CHARLES D. AMSLER, JAMES B. MCCLINTOCK, and DOMINIC TEDESCHI, Department of Biology, University of Alabama at Birmingham, Birmingham, Alabama 35294-1170
KENNETH H. DUNTON, Marine Science Institute, University of Texas at Austin, Port Aransas, Texas 78373
BILL J. BAKER, Department of Chemistry, Florida Institute of Technology, Melbourne, Florida 32901
The diverse assemblage of macroalgae that occurs along the Antarctic Peninsula frequently dominates shallow marine communities on hard substrates (e.g., Neushul 1965; Richardson 1979; Klöser et al. 1994). These plants form undersea forests and often cover over 80 percent of the bottom with standing biomass levels exceeding 8 kilograms per square meter. These levels of coverage are comparable to algal bio-mass levels in temperate kelp forests (cf. Amsler et al. 1995; Brouwer et al. 1995).
The majority of this standing biomass appears to enter the detrital food chains (references in Amsler et al. 1995; Amsler, McClintock, and Baker in press). Iken (1994, 1995) tested the ability of several potential herbivores to consume a number of antarctic macroalgae in aquarium studies and demonstrated that macroalgae can constitute a significant proportion of the gut contents of such animals. However, there is little evidence of herbivory in the field (references in Amsler et al. in press), and we are aware of no reports of substantial grazing on antarctic macroalgae. Macroalgae have been reported only rarely and, primarily, in small amounts in the guts of potential herbivores in the field (references in Amsler et al. in press). Therefore, even though macroalgae represent a large potential food source that some animals are capable of eating, few appear to do so. This raises the question, why is so little of this potential resource consumed?
One way in which macroalgae defend themselves against herbivory is by chemical means such as production and sequestration of metabolites that deter feeding (reviewed by Hay and Fenical 1992; Paul 1992; Hay 1996). Recent research has revealed that chemical defenses are not uncommon in antarctic invertebrates (reviewed by McClintock and Baker 1997). We have recently shown that thallus disks of the antarctic red macroalgae Iridaea cordata and Phyllophora antarctica were rejected by the sea urchin Sterechinus neumayeri in a phagostimulation assay (Amsler et al. in press). This finding is consistent with previous observations that although the urchins use these macroalgae for cover, they do not consume them in significant quantities. Nonpolar and polar extracts of both macroalgal species were also strongly rejected by the animals (Amsler et al. in press), indicating that the unpalatability of the intact plants is due, at least in part, to defensive chemistry. This definitive evidence of chemical defenses against herbivores in antarctic macroalgae is, to our knowledge, the first, but we believe that chemical defenses against herbivory are likely to be much more common in antarctic macroalgae than have been recognized to date.
Brown macroalgae similar to those that dominate communities along the Antarctic Peninsula are often chemically defended against herbivory via sequestration of phlorotannins (polyphenolics) (reviewed by Steinberg 1992). Physodes that may contain phlorotannins are produced by antarctic members of the Phaeophyceae (Anderson 1985; Moe and Silva 1989), and Iken (1994, 1995) has detected phlorotannins and suggested that they may be involved in deterring grazing, but we are aware of no published reports that provide quantitative data on the concentrations of these defensive compounds in antarctic macroalgae. The goal of this article is to provide preliminary, quantitative estimations of phlorotannin levels in three large and ecologically important antarctic brown macroalgae.
Individual plants of the brown macroalgae Desmarestia menziesii, Himantothallus grandifolius, and Cystosphaera jacquinotii were collected near Palmer Station on Anvers Island, Antarctica (64°S 64°W). Triplicate samples of each plant were analyzed for phlorotannins following the method of Arnold, Tanner, and Hatch (1995). C. jacquinotii was subdivided into reproductive receptacles and into individual vegetative components (blade, air bladders, and lower stipe for analysis). Levels detected in vegetative tissues of D. menziesii and H. grandifolius and in the reproductive tissues (receptacles) of C. jacquinotii (figure) were very high relative to levels known to deter feeding by herbivores (cf. Steinberg 1992). Conversely, vegetative tissues of C. jacquinotii contained levels of phlorotannins that were very low and that would be unlikely to prevent herbivory.
These results indicate that antarctic brown macroalgae produce phlorotannins at concentrations that would provide defense against a wide variety of herbivores in lower latitude communities (cf. Steinberg 1992). We postulate that the phlorotannins are likely to play this same role in antarctic brown macroalgae. Our observation that C. jacquinotii differentially allocates phlorotannins to its stalked (and, therefore, very exposed) reproductive structures (figure) is consistent with the "optimal defense theory" of plant chemical defense (Rhoades 1979). This species appears to invest in defenses of its vulnerable and, presumably, high energy content reproductive structures even though it does not seem to defend its vegetative tissues.
We are grateful to J. Heine for collections and to A. Boettger for assistance with German translations. This work was supported by National Science Foundation grants OPP 95-30735 to James B. McClintock, OPP 95-26610 to Bill J. Baker, and OPP 94-21765 to Kenneth H. Dunton. Dominic Tedeschi was a participant in the National Science Foundation's "Teachers Experiencing the Antarctic/Arctic" program.
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