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Distribution and Dynamics of an Intertidal Ascidian Pseudopopulation

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Distribution and Dynamics of an Intertidal Ascidian Pseudopopulation BULLETIN OF MARINE SCIENCE, 45(2): 288-303, 1989 DISTRIBUTION AND DYNAMICS OF AN INTERTIDAL ASCIDIAN PSEUDOPOPULATION Craig M, Young ABSTRACT Molgula occidentalis. an abundant ascidian in the shallow subtidal zone of the northern Gulf of Mexico, occurs seasonally on intertidal sandbars. One such population was studied over a 5-year period on Bay Mouth Bar, Franklin County, Florida. Most years, ascidians recruited during the spring and summer months, then died during early morning low tides in January and February. Predation by Fasciolaria hunteria (Gastropoda) accounted for a negligible portion of the mortality. Most mortality occurred when strong north winds caused freezing temperatures and prolonged exposure in the intertidal. Molgula occidentalis aggregate at several spatial scales. They occur most abundantly in areas where seagrasses (predominantly Ruppia maritima) stabilize the sediment. Sand movements and excessive siltation have adverse effects on adults and juveniles. Although larvae are capable of attaching to uncon- solidated sand, they prefer to settle on the consolidated sands from the adult habitat. Small- scale distribution is also related to elevation. Few ascidians occur in the bottoms of deep pools or on high sand ridges. Highest densities are attained near the edges of pools, no more than a few em above or below the seawater table at low tide. Because adults can survive in the bottoms of pools, it is suspected that this distribution pattern is established by larval choice andjuvenile mortality. Intertidal Molgula occidenta/is are not true populations because they are not self-sustaining. Recruitment depends entirely on larvae emigrating from subtidal populations. Individual organisms sometimes colonize regions beyond the usual geographical range of their species. Often, however, such individuals either fail to reproduce because of physiological stress, or die because of inimical biological or physical conditions before attaining reproductive maturity (Thorson, 1946; Mileikovsky, 1961). Thus, the species border may shift slightly with habitat fluctuations from year to year or from season to season, but tends to remain more-or-less constant over the long term (Mayr, 1970). Portions of a population established and sus- tained only by outside propagules are sometimes known as pseudopopulations. Most benthic marine invertebrates with pelagic larvae form "open" populations, meaning that colonizing larvae may originate either from within or from outside of a particular part of the range (Roughgarden et al., 1985). Thus, the distinction between an open population (Roughgarden et al., 1985) and a pseudo population (Mileikovsky, 1961) is tenuous. For the purpose of the present paper, I define pseudopopulations as portions of open marine populations occurring so near the species boundary that individuals either fail to reproduce or persist only tem- porarily. Virtually all individuals in a pseudopopulation are immigrants. The dynamics of pseudo populations are instructive from an evolutionary stand- point; they provide clues as to the factors controlling the species border and thus constraining horizontal gene flow. Nevertheless, most ecological studies focus on the self-perpetuating or otherwise permanent portions of populations geograph- ically removed from the species border. In the marine environment, species have both horizontal and vertical bound- aries. Pseudopopulations frequently occur in deep water, where recruitment de- pends on larvae washed offshore and reproduction is precluded by cold water temperatures and other environ mental conditions (reviewed by Mileikovsky, 1961). Physiological limits of tolerance to physical factors are generally thought to control 288 YOUNG: ASCI DIAN PSEUDOPOPULATION DYNAMICS 289 the upper limits of distribution for intertidal and shallow subtidal species (Connell, 1972; Underwood, 1985). For example, both hot and cold weather have adverse effects on molluscs occurring high on the shore (reviewed by Newell, 1979). Horizontally, populations of some benthic invertebrates are repopulated from outside sources because of the directions of prevailing currents. One example is in the Oresund of Scandinavia, where many species are replenished by supplies of larvae originating in the Kattegat or in the Baltic Sea (Thorson, 1946). Ascidians, being soft-bodied, are typically subtidal animals. Intertidal ascidians generally occur either in tidepools or in shaded habitats far down on the shore. The most notable exceptions are stolidobranchs in the genus Pyura, which char- acteristically have thick, leathery tunics (Paine and Suchanek, 1983; Stephenson, 1942; Underwood and Fairweather, 1986). Extensive long-term data on the dy- namics of natural ascidian populations have only been collected from subtidal Scandinavian fjords (Svane, 1983; Svane and Lundalv, 1982a, 1982b). Many members of the family Molgu1idae occupy soft bottoms. Use of sedimentary habitats by these organisms has been facilitated by apparent adaptations in de- velopmental mode, larval structure, larval behavior, and adult morphology (Ber- rill, 1931; Young et ai., 1988). Specifically, it has been suggested that adaptations for habitat selection have been lost in soft-sediment molgulids because habitat homogeneity renders such traits unnecessary (Berrill, 1931). However, neither population biology nor small-scale distribution of soft-sediment molgulids has been studied. In the northern Gulf of Mexico, Molgula occidentalis and Styela plicata are two dominant ascidians in the shallow subtidal zone. Both species occur in dense aggregations, occasionally attaching to seagrasses, shells, and other ascidians, but more often resting on the sandy or muddy substratum. Of the two species, only Molgula occidentalis is found in large numbers in the intertidal zone. Although it attains local densities as high as several hundred per square meter, intertidal populations are ephemeral. In this paper, I report quantitative and anecdotal observations on the dynamics of Molgula occidentalis at a single intertidal site over a 5 year period. I also report their intertidal spatial pattern on several scales, document behavioral attributes of the larvae, and discuss factors that control patterns of distribution and abundance. MATERIALS AND METHODS Study Site. -Pseudopopulations of Molgula occidentalis were studied primarily on Bay Mouth Bar, a large intertidal sandbar, several kilometers long by approximately 400 m wide, located at the mouth of Alligator Harbor, Florida (Fig. ]). The bar stretches north and south; the east side, where most ascidians were located, was on the leeward side of the bar except when winter cold fronts passed through from the north. Population Dynamics and Sources of Mortality. - During the winters of 1982-] 983 and 1983-1984, the corners of permanent sampling plots were marked with wooden stakes near the eastern shore of the sandbar where Molgula occidenta/is were abundant. All individuals were counted on each sub- sequent visit to the intertidal. Cohort survivorship curves were correlated with low tide predictions taken from NOAA tables and with National Weather Service air temperature data taken at the Tallahassee airport, approximately 50 km north of the study site. Surface seawater temperatures were taken at the Florida State University Marine Lab, located approximately 8 km west of the study site. The potential role of predators was examined by comparing survivorship of caged individuals, individuals surrounded by fences, and uncaged, marked individuals. Circular cages and fences 15 cm in diameter were constructed of I-cm mesh galvanized hardware cloth. Each cage or fence contained only a single individual in order to eliminate any confounding density-dependent effects. One set of treatments was set adjacent to each of the permanent sampling plots. Whenever mortality occurred, the cages were placed over new sets of animals, so data values represent mortality occurring between sampling dates, not cohort survivorship. 290 BULLETIN OF MARINE SCIENCE. VOL. 45. NO.2. 1989 ~,r" FS.U. Morine Lob Figure I. Map of the study area in the northern Gulf of Mexico. Adult Molgula occidentalis cover themselves with a thick layer of sand, which is held in place by hair-iike extensions of the tunic. I investigated the potential role of this sand covering as a defense mechanism in laboratory experiments. In each of three experimental runs, either 4 or 10 adult ascidians were maintained in running seawater tables with 3 adult Fasciolaria hunteria. Half of the ascidians were scraped gently to remove most of the sand and to expose the bare tunic. After 24 to 48 h, mortality in the two groups was tabulated. Because each run had a small sample size and the results were similar for all runs, the data were pooled for analysis. Three aggregations of adult ascidians were transplanted to the bottoms of deep pools in December 1983 in order to determine if environmental conditions in pools were suitable for adult survival. The ascidian clumps were held in placc with large brass staples and were monitored biweekly until 19 January 1984. A single attempt to monitor early juvenile survivorship and growth by means of field transplants was unsuccessful. Fortuitously, heavy recruitment in a shallow running seawater system during the spring of 1983 enabled me to monitor juvenile mortality under high and low flow conditions. The seawater hose was directed at three marked quadrats, each to em on a side, near the standpipe drain. Individuals in these quadrats
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