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

Dissolved chemical signals mediate many ecological interactions in the mari ne environment, but little is known about how rates of production and distr ibution of waterborne cues affect life-history processes in the field. Larv ae of the specialist marine herbivore Alderia modesta settle and metamorpho se in response to complex carbohydrates produced exclusively by the adult h ost alga Vaucheria longicaulis, but the natural cue cannot be detected in s ea water by current methods of analytical chemistry. The simple carbohydrat es mannitol and glucose, which are highly concentrated in V. longicaulis ti ssue, 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 w ater over time, as did bioactivity due to the settlement cue. Pore water tr apped within patches of V. longicaulis during low tides contained high conc entrations of mannitol and glucose, and induced a high level of larval meta morphosis even at a 1:5 dilution. The bioactive pore water was released fro m algal patches into overlying water following immersion by a flood tide; w ater collected above the surface of V. longicaulis induced significant meta morphosis and changes in larval swimming behavior. Glucose content was sign ificantly correlated with bioactivity in water collected above algal mats t hroughout 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 w ater above the center of an algal patch, but diminished rapidly at the edge s and outside of the patch. Pore water collected from mats of the co-occurr ing alga Enteromorpha clathrata did not induce metamorphosis or changes in larval swimming behavior, and contained only background levels of the marke rs. 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 tha t settlement rates of larvae of A. modesta may vary widely during a tidal c ycle, as a function of the release and subsequent hydrodynamic transport of waterborne cues from the host alga.