Distribution and Ecologic Aspects of Central Pacific Salpidae (Tunicata) 1 JAMES 1. YOUNT 2 THE PRESENT REPORT is a continuation of a carried out, insofar as the available data general salp study, the first section of which would permit, in an attempt to elucidate already has been reported (Yount, 1954). It these points. In addition, little information concerns the distribution and ecology of the exists defining the actual role or position of salps collected during two cruises by the salps in the plankton community (the niche "Hugh M. Smith" of the Pacific Oceanic as defined by Elton, 1927); consequently, the Fishery Investigations of the United States niche of the different salp species was inves­ Fish and Wildlife Service (referred to in the tigated as thoroughly as possible. remainder of this report as POFI) in the central Pacific Ocean . Nineteen out of a total ACKNOWLEDGEMENTS of 22 recognized world species were found in I wish to express my gratitude to R. W. these collections. Hiatt and 1. D. Tuthill for their help through­ Knowledge of the distribution of salps was out this investigation. O. E. Sette of POFI, reviewed by Traustedt (1885), Apstein (1894, in addition to permitting me to use the plank ­ 1906), Metcalf (1918), Ihle (1935), and ton collections, reviewed the manuscript in Thompson (1948). Various other investiga­ part . I also wish to thank T. S. Austin of tors have added to this general knowledge in POFI for his help with regard to the section regard to certain species. Certain points in Physicochemical Factors of the Environment. ]. regard to geographic distribution, however, E. King of POFI helped by furnishing in­ needed clarification. Among these was an in­ formation on total plankton volumes and vestigation of the usefulness of salps as in­ numbers of the collections examined . dicator species of oceanic currents within the area of study. Moreover, knowledge of sea­ MATERIALS AND METHODS sonal effects on salps, of the relation of salp abundance to the abundance of other zoo­ The animals used were taken from a large plankton, and of the relation of salp abun­ series of plankton captures made by POFI in dance to physicochemical environmental an area that extended from 270 N . to 150 S. factors was very limited. Thus studies were and from 1760 W. to 1550 W. In this area, the "Hugh M . Smith" made cruise 5 inJune, 1 Contribution No . 99, Hawaii Marine l aborato ry, July, and August of 1950, and cruise 8 in in cooperation with the Department of Zo ology and January, February, and March of 1951, thus Entomology, University of Hawaii. Part of a thesis submitted to the Department of Zoology, Uni versity permitting seasonal comparisons. The study of Hawaii, in partial fulfillment of the requirements for of abundance and distribution was confined the degree of Doctor of Phil osophy. to cruises 5 and 8; of these cruises, all samples 2 Department of Biology, University of Florida , Gainesville, Florida. Manuscript received August 27, (51) from cruise 5, and 30 samples out of 106 1956. from cruise 8, were studied. 111 112 PACIFIC SCIENCE, Vol. XII, April, 1958 The total plankton data used here were to them is the distribution of temperature, reported by Kin g and Demond (1953); Crom­ density, currents, salinity, and oxygen. On well (1953, 1954) reported all hydrographic the other hand, distribution of light and of observations used here. As King and Demond nutrient chemicals, factors which influence stated, the plankton net descended obliquely phytoplankton production, is probably only to 200 meters, then made an oblique ascent indirectly important to salps. Which among to the surface. Thus the net strained , during these are limiting factors for salps is almost approximately 30 minutes, a total of about wholly unknown. 1,000 cubic meters of water. This net was In the open ocean, salinity is presumed not described in King and Demond's report as limiting because of its small variation, from being 1 meter in diameter, with a mesh width about 34 to 36 0/00' Salps seem to be absent of 0.65 mm ., but I should point out here that from areas of very low or high salinity, such it was an open net. For this reason, vertical as may be foun d in certain waters. For ex­ distribution of the captured animals is un­ ample, Apstein (1906) reported no salps from known. the region off the mouth of the Congo River All animals larger than about 5 cm. were in Africa, a situation apparently related to the removed from each POFI plankton sample low salinity (30.4 0/ 00) in the area; and Ihle (King and Demond, 1953). The sample was (1935) noted that some species of salps were then divided into two equal portions, one of absent from the eastern part of the Mediter­ which was not used by POFI personnel. The ranean Sea, perhaps indicating a relationship latter portions were used for the present study. to the high salinity (up to 40 0/ 00 off Syria). Therefore, in calculations where comparisons In regard to temperature, also, little is were necessary, salp numbers and volumes known abou t limits of tolerance for the va­ were estimated by doubli ng the figure ob­ rious species, although all salp species-ex­ tained from each half sample. In addition , cept an antarctic form, Ihlea magalhanica- . many of the removed salps larger than 5 cm. are found principally in warm regions of the were examined and identified. Number deter­ ocean. Occasionally they are found in high minations ofsalps were made by direct count­ latitudes, presumably carried there by current ing, whereas volume determinations of the tongues, for example, in the Bering Sea (Met­ salps were made for each plankton sample by calf, 1918), the Gulf of Maine (Bigelow, . means of water displacement in a graduated 1926), the North Sea (Fraser, 1949, 1954), cylinder. In order to determine the relation and waters of southern Australia (Thompson, between salp volume and volume ofthe other 1948). Salps have also been found at great plankton in the same sample, the salp volume depths (more than 1,000 meters) where the determinations were divided by the amount temperature was low (Apstein, 1906; Michael, of water strained (taken from King and De­ 1918; Sewell, 1926, 1953; Leavitt, 1935, 1938). mond) and compared with the total volumes The limits of tolerance for both salinity and of zooplankton, less salps, reported by King temperature in the various species are thus and Demond. unknown and can only be inferred from re­ ABUNDANCE OF SALPS ports of distribution. Factors that influence variation in organi c Physicochemical Factors of the Environment productivity must also be considered for a A study of the ecology of salps should clear understanding of variation in salp abun­ .consider all aspects of their environment such dance. Areas ofmaximum biological produc­ as food, enemies, and the distribution of tivity are found in coastal waters, in temperate physicochemical factors of the waters in which and higher latitudes, and in regions of up­ they are found . Of possible direct importance welling. Productivity in these areas is based Central Pacific Salpidae - YOUNT 113 chiefly on vertical mixing of the waters meridians studied are in a continuous state of (Sverdrup, et al., 1942: 785; Tait, 1952: 76; upwelling they are "young ," that is, they Harvey, 1955: 99) and additions of dissolved have a high concentration of nutrient chemi­ nutrients from the land (Kiinne, 1950: 57; cals, being more or less continuously replen ­ Dakin, 1952: 7), which bring nutrients to the ished from below . euphotic zone where they can be used . In Another factor should be considered in temperate waters, vertical mixing and result­ regard to enrichment: phosphates probably . ant enrichment are useful for production prin­ affect salps only indirectly. As only plants are cipally in the spring and fall; in midwinter the considered able to utilize dissolved nutrients standing crop of organisms is low, due pri­ such as phosphates for biological production, marily to insufficient light intensity, while in salps are prob ably affected by phosphates only midsummer the low standing crop is appar­ by way of their food, which consists primarily ently the result of depletion of nutrients in ofphytoplankters. Salps thus stand one troph­ the euph otic zone (Russell and Yonge, 1936: ic level away from the dissolved nutrients as 246). In tropical waters, on the other hand , shown by their trophic relationships: dis­ there is generally a low concentration of nu­ solved nutrients ~ phytoplankton (produc­ trients in the euphotic zone with an attendant ers) ~ herbivorous zooplankton (primary low standing crop throughout the year, ap­ consumers). Time , therefore, is required for parently because upper tropical waters (to a the phosphates to be used by the phyto­ depth of about 300 meters) are characterized plankton in order that the primary consumers by a three-layer system-an upper warm layer, can be affected by the increased production. a transition layer, and a lower cool one­ For this reason, it is possible that the salps present during a large portion of the year. captured at the same time would show little Thus sinking of decomposing substances from correlation, station to station, with the phos ­ the surface layer to a depth below the transi­ phates. However, the increased phosphate tion layer results in a removal of nutrients found in the region of upwelling is a zonal from the euphotic zone until vertical mixing cond ition and thus not transient, and results of the waters again brings the nutrients back. in generally larger standin g crops of plankton The enrichment in the zone between about in this region (Cromwell, 1953; King and 2° N .