Pacific Science (1993), vol. 47, no. 3: 256-262 © 1993 by University of Hawaii Press. All rights reserved Behavioral Basis of Depth Regulation in the First Zoeal Stage of the Pacific Shore Crab, Hemigrapsus oregonensis (Brachyura: Grapsidae)l 2 MARIELISE ARANA 2 ,3 AND STEPHEN SULKIN ,4 ABSTRACT: The behavioral basis ofdepth regulation is determined for the first pelagic larval stage ofthe shore crab Hemigrapsus oregonensis Dana. Larvae are negatively buoyant, passively sinking at 0.79 em/sec in 25 parts per thousand (ppt) salinity (S) seawater and at 0.67 em/sec in 30 ppt S. At 30 ppt S, larvae are negatively geotactic and move upward. At 25 ppt S, larvae remain negatively geotactic, but a low level of locomotor activity results in net downward move­ ment. Swimming speed is higher at 30 ppt S than at 25 ppt S; however, there is no response to incremental increases in hydrostatic pressure up to 0.8 atm at either salinity. Behavioral responses should promote upward migration ofthe hatching stage similar to the case with other intertidal crab species; however, low precision in depth regulation contrasts with results from other species. IN MANY BENTHIC marine invertebrates, in­ ment into the water column, a response that cluding most brachyuran crabs, adult distri­ presumably results in dispersal of offspring bution is influenced by dispersal ofthe pelagic from the site of hatching. Hatching-stage larval stage. The pattern of larval dispersal is larvae generally respond to the higher pres­ regulated by a number of factors, including sures and salinities present at depth by in­ larval morphology, duration, and behavior. creasing locomotor activity. This locomotor In estuarine and coastal marine environ­ activity is oriented by negative geotaxis such ments, current speed and direction often vary that upward migration results. Key behav­ with depth. As a consequence, patterns of ioral elements of this response thus include vertical distribution can significantly influence negative geotaxis and high barokinesis (Sulkin larval dispersal (e.g., Bousfield 1955, Thorson et al. 1980, Jacoby 1982, Kelly et al. 1982, 1964, Cronin 1982). It is well documented that Schembri 1982). The effects of other environ­ larval crabs possess behavioral traits that mental parameters, such as salinity, on larval contribute to depth regulation (reviewed by behavior also have been documented (Latz Sulkin 1984), thus producing characteristic and Forward 1977, Sulkin et al. 1980). patterns ofvertical distribution and dispersal. With the exception ofJacoby's (1982) study Particular attention has been paid to the of larval behavior in Cancer magister Dana, behavioral response of the first zoeal stage of most research has been conducted either on many species of crabs (Sulkin 1984). Such shallow-water, estuarine species (Sulkin 1984) studies have documented the presence of be­ or on oceanic species whose adults occupy havioral traits that promote upward move- deep water (Kelly et al. 1982). Both groups Rossess larval adaptations likely to promote upward migration upon hatching, althougfi some differences in larval behavior between 1 The first author was supported on this project by a grant to the Shannon Point Marine Center from the shallow-water and oceanic species have been National Science Foundation as part of the Research noted. Experiences for Undergraduates Site Program (OCE­ The study reported here examined the be­ 9000676). Manuscript accepted 8 October 1992. havioral responses to gravity and hydrostatic 2 Shannon Point Marine Center, 1900 Shannon Point Road, Anacortes, Washington 98221. pressure in the hatching stage of the crab 386-24 144th Street, Briarwood, New York 11435. Hemigrapsus oregonensis Dana, a common 4 Correspondent. shore crab that inhabits rocky intertidal Depth Regulation in Pelagic Larvae of Hemigrapsus-ARANA AND SULKIN 257 beaches along the eastern Pacific Ocean. In marked offin the middle ofthe cylinder. Mean the Puget Sound basin, adults occupy the values were compared using a Student's t test. shallow waters of the lower intertidal, but larvae are readily dispersed to nearby waters Geotaxis exceeding 100 m in depth. Results are com­ pared with those reported in the literature for Geotaxis, or gravity orientation, is defined shallow-water species whose larvae are re­ as oriented movement in the vertical plane tained in shallow water and for oceanic spe­ in the absence of other environmental cues cies whose larvae are distributed in waters of (Fraenkel and Gunn 1961). Two methods substantial depth. were employed to measure geotaxis in H. oregonensis. The first method involved recording the response of individual larvae. A larva was MATERIALS AND METHODS pipetted into a test tube (150 mm long by 15 Experimental Animals mm diam.) filled with seawater of either 25 ppt S or 30 ppt S. The test tube was sealed Ovigerous H. oregonensis were collected and placed in the horizontal position on a from the intertidal zone at the Shannon Point ringstand, with a rotating clamp. The larva Marine Center near Anacortes, Washington. was retained in this position for 75 sec in the Adult crabs were held in large glass bowls dark (Forward 1985). The tube was then containing filtered seawater at 30 parts per rotated 90° to the vertical position and the thousand (ppt) salinity (S) and 15°C, until individual was monitored for movement in larvae hatched. Larvae from three broods the vertical plane for the first 10 sec. Ifa larva were used in the experiment, with larvae moved upward (±22.5° from the vertical from each brood maintained separately. Up­ axis), it was defined as exhibiting negative on hatching, larvae from each brood were geotaxis; ifit moved downward (±22,SO from transferred by pipette to bowls containing the vertical axis), it was defined as showing clean seawater. Each culture bowl contained positive geotaxis. At each salinity, a total 100 larvae. Larvae were transferred daily to of 25 individual measurements were made for clean seawater (30 ppt S; 15°C) and fed newly each of three broods (n = 75). The numbers hatched nauplii of brine shrimp, Artemia sp. responding as negative, positive, or neutral Larvae to be used in the various behavioral are presented as percentages. experiments were drawn randomly from the A second method of testing for geotaxis mass cultures. Larvae used in all experiments involved placing a sample of 10 larvae into a that called for 25 ppt S seawater were placed vertically oriented acrylic observation cham­ in this lower salinity for at least 12 hr before ber (30 cm long by 5 cm diam.), divided the experiment. All experiments were con­ vertically into three sections of equal length. ducted on larvae within 24 hr of hatching. The larvae were introduced carefully by pi­ pette into the middle section after dark-adap­ Buoyancy tation for 75 sec. Distribution oflarvae among the three sections was determined every 5 min Buoyancy was measured on anesthetized for 30 min. A deep red backlight was used to r-vae-that-had-btl~n-{}X-fl0secl-te--seawater-()f-silhouette-thdaTvae-fur-c-o-onttng4'noe"'e"'x"'p"'eri'tI - the appropriate salinity to which a few crystals ment was performed with larvae from each of of magnesium sulfate had been added. The the three broods at each salinity, and the data larvae were then pipetted gently into a vertical were pooled for analysis. cylinder filled with seawater of either 25 ppt or 30 ppt S as appropriate. Descent rates for Barokinesis 50 larvae were measured in both 25 ppt and 30 ppt S by determining the time required for Swimming speed as a function of hydro­ individuals to traverse a 10-cm-Iong distance static pressure was determined according to 258 PACIFIC SCIENCE, Volume 47, July 1993 the method of Kelly et al. (1982). Approxi­ TABLE I mately 25 larvae were placed in the acrylic PERCENTAGE OF INDIVIDUAL LARVAE MOVING UP, observation chamber, and the chamber was DOWN, OR NOT MOVING IN EITHER DIRECTION filled with seawater, sealed, and attached to a (NEUTRAL) IN THREE EXPERIMENTS USING LARVAE FROM closed system in which pressure can be in­ EACH OF THREE BROODS, CONDUCTED AT 30 PPT SALINITY creased. The pressure generator consists of a AND AT 25 PPT SALINITY piston screw pump that develops pressure ppt 25 ppt within a small volume by fluid compression. 30 In these experiments, larvae were subjected to DIRECTION EXPERIMENT NO. EXPERIMENT NO. stepwise increments of0.2 atmospheres (atm), OF MOVEMENT I 2 3 I 2 3 up to a total of 0.8 atm. Larvae were induced to swim along the axis of the horizontally Up 88 96 44 0 32 36 Neutral 12 4 44 8 12 4 oriented chamber toward a dim white light. Down 0 0 12 92 56 60 Individuals were timed as they swam across a 5-cm space marked off in the middle of the chamber. After 10 measurements were made, the pressure was increased by 0.2 atm to the bution determined at 5-min intervals. Shown next increment, and 10 larvae again were are the results at 5, 15, and 30 min. After only timed. The complete series of measurements 5min, a stable distribution was obtained, with was repeated three times at each salinity. For the majority moving to the top section of the each salinity, the mean swimming speed at cylinder. each pressure was calculated and compared Results at 25 ppt S were more equivocal. among pressures. As shown in Table I, initial movement of a majority of individuals was downward, al- though in two of the experiments, approxi­ mately one-third of the larvae did move up­ RESULTS ward. In the experiments using the vertical Buoyancy cylinder, all larvae had positioned themselves in the bottom third ofthe chamber after 5 min Anesthetized larvae were negatively buoy­ and remained there (Figure I).