On the Occurrence of the Engraulid Fish, Stolephorus buccaneeri Strasburg, in the Oceanic Region of the Equatorial Western Pacific : Life History and Distribution

Abstract

Larvae of Stolephorus buccaneeri previously reported only from coastal waters, were collected dominantly with larva nets in the oceanic regions of the Equatorial Western Pacific during the cruises of R/V Shunyo-maru and R/V Hakuho-maru. The development from larva (3.2 mm SL) to adult (39.5 mm SL) is described. The following changes of body form are characteristic of engraulid fishes; slender body deepens, head becomes round and mouth inferior, dorsal and anal fins shift anteriorly; with rapid changes shortly after 15 mm SL, the juvenile assumes almost the adult form. The melanophores typical of the postlarva are restricted to the ventral surface and show strongly throughout development; those of the adult first appear at a late postlarval stage. It was found that after the late postlarval stage the developmental sequence is subject to intraspecific variation correlated with distribution, the more oceanic the fish the earlier the changes to adult form occur. In Hakuho-maru samples, postlarva of just over 15 mm SL changes morphologically more rapidly, to reach the juvenile stage at about only 25 mm SL; the size of the adults is less than 40 mm SL. Brain form of S. buccaneeri was compared with that of four other Stolephorus species taken in the neritic waters. The oblong telencephalon and relatively long bulbus olfactorius were found to be characteristic of S. buccaneeri, although intraspecific variation was found. The brain of the oceanic specimens is well separated from the skull and has a slender telencephalon, in contrast with that abutting on the skull and the oblong telencephalon in the neritic specimens. The larval occurrence was independent of the direction of the surface currents and the composition of samples was similar in the oceanic regions. The results of collections by MTD horizontal closing nets in the upper 500 m indicate that these larvae are limited to surface water. It is concluded that S. buccaneeri probably inhabits and completes its life history both in oceanic and neritic waters throughout the whole West Pacific region. According to analysis by t-test, meristic counts did not show highly significant differences (P<O.Ol) among 6 samples taken in different areas (three oceanic, the others neritic). The results support the assumption that the species may occur as an immigrant into neritic waters.