Offshore–inshore and vertical distributional patterns of heteropod mollusks off leeward Oahu, Hawaii

Abstract

Replicated zooplankton tows were taken during day and night periods at neritic (1 nmi offshore) and oceanic (5 and 15 nmi offshore) stations off the leeward side of Oahu, Hawaii during fall (September) and spring (April). Plankton samples were obtained with a 1-m2 MOCNESS net system towed obliquely through seven depth strata to 80 m (neritic station) and eight depth strata to 200 m (oceanic stations). Five more species were collected in April (17) than in September (12). During both months the number of species decreased from the 15 to the 1 nmi stations; from 11 to 9 in September and 15 to 11 in April. Species diversity, as measured by species accumulation (rarefaction) curves, was higher at the 15 than the 1 nmi stations during both months. Total nighttime water column density in April was about double that in September. In September, the density decreased sevenfold from the 15 to 1 nmi stations, but in April densities were comparable among the three stations; mainly as a result of the shoreward increase in density of Atlanta plana. The most abundant species in both months were A. plana, A. lesueuri, A. inflata, A. peroni, and Protatlanta souleyeti, which in combination accounted for 93% (September) and 91% (April) of the total nighttime water column densities. The vertical ranges of six species were limited to the upper 100 m, and nocturnal migration was suggested for two (A. helicinoides during both months and A. turriculata in April). The ranges of the remaining 11 species extended to 160 or 200 m, and among them nocturnal migration was suggested for five (A. plana, A. meteori and Pterotrachea hippocampus during both months, and A. peroni and Protatlanta soueleyti in April).