The Distribution of the Planktonic Chaetognatha in the Pacific and their Relationship to the Water Masses1

Abstract

The geographic extents of 27 species of Chaetognatha in the Pacific have been inferred from analyses of 2,900 plankton samples taken at more than 900 stations.Two species of chaetognaths—Sagitta elegans and S. maxima—are found in the Pacific only in the Subarctic Water Mass. Four species—Sagitta enflata, S. hexaptera, S. pacifica, and Pterosagitta draco—occur in the Pacific Equatorial and Central Water Masses. Seven species—Sagitta robusta, S. regularis, S. ferox, Krohnitta pacifica, Sagitta bedoti, S. neglecta, and S. pulchra—occur in the Equatorial and Western Central Water Masses. The latter three species appear to be disjunct equatorial species that are absent from the central equatorial Pacific. Sagitta pseudoserratodentata occupies the North Pacific Central Water Mass. S. californica, essentially a warm‐water cosmopolite, is very rare in the Equatorial Water Mass. The Peru and California Currents are inhabited by Sagitta friderici. A close relative, S. crassa, lives in the coastal waters of Japan. Sagitta minima, Sagitta species (serratodentata group), Sagitta decipiens, S. lyra, and Krohnitta subtilis, species which have their maximum abundance at about 200 meters (somewhat deeper than all of the preceding), appear to be limited in the north at about 45° N. Lat. Sagitta planctonis, S. macrocephala, and Eukrohnia fowleri, living primarily at depths of 500 meters and greater, have been taken in the present study as far north as 55° N. Lat., both in the Bering Sea and Gulf of Alaska. The very deep living species, Heterokrohnia mirabilis, was taken at a single station in the Eastern Equatorial Pacific. The sharpest chaetognath boundary yet found in the Pacific occurs in the Eastern Pacific at 18° to 20° N. Lat. and is associated with pronounced temperature, oxygen, and probably salinity gradients.Species distributions analogous to those of the Chaetognatha are found in the euphausiids, copepods, and dinoflagellates among other groups of organisms.