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Dendraster Excentricus MARINE ECOLOGY PROGRESS SERIES Vol. 31: 245-254. 1986 - Published July 24 Mar. Ecol. Prog. Ser. l Larval production, dispersal, and growth in a fjord: a case study on larvae of the sand dollar Dendraster excentricus Richard B. Emlet* Dept. of Zoology and Friday Harbor Laboratories, University of Washington. Seattle. Washington 98195. USA ABSTRACT: In a small shallow fjord with predominantly wind-driven circulation, larvae of the sand dollar Dendraster excentricus (Eschscholtz) do not persist for longer than 2 wk after a local population in the fjord spawns. Changes in distnbuhon and abundance of larval cohorts indicate that larvae are advected out of the fiord and thus contribute to a regional larval pool. A series of plankton samples at the start of 2 reproductive seasons showed that cohorts of larvae are produced during spring tides that expose the intertidal adult population to air. Growth rates of 1 to 3 wk old larvae of D. excentricus and Strongylocentrotus franciscanus in the fjord are comparable to those reared in the laboratory. Later in the season, advanced-stage larvae of D. excentricus are found in surface waters in the fjord; this may enhance their chances of locating suitable habitat for settlement. Many of these advanced-stage larvae have shortened larval arms that may indicate delayed metamorphosis. INTRODUCTION are transported, how fast they grow, how readily they are eaten while in the plankton and what mechanisms A substantial portion of the temperate coastal ocean bring larvae into contact with suitable benthic habitats consists of fjords, bodies of marine water that are partly are basic questions in population ecology and life enclosed by land and whose water exchange with the history biology, yet they are rarely studied in the field. open sea is restricted by sllls (Pearson 1980). The Because fjord populations provide known sources of oceanographic processes controlling flushing and mix- larvae and because oceanographic processes in fjords ing in these water masses are relatively well under- are relatively simple, sampling a larval population is stood (e.g. Freeland et al. 1980) and must ultimately facilitated. influence the retention or dispersal of larvae of benthic In this paper I present results of a field study of marine invertebrates to fjord habitats suitable for adult larvae of the sand dollar Dendraster excentricus life, Pearson (1970), Gage (1972), and Rasmussen (Eschscholtz) produced by a population in a fjord (1973) have all remarked that the abundance of marine (Fig. 1). The information answers the first 5 of the organisms in fjords and the often broad habitat dis- following 6 questions. Data on larval size and form also tributions of benthic species relative to non-fjord areas bear on the last question. (1)Are planktonic larvae may be due to the retention of larvae and their subse- retained in the immediate area after release from a quent abundant settlement in fjords. population of benthic adults? (2) How variable Larval supply, settlement and post-settlement sur- between seasons and lunar-tidal cycles is onset of vivorship all may influence structure of benthic com- larval production? (3) How do development rates infer- munities (e.g. Keough & Downes 1982, Underwood & red from plankton samples compare with those in Denley 1984, Gaines & Roughgarden 1985). Larval laboratory rearing? (4) Are early-stage larvae concen- supply is notoriously hard to measure. Where larvae trated at certain depths and how could depth distribu- tion affect retention? (5) Are late-stage larvae concen- ' Present address: Dept. of Invertebrate Zoology, Natural trated at certain depths and could this affect return to History Museum, Smithsonian Institution, Washington, D.C. suitable adult habitat? (6) Is there evidence for delay of 20560, USA metan~orphosisby larvae in the plankton? O Inter-Research/Printed in F. R. Germany Mar. Ecol. Prog. Ser 31: 24.5254, 1986 STUDY ORGANISM AND STUDY AREA Laboratories (FHL) in the 1970's (summarized by King 1976) showed that circulation in East Sound is primar- Dendraster excentn'cus occurs in dense aggregahons ily wind-driven. Though a 2 to 3 m tide (mixed, semi- (200 to 1000 m-2) in shallow subtidal sandy habitats on diurnal cycle) in the area flushes water back and forth the outer coast and some protected intertidal habitats over the sill, tidal currents in East Sound are neglig- from Baja California to Southern Alaska (Menill & ible. Wind-driven circulation moves surface waters in Hobson 1970). In Puget Sound, Washington, USA, or out of the sound depending on wind direction and populations (called beds) occur intertidally in sandy sets up a deeper current whch moves in the direction embayments; some of these populations have been in opposite to the wind-driven surface waters. In the existence for a minimum of 53 to 78 yr (Bovard & summer, water is weakly stratified with a pycnocline Osterud 1918, Birkeland & Chia 1971). Throughout its approximately 10 m deep. The highest land in the San range, D. excentncus releases gametes during the Juan Archipelago, Mount Constitution (elevation summer and fall into the water where fertilization 700 m) forms the eastern shore of East Sound and occurs (Niesen 1977, Cameron & Rumrill 1982, author's affects wind patterns on the sound. Due to the moun- obs.). Feeding larvae take 3 wk to 2 mo to develop to tain, wind on the sound is restricted to north or south the point where they are competent to metamorphose direction, and can be different in magnitude from the (Strathmann 1978, Cameron & Rumrill 1982, High- wind over surrounding waters. smith 1982, author's obs.). Larvae are induced to There are 2 populations of Dendraster excentricus in metamorphose in the presence of adults by a low East Sound (Fig. 1). The larger of the two and the focus molecular weight compound associated with the sand of this study is located at the extreme North end of the in which adults Live (Highsmith 1982, Burke 1984). sound at Crescent Beach. This population extends East Sound, Washington, is a shallow fjord of 30 m along 500 m of beach and is located at 0.0 m (MLLW) mean depth. approximately 10 km length, and 1.5 km k0.2 m. Estimated population size is 5 X 106 indi- average width. At the southern entrance to the sound a viduals k 20 O/O (author's unpubl, obs.). The second sdl, 10 to 15 m shallower than surroundng bottom, population is located more than 10 km away at Olga, extends approximately half-way across the mouth. East on the eastern shore just outside of the entrance to East Sound has minimal freshwater input and lacks a lower Sound. This population is restricted to approximately salinity surface layer common to many fjords. A brief 40 m of shoreline also at about MLLW and is estimated study of circulation (Rattray 1967) and 4 separate sum- to contain 2.5 to 5 X lo5 individuals (author's unpubl. mer profhng studies conducted by biological obs.). A third, extremely small population (ca 5000 oceanography courses at nearby Friday Harbor individuals) exists 6.5 km away from East Sound in Flg. 1. Composite map of study area. Re- gonal map of Western Washington, San Juan Archpelago, and southern Van- couver Island. SJC indicates site in San Juan Channel where reference plankton samples were collected. Inset map is Or- cas Island. Collection sites in East Sound are indicated (A, A', B, C, D. E) and the locations of the adult sand dollar bed are shown by arrows. Double arrows: the large Crescent Beach population; single arrow: the Olga population Emlet: Production, dispersal and growth of sand dollar larvae Shoal Bay on Lopez Island. Other known local popula- originate in East Sound and so indicates supply of tions are more than 20 km from the entrance to East larvae from the surrounding region. Sound. In all 1982 samples, number and stage of larvae of Dendraster excentxicus and of the red urchin, Strongy- locentrotus franciscanus (A. Agassiz) were determined METHODS for the entire sample. Larvae of S. franciscanus were counted because they were abundant and indicate Plankton was sampled regularly in East Sound and water from outside East Sound (see below). The 1983 at 1 site in San Juan Channel during the beginning of samples were split with a Folsom plankton splitter and the sand dollar reproductive season (May and June) half of each was counted. Numbers of larvae (rounded and occasionally later in the season during 1982 and to the nearest integer) are reported as no. per vertical 1983. All samples were fixed in 3 O/O buffered formalin tow in 1982 and no. m-3 in 1983. immediately upon collection. The skeleton of echinopluteus larvae is composed of In 1982, samples were taken with a 125 Km mesh net calcareous rods and can be used to identify species with a %m diameter opening. Vertical tows were (Mortensen 1921). The larvae of Dendraster excen- taken at established sites along the axis of East Sound tricus were identified by their transparent larval body (Fig. 1). One to 3 vertical tows were collected at each and fenestrated arm rods (Strathmann 1979).The larval site. Forty-six site-dates are reported; 10 of these have skeleton of Stronyylocentrotus franciscanus possesses 3 replicate tows and 9 others have 2 replicate tows a recurrent body rod that makes this species easily incorporated into the mean values reported (Table 1A, distinguishable from larvae of other Strongylocen- B). Replicates were restricted primarily to Sites A, B, trotus species (Strathmann 1979). Other Strongylocen- and C and never occurred at Site D or at the SJC site. trotus larvae, such as those of S. droebachiensis and S. AU net tows were made from a depth of 25 m to the purpuratus, were seen only rarely during the study.
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