Geographic Differences in Herbivore Impact: Do Pacific Herbivores Prevent Caribbean Seaweeds From Colonizing Via the Panama Canal?

Several dominant, shallow-water seaweeds from the Caribbean are physiologically capable of transiting the freshwater of the Panama Canal, surviving both the marked salinity changes and the required time spent in freshwater. Some of these species are known to settle and grow on boat bottoms, thus providing a direct method for transport through the canal. However, they are rare or absent on the Pacific coast. The primary barrier to the successful transport and establishment of these species appears to be herbivore activity and the lack of reef-generated refuge areas on the Pacific coast, not the salinity of the Panama Canal. IN THE 70 YEARS THAT THE PANAMA CANAL HAS LINKED THE WESTERN CARIBBEAN AND THE EASTERN PACIFIC, very few interoceanic colonizations have been documented. It is generally assumed that the major barrier to transfer of marine species between oceans has been the freshwater in the canal. Thus, the discussions of potential colonizations have focused primarily on transfers that might occur if a new, sea-level canal were constructed or on species that might be carried in the ballast tanks of ships (Briggs 1968, 1969; Rubinoff 1968; Topp 1969; Springer 1973; Glynn 1982; but see Menzies 1968). Here we focus on direct transfer of adult seaweeds, many of which are shown to be tolerant of long exposure to freshwater. Disjunctions in herbivore impact often determine the local distribution of plant populations or community types in marine environments (Kitching and Ebling 1961; Randall 1965; Jones and Kain 1967; Ogden et al. 1973; Lubchenco and Menge 1978; Lubchenco 1980; Hay 198 la, b, c; Underwood and Jernakoff 1981; Hay et al. 1983). Herbivory also appears to be a primary factor maintaining the between-habitat diversity of seaweeds on coral reefs (Hay 1981a, Hay et al. 1983) and has been hypothesized to be important in maintaining distributional boundaries on a geographic scale (Lubchenco 1980, Hay 1981a, Lubchenco and Gaines 1981, Gaines and