Discarded appendicularian houses as sources of food, surface habitats, and particulate organic matter in planktonic environments

Observations using SCUBA reveal that macroscopic organic aggregates produced by appendicularians reached densities of 44 to 1,130 m-’ in the Gulf of California. Though these aggregates constituted less than 5% of total particulate carbon, their carbon : nitrogen ratio was twice that of particles in the surrounding seawater. The C : N ratio of the total particulate matter increased directly with the density of appendicularian produced aggregates. Aggregates were utilized extensively as food by euphausiid larvae, copepods, and planktivorous fish. Such aggregates affect the adaptive strategies and distribution of the plankton by introducing spatial heterogeneity and physical structure into an environment previously considered unstructured and may substantially alter the chemical composition of particulate organic matter, Little is known of the role of planktonic organisms in forming particulate organic matter in the sea. Many zooplankton, cspecially pteropods ( Gilmcr 1972)) gastropod and annclid larvae ( Hamner et al. 1975)) salps ( Madin 1974), and appendicularians (Alldredge 1972) product macroscopic, mucus aggregates ranging from a few millimeters to 2 m in diameter. Since the fragility, size, and patchy distribution of these macroscopic aggregates makes them difficult to sample using standard ship-board tcchniqucs, little is known of their ecology. Yet macroscopic aggregates may be disproportionately important to the pelagic community as sources of food (Parsons and Strickland 1962; Alldrcdge 1972)) surface microhabitats ( Pomeroy and Johanncs 1968), and as components potentially affecting the chemical composition of total particulate organic matter (Riley 1970). Appendicularians, or larvaccans, are pelagic tunicates which filter particles through the “house,” a unique feeding structure secreted around the animal by glandular epithelium on the body. When the external filters become clogged with phytoplankton or particulate matter, the house is discarded but remains in the water column as a recognizable organic aggregate. A single animal ’ Present address: Department of Biological Sciences, University of California, Santa Barbara 93106. may build and discard six houses per day ( Lohmann 1909). The following study describes the abundance, organic composition, and significance of macroscopic aggregates produced by appcndicularians as an example of the possible impact of mucusproducing zooplankton on particulate organic matter and trophic relationships in the sea. I thank R. Laskcr and R. W. Eppley for the USC of their CHN analyzers and E. Brinton, M. Knight, A. Flemingcr, and R. Bauer for aid with identification of crustaceans. I am grateful to M. M. Mullin and P. J, Richerson for critical review of the manuscript and especially to W. M. Hamner for many helpful suggestions throughout this study. Materials and methods Field data were collected at depths of 5 to 25 m in the Florida Current lo-15 km west of Bimini throughout 1971-1972 and in the Gulf of California 2-5 km east of Puerto Escondido during July and August 1973 and 1974. Conventional SCUBA techniques modified for open-ocean research diving (IIamncr 1975) were used. I collected three catcgorics of field data: visual dctermina tion of appendicularian and discarded house densities; hand collection of animals and houses; hand collection of water samples for chemical analysis. LIMNOLOGY AND OCEANOGRAPHY 14 JANUARY 1976, V. 21(l) Discarded appendicularian houses 15 The size and shape of the incurrent and feeding filters permitted specific idcntification of discarded appendicularian houses. I counted appcndicularians and houses visually using a handheld grid 10 X 20 X 20 cm ( l/250 rn” ) made of Plexiglas rod. I was tied to a central pivot by a cord and swam in a large arc, stopping every two kicks to count the contents of the grid. The occurrence of zooplankton on discarded houses, the number of houses damaged by foraminifera and Trichodesmium ( a blucgreen alga), and the number of occupied houses wcrc noted, Data were recorded on an underwater tape recorder equipped with a throat microphone. I measured house diameters from enlargcd photographs of occupied houses taken while diving ( Alldredgc 1972). IIouse volumes were calculated assuming the houses to be spheres. (The torpcdoshaped house of Megalocercus huxleyi was assumed to bc a cylinder. ) Houses were collected by hand in individual collecting jars to determine the contribution of discarded appendicularian houses to total particulate carbon and nitrogcn in seawater. Random collection was assured by taking the first 15-30 houses I encountered regardless of size or species. Discarded houses identifiable to species were also collcctcd for laboratory analysis. Replicate water samples were collected by hand for analysis of total particulate carbon and nitrogen, simultaneously with the visual counts and house collections, in a clean S-liter, collapsible polyethylene “cubctainer.” Care was taken to exclude appcndicularian houses from thcsc samples. The accuracy of direct visual counting of these small organisms by SCUBA divers was determined through a program of simultaneous net tows and visual counts. While the population density of appendicularians was being determined visually at 10 to 15 m, a calibrated Clarke-Rumpus net of 0.15-mm mesh size was simultaneously towed in a 200-400-m-diameter circle around the diver.