SL. In juveniles, additional large spines are present on the lacrimals, posttemporals, supracleithra, and cleithral lobes. A 7.4-mm SL Histiopterus typus larva from western Australia has similar head spines but strikingly elongated dorsal- and pelvic-fin elements, more evenly distributed pigment, a larger head, and a more compressed body, compared to Ps. wheeleri. Juvenile Ps. wheeleri can be differentiated from juveniles of the other North Pacific pentacerotids, Pentaceros japonicus and Evistias acutirostris, by numbers of fin rays and from the very similar Southern Hemisphere Ps. richardsoni by vertebral count, interpelvic width, and the absence of anterior spines on the supraoccipital crest. All known armorhead juveniles share a distinctive pattern of cranial spines that includes spines on the lacrimals, large spines in laterally projecting supraorbital ridges, and an enlarged, serrated supraoccipital crest, in addition to the spines on the preopercles, opercles, posttemporals, supracleithra, and pterotics found in many percomorph larvae. Pelagic young of Ps. wheeleri were collected only in neuston tows in winter. The neustonic distribution of small pelagic armorhead may contribute to their advection from seamount spawning sites.