Stachowicz John J, Hay Mark E
Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell St., 28557, Morehead City, NC, USA.
Oecologia. 1996 Feb;105(3):377-387. doi: 10.1007/BF00328741.
Because encrusting coralline algae rely on herbivory or low light levels to prevent being overgrown by competitively superior fleshy algae, corallines are relatively rare in shallow areas with low rates of herbivory. In contrast to this general trend, the branching coralline alga Neogoniolithon strictum occurs primarily in shallow seagrass beds and along the margins of shallow reef flats where herbivory on macrophytes is low. This alga apparently persists in these habitats by providing refuge to the herbivorous crab Mithrax sculptus at mean densities of 1 crab per 75 g of algal wet mass. When crabs were removed from some host corallines, hosts without crabs supported 9 times the epiphytic growth of hosts with crabs after only 30 days. Crabs without access to a coralline alga were rapidly consumed by reef fishes, while most of those tethered near a host alga survived. These results suggest that the crabs clean their algal host of fouling seaweeds and associate with the host to minimize predation. However, to effectively clean the host, the crab must consume the wide array of macroalgae that commonly co-occur with coralline algae in these habitats, including chemically defended species in the genera Halimeda, Dictyota, and Laurencia. Crabs did readily consume these seaweeds, which were avoided by, and are chemically defended from, herbivorous fishes. Even though crabs readily consumed both Halimeda and Dictyota in whole-plant feeding assays, chemical extracts from these species significantly reduced crab feeding, suggesting that factors other than secondary chemistry (e.g., food value, protein, energy content), may determine whole-plant palatability. Having the ability to use a wide variety of foods, and choosing the most profitable rather than the least defended foods, would diminish foraging time, increase site fidelity, and allow the crab to function mutualistically with the host alga. Despite the obvious benefit of associating with N. strictum, M. sculptus did not prefer it over other habitats offering a structurally similar refuge, suggesting that these crabs are not N. strictum specialists, but rather occupy multiple habitats that provide protection from predators. Structurally complex organisms like N. strictum may commonly suppress competitors by harboring protective symbionts like M. sculptus. It is possible that diffuse coevolution has occurred between these two groups; however, this seems unlikely because both herbivore and host appear to respond most strongly to selective pressures from predators and competitors outside this association.
由于结壳珊瑚藻依靠食草作用或低光照水平来防止被竞争优势更强的肉质藻类过度生长，因此在食草率较低的浅水区，珊瑚藻相对较少。与这一普遍趋势相反，分支状珊瑚藻Neogoniolithon strictum主要出现在浅海草床以及浅礁坪边缘，这些地方大型植物上的食草作用较弱。这种藻类显然通过为食草性螃蟹Mithrax sculptus提供庇护而在这些栖息地中得以存续，平均密度为每75克藻类湿重有1只螃蟹。当从一些宿主珊瑚藻上移除螃蟹后，仅30天后，没有螃蟹的宿主上附生植物的生长量是有螃蟹宿主的9倍。无法接触到珊瑚藻的螃蟹会迅速被礁栖鱼类吃掉，而大多数拴在宿主藻类附近的螃蟹存活了下来。这些结果表明，螃蟹会清理其藻类宿主上的污损海藻，并与宿主共生以尽量减少被捕食。然而，为了有效清理宿主，螃蟹必须吃掉这些栖息地中通常与珊瑚藻共生的各种大型藻类，包括Halimeda属、Dictyota属和Laurencia属中具有化学防御能力的物种。螃蟹确实很容易吃掉这些海藻，而食草性鱼类会避开这些海藻，并且这些海藻具有化学防御能力。尽管在全株喂养试验中螃蟹很容易吃掉Halimeda和Dictyota，但这些物种的化学提取物显著减少了螃蟹的摄食，这表明除了次生化学物质（如食物价值、蛋白质、能量含量）之外的其他因素，可能决定了全株的适口性。有能力利用多种食物，并选择最有利可图而非防御性最弱的食物，将减少觅食时间，提高栖息地忠诚度，并使螃蟹能够与宿主藻类互利共生。尽管与Neogoniolithon strictum共生有明显的好处，但Mithrax sculptus并不比提供结构相似庇护所的其他栖息地更偏好它，这表明这些螃蟹不是Neogoniolithon strictum的专性物种，而是占据多个能提供免受捕食者侵害保护的栖息地。像Neogoniolithon strictum这样结构复杂的生物可能通常通过容纳像Mithrax sculptus这样的保护性共生体来抑制竞争者。这两个群体之间可能发生了扩散式协同进化；然而，这似乎不太可能，因为食草动物和宿主似乎对这种共生关系之外的捕食者和竞争者的选择压力反应最为强烈。
