MEPS 207:283-296 (2000)  -  doi:10.3354/meps207283

Larval settlement: chemical markers for tracing production, transport, and distribution of a waterborne cue

Patrick J. Krug*, Richard K. Zimmer

Department of Biology, University of California, Los Angeles, California 90095-1606, USA

ABSTRACT: Dissolved chemical signals mediate many ecological interactions in the marine environment, but little is known about how rates of production and distribution of waterborne cues affect life-history processes in the field. Larvae of the specialist marine herbivore Alderia modesta settle and metamorphose in response to complex carbohydrates produced exclusively by the adult host alga Vaucheria longicaulis, but the natural cue cannot be detected in sea water by current methods of analytical chemistry. The simple carbohydrates mannitol and glucose, which are highly concentrated in V. longicaulis tissue, were tested as possible markers for the settlement cue in laboratory and field experiments. In production experiments, both mannitol and glucose were released by patches of the algae and accumulated in the surrounding water over time, as did bioactivity due to the settlement cue. Pore water trapped within patches of V. longicaulis during low tides contained high concentrations of mannitol and glucose, and induced a high level of larval metamorphosis even at a 1:5 dilution. The bioactive pore water was released from algal patches into overlying water following immersion by a flood tide; water collected above the surface of V. longicaulis induced significant metamorphosis and changes in larval swimming behavior. Glucose content was significantly correlated with bioactivity in water collected above algal mats throughout the first 30 min of a flood tide, and also 2 h later, during the peak of a high tide. Mannitol and glucose concentrations were high in sea water above the center of an algal patch, but diminished rapidly at the edges and outside of the patch. Pore water collected from mats of the co-occurring alga Enteromorpha clathrata did not induce metamorphosis or changes in larval swimming behavior, and contained only background levels of the markers. The combined results show that mannitol and glucose are indeed released and transported along with complex carbohydrates from V. longicaulis, and can be used to define patterns of distribution of the dissolved settlement cue on both spatial and temporal scales. Ecologically, the data suggest that settlement rates of larvae of A. modesta may vary widely during a tidal cycle, as a function of the release and subsequent hydrodynamic transport of waterborne cues from the host alga.

KEY WORDS: Larval settlement · Chemical cue · Metamorphosis · Recruitment · Alderia modesta · Vaucheria longicaulis

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