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Ecology of <I>Priapulus Caudatus</I> Lamarck, 1816 (Priapulida) in An

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Ecology of <I>Priapulus Caudatus</I> Lamarck, 1816 (Priapulida) in An BULLETIN OF MARINE SCIENCE, 47(1): 149-158, 1990 ECOLOGY OF PRJAPULUS CAUDATUS LAMARCK, 1816 (PRIAPULIDA) IN AN ALASKAN SUBARCTIC ECOSYSTEM Thomas C. Shirley ABSTRACT Priapulid worms (Priapulus caudatus) were a conspicuous component of the meiofauna «0.500 mm) and macrofauna (>0.500 mm) in Auke Bay, Alaska (58°N, I34°W) from 1985- 1988. The smallest priapulid larva collected had a lorica of 0.050 mm length, while total lengths of adults exceeded 150 mm. Priapulids were usually the third most abundant mei- ofaunal organisms, following nematodes and harpacticoid copepods, with densities to 58,000' m-2 at subtidal stations of 25-55 m depth. Priapulids were much less abundant in the macrofauna, never exceeding 85, m-2• Large interannual variation in densities of larval pria- pulids occurred. The smallest larval stages were found during the winter months, the apparent spawning period, with greatest densities of larvae occurring in early spring. Growth rates suggested by length-frequency distributions support a 2-year larval period. The Priapulida are a small phylum of benthic, marine worms of obscure phy- logenetic affinity that have been allied with a variety of phyla (Candia Carnevali and Ferraguti, 1979; Conway Morris, 1977; Lang, 1963; Malakhov, 1980; Por and Bromley, 1974; Shapeero, 1961); most recently they have been aligned with the Kinorhyncha (Land and Norrevang, 1985). Approximately 15 extant and 6 fossil species have been described (Land and Norrevang, 1985; Conway Morris, 1977), with Priapulus caudatus Lamarck undoubtedly the best studied (Carlisle, 1958; Lang, 1939, 1948a; 1948b; Nyholm and Borno, 1969; Shapeero, 1962a; Wesenberg-Lund, 1929). Priapulids are generally considered rare and biologically important only in marginal environments (Calloway, 1982; Land, 1970). However, in a long-term benthic study in Auke Bay, Alaska (Shirley et al., 1988), Priapulus caudatus were common and conspicuous members of the meiofauna as larvae and macrofauna as adults. Auke Bay is not a marginal environment, but has a diverse ichthyofauna (Haldorson et al., 1988), macrofauna and meiofauna (Shirley et al., 1987, 1988; Coyle and Shirley, in press). Despite its widespread northern circumpolar distri- bution (Land, 1970), few quantitative data exist regarding the ecology of P. cau- datus. New quantitative data available on long term densities and seasonal size- frequency distributions of priapulids were the impetus for the present paper. MATERIALS AND METHODS Sampling was conducted in Auke Bay, Alaska (58°22'N, 134°40'W), approximately 20 km north of Juneau (Fig. I). Auke Bay is a partially enclosed embayment with an area of approximately 16 km2 in a system of fjords connected with the Pacific Ocean. Approximately 50% of the bay is between 40- 60 m depth and has a muddy substrate. The bay has semidiumal tides with amplitudes of 3-7 m. Bottom water temperatures at 25 m depth vary between 3-7°C annually and salinities vary between 29 and 320/00annually (Coyle and Shirley, in press). A modified Kajak corer, the Tom-Tom corer, was used to collect meiofauna samples. The corer 149 ISO BULLETIN OF MARINE SCIENCE, VOL. 47, NO.1, 1990 STI:PHENS PASSAGE o 2 • I KM Figure I. Map of Auke Bay, Alaska with locations of sampling stations: ABM = Auke Bay Monitor, E = East, FC = Fritz Cove, I = Intertidal, N = North, NE = Northeast, UAF = Juneau Center for Fisheries and Ocean Sciences" University of Alaska Fairbanks. collects four replicate cores 2.5 cm in diameter each time it is deployed and has proved to be reliable and effective (Chandler et aI., 1988). All cores were collected between 0800 and 1100. Macrofauna were retained fi'om meiofauna core samples on all sampling dates in 1986 and examined for priapulids. Additional macrofauna samples were collected with four 0.1 m' Petersen grabs on the same dates. Meiofauna samples werc collected from four subtidal stations in Auke Bay and one station in adjacent Fritz Cove (Fig. I) from 1985-1988: East (E, 27 m water depth), Auke Bay Monitor (ABM, 55 m), North (N, 42 m), Northeast (NE, 42 m) and Fritz Cove (FC, 25 m). In 1985 eight cores were collected at 14-d intervals from E and FC from 3 April-IO June, and in 1986 at lO-d intervals from 8 May-7 August at E. The most intensive sampling was conducted in 1987 from ABM, E, Nand NE: four replicate cores were colle(:ted at 2-week intervals prior to the spring phytoplankton bloom, weekly during the bloom and for 6 weeks after its sedimentation. Cores were collected at 2-week intervals thereafter throughout the summer. Samples were collected monthly during the fall and winter from E and ABM, yielding a total of 20 sampling dates during 1987. During 1988, four cores were collected weekly from E and ABM from 16 March to 16 June, a total of 14 sampling dates. Priapulid data were therefore collected during 13 of 15 months from April 1987 to June 1988 on 34 sampling dates. Intertidal meiofauna was sampled at 2-week intervals from March 1987 to July 1988 from a single station (0 m) in Auke Bay with four cores collected with hand-held 50-cc syringes (McGregor and Shirley, 1988). The upper 5 cm of sediment from all cores and the overlying water was returned to the laboratory for processing where samples were sieved with seawater, except in 1986, when the entire core was processed. All material passing through a 0.500 mm mesh sieve but retained on a 0.063 mm mesh sieve was then extracted live with sucrose centrifugation (Fleeger, 1979). Sediment pellets resulting from centrifugation were examined weekly for sorting efficiency, which was estimated to exceed 95%. Macrofauna remaining on thc~0.500 mm sieve were narcotized with MgCl and preserved in 10% buffered formalin. All organisms were identified to major taxon under a stereo-dissection microscope. Nematodes were subsampled with sorting trays and a technique which employs a triply-balanced square design described by Sherman et al. (1984). Lorica lengths of all available priapulid larvae to a maximum of 50 were measured with an ocular micrometer each month. SHIRLEY: ECOLOGY OF AN ALASKAN PRIAPULID lSI RESULTS Priapulid larvae and adults were present at all subtidal stations sampled in Auke Bay and Fritz Cove; none were present in intertidal samples (Fig. 1). Densities ofpriapulids were not significantly different by depth among the subtidal stations. Priapulid larvae were present only in meiofaunal samples, as they were not re- tained on macrofauna sieves (0.500 mm mesh). Small numbers of metamorphosed priapulids having adult morphology, e.g., lacking the lorica and possessing ru- dimentary caudal appendages, were also present in meiofaunal samples. Large interannual variations in mean density of larval priapulids, from < 1.0' 10 cm-2 in 1985 to >50.0· 10 cm-2 in 1988, were recorded from the single station (E) sampled all 4 years (Fig. 2), with densities generally increasing from 1985 through 1988. Intraannual variation also occurred, with larval densities usually being highest in the early spring and declining in the summer months, although large variation often existed between dates. Densities of the smallest size class of larvae increased in December, 1987, the only year that winter samples were collected (Fig. 3a). Total meiofauna also had interannual variations in mean density during the 4 years. Mean total meiofauna density varied by an order of magnitude at E from 1985-1988, from a minimum of 520·10 cm-2 in 1987 to a maximum of 5,900' 10 cm-2 in 1986 (Fig. 2). Total meiofauna were heavily predominated at all subtidal stations by nematodes, which comprised 97% of the total meiofauna across all dates and stations (Shirley et aI., 1989). Harpacticoid copepods were usually the second most numerically abundant taxon at subtidal stations, even though they rarely comprised more than 2% of the total meiofauna. The percent composition of harpacticoids in the total meiofauna was consistently higher from late May through August. Standard errors were unusually low for meiofauna, seldom exceeding 15% of the mean, despite the average sample size of only 4 cores per station per collection date in 1987 and 1988 (Shirley et aI., 1988; Shirley et aI., 1989; Fleeger and Shirley, 1990). The density relationships between the predominate meiofaunal components, excluding nematodes, did not vary greatly between years or subtidal stations (Walls et aI., 1987). At E, the numerically predominate meiofauna, in order of abundance, were harpacticoids, priapulid larvae, polychaetes, and ostracods. After the har- pacticoids, only priapulids contributed significantly to total meiofaunal density and at times replaced harpacticoids as the second most abundant taxon. The same relationships were true at ABM, with the exception that kinorhynchs supplanted ostracods in abundance, and at Nand NE stations, where the maximal density of the less abundant taxa was <2.5·10 cm-2• Approximately 35 larval size classes, lorica total lengths ranging from 0.050 to 1.050 mm, were found in Auke Bay (Fig. 3). Many of the larval sizes undoubtedly represent variation around modes, and many were not present in all months. The smallest size class of priapulid larvae first appeared in December (Fig. 3a). A comparison oflength-frequency distributions ofpriapulids in December and March implies that the larvae of the predominate size classes had increased by 0.100 mm (Fig. 3a). The 0.400 mm shift in size-frequency distributions of the predom- inate size class between April and August similarly presents a tenable growth schemata representing four molts with approximately a 25% molt increment (Fig. 3b). Nearly identical size-frequency distributions ofpriapulid larvae, particularly with the predominate size class, were present in the same months in 1987 and 1988 (Fig. 3c). Bottom water temperatures and salinities varied between years in Auke Bay.
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