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found mostly in the upper intertidal zone, emerged in a circadian rhythm, which ... swash-rider organisms from microtidal beaches, where sea level is difficult to ...

Marine Biology (2001) 138: 187±197

Ó Springer-Verlag 2001

B. Yannicelli á R. Palacios á L. GimeÂnez

Activity rhythms of two cirolanid isopods from an exposed microtidal sandy beach in Uruguay

Received: 23 March 2000 / Accepted: 30 August 2000

Abstract Activity rhythms of two cirolanid isopods, Excirolana armata and Excirolana braziliensis, were studied based on both seasonal ®eld observations and laboratory experiments, at an exposed microtidal sandy beach in Uruguay. The natural emergence patterns of both species were observed in the ®eld for 1 year, twice in each season, and correlated to sea level, expected tidal cycles and diel cycles. Laboratory experiments were carried out in order to detect endogenous rhythms of activity and observe how emergence of both species was a€ected by changes in light and/or sediment thixotropy. We also compared behavioral strategies of sympatric species that occupy di€erent beach levels. Sea level (and thus swash zone position) during ®eld sampling did not follow expected tidal cycles for most sampling occasions. E. armata was observed in activity most of the time, but activity only correlated with sea level on three out of eight occasions, and only once was correlated to expected tidal cycle. Laboratory results showed that emergence under constant conditions was rare; changes in sediment thixotropy stimulated emergence, but the response was not cyclical; light had little e€ect on this response. On the other hand, E. braziliensis was fairly scarce in the water column, but swimming individuals were observed always during the night. They displayed an endogenous circadian activity pattern in the laboratory which augmented in response to changes in sediment thixotropy. The natural light/dark cycle modulated both spontaneous and response emergence by increasing day/night di€erences in activity. In this study E. armata, a midlittoral species Communicated by O. Kinne, Oldendorf/Luhe B. Yannicelli (&) á L. GimeÂnez SeccioÂn Oceanografõ a, Facultad de Ciencias, Universidad de la RepuÂblica, Montevideo, Uruguay Tel.: +598-2-5258618; Fax: +598-2-5258617 e-mail: [email protected] R. Palacios Programa de Desarrollo de las Ciencias BaÂsicas (PEDECIBA), Universidad de la RepuÂblica, Montevideo, Uruguay

more exposed to sea level variations, seemed to rely entirely on di€erent physical and/or biological cues to trigger emergence at the appropriate time. E. braziliensis, found mostly in the upper intertidal zone, emerged in a circadian rhythm, which was stimulated by changes in sediment thixotropy and reinforced by light cycles. The results of this study led us to conclude that on microtidal, unpredictable beaches, local physical and biological factors can combine to determine di€erent activity strategies in organisms from di€erent intertidal levels.

Introduction Cirolanid isopods are often found on exposed sandy beaches around the world (McLachlan and Jaramillo 1995), being in many cases the dominant species in terms of biomass and/or abundance (Glynn et al. 1975; Dexter 1977). Di€erent species have been reported across the entire beach pro®le or occupying high, middle or low intertidal levels, depending on the beach studied (Jones 1974; Jaramillo 1982, 1987; Bally 1983; De Ruyck et al. 1992; Jones and Pierpoint 1997). It is common for cirolanids to remain buried during low tide, emerging to search for food as the sea level rises. They migrate up and down the beach, returning to their usual burial zone as the tide recedes. They use the swash as a physical means of transportation (SMAM swash riders, sensu GimeÂnez and Yannicelli 1997), maintaining zonation through complex systems of behavioral responses (McLachlan and Jaramillo 1995) which include endogenous activity rhythms and response plasticity. These behaviors allow cirolanid isopods to emerge and re-bury in the appropriate time period and zone, and must have been favored during the evolution of intertidal macrofauna (Brown 1996). Cirolanids are known to display distinct rhythms of activity at di€erent intertidal levels and tidal regimes (De Ruyck et al. 1991). In meso- and macrotidal environments, species from the lower zones display circatidal rhythms of emergence (Klapow 1972; Jones and


Hobbins 1985; Reid 1988). However, it has been observed that those same species display circadian rhythms when they occur on tideless beaches (Macquart-Moulin 1977). Activity rhythms of cirolanids from beaches with intermediate tidal range and high energy are not well de®ned (De Ruyck et al. 1991). Cirolanids can perceive and respond to physical stimuli such as light, sound, turbulence and changes in sediment thixotropy (Jones and Naylor 1970; Enright 1972; Hastings 1981; Forward 1986), allowing them to: (1) synchronize rhythms of activities to environmental regimes (such as ¯ooding tides) to move up or down the beach, and (2) respond to unpredictable events (such as storms) that may jeopardize survival. Uruguayan beaches experience a semidiel tide, with a range of

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