Latitudinal gradients of seaweed nutritional content and palatability to generalist marine herbivores
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Over the past century there has been a rise in global ocean temperatures as a result of increased anthropogenic activity. The increase in ocean temperatures has already begun to facilitate the movement of tropical marine herbivores and seaweeds into temperate systems with consequences to ecosystem dynamics, function, and overall productivity. In an effort to gain insight into the seaweed-herbivore interaction over a latitudinal gradient, we assayed the palatability of 50 seaweeds (31 Rhodophyta and 19 Phaeophyta) collected from polar (Antarctica; 66ºS), temperate (California; 33 and 38ºN), and tropical (Fiji; 18ºS) locations to generalist crab and urchin herbivores from the Atlantic Ocean that were naïve to these seaweeds. Overall, seaweeds from polar and temperate locations were more palatable than seaweeds from the tropical location. This latitudinal increase in apparent palatability is largely explained by the response of urchins to Rhodophytes. In contrast, the palatability of Phaeophytes did not significantly differ across latitude and crabs showed no latitudinal preference in Rhodophyta or Phaeophyta seaweed. Some of the latitudinal increase in palatability was attributable to latitudinal increases in nutritional content, as measured by percent carbon, nitrogen, and ash-free dry mass. In contrast, protein and polyphenolic content (a putative chemical defense) did not significantly vary with either latitudinal origin or palatability. After accounting for the positive effect of organic content on palatability, a residual analysis indicated that tropical red seaweeds were significantly less palatable than temperate and polar reds than was expected. This suggests that there is an undescribed component of tropical red seaweeds that lowers their palatability to generalist herbivores. To our knowledge, this represents the first study to assess palatability of any group of primary producers across a broad latitudinal gradient. It also suggests that the diffuse coevolution of seaweed-herbivore interactions may be intensified within tropical habitats. We predict that ongoing movement of marine herbivores and seaweeds into temperate habitats may increase both the relative success of herbivores with higher tolerance for chemical defenses and seaweeds with lower palatability.