Trepezia flavopunctata. The largest species of guard crab.
Photo: Seabird McKeon
A stunning example of mutualism - the sort of cooperation which 'selfish gene' theory predicts - was published recently by Smithsonian.com - the online science magazine of the Smithsonian Insitute.
It shows how a complex ecosystem like a coral reef has evolved a defensive system of guard crabs which receive shelter in crevices in the coral and nutrients in return for vigorously defending the living coral polyps from attack by predatory molluscs and starfish. Its not just the presence of the crabs themselves which is important but the presence of several different types, each with a particular role.
While the relationship between coral and crab has been known for a while, researchers have now found that it takes more than just one rank in the guard-crab army to keep a location secure. The discovery highlights the importance of biodiversity not just across an array of animal types, but also within a group of seemingly similar species.
More than 20 coral guard-crab species exist, and they come in many shapes and sizes. Often more than one crab species will occupy a single coral, and the diversity of pincers and claws suggests that the crabs each have unique defensive strategies.
The article is based on findings by Seabird McKeon and Jenna Moore, of the National Museum of Natural History’s Smithsonian Marine Station, published in PeerJ:
Coral guard-crabs in the genus Trapezia are well-documented defenders of their pocilloporid coral hosts against coral predators such as the Crown-of-Thorns seastar (Acanthaster planci complex). The objectives of this study were to examine the protective services of six species of Trapezia against corallivory, and the extent of functional diversity among these Trapezia species.
Studies conducted in Mo’orea, French Polynesia showed the Trapezia—coral mutualism protected the host corals from multiple predators through functional diversity in the assemblage of crab symbionts. Species differed in their defensive efficacy, but species within similar size classes shared similar abilities. Smaller-size Trapezia species, which were previously thought to be ineffective guards, play important defensive roles against small corallivores.
We also measured the benefits of this mutualism to corals in the midst of an Acanthaster outbreak that reduced the live coral cover on the fore reef to less than 4%. The mutualism may positively affect the reef coral demography and potential for recovery during adverse predation events through shelter of multiple species of small corals near the host coral. Our results show that while functional diversity is supported within the genus, some Trapezia species may be functionally equivalent within the same size class, decreasing the threat of gaps in coral protection caused by absence or replacement of any single Trapezia species.
- These results suggest ecological complementarity, as well as a hierarchy of defensive effectiveness among different species and sizes of Trapezia. Small crabs effectively defend their hosts against the small predator Drupella, but fail against larger predators such as Culcita: small Pocillopora were always consumed entirely by Culcita, regardless of the presence or absence of small Trapezia. The crabs flee the coral, or are consumed along with the host.
- Medium sized crabs are effective against Culcita, with effectiveness varying among species, but they do not defend against Drupella or Acanthaster. The largest crabs actively defend against Acanthaster, but their efficacy against the other coral predators remains untested. Thus, barring the untested possibility of negative interactions, a coral harboring all of these symbionts may be defended against all three corallivores, while a coral with a lesser complement of Trapezia species may remain vulnerable to some predators. An additional aspect of this within guild complementarity, or ‘species stacking’, is that several species may create additional synergistic defensive effects even against the same coral predator (McKeon et al., 2012).
So, with nothing more than natural selection selecting for whichever variants give most offspring in that environment, nature, with no plan, no sense of right and wrong and no magic at all, has evolved corals with niches in which different types of crabs can live, and which, because it produces more offspring, vigorously defend their environment (the coral) from predators, and apparently altruistically because the corallivores are not after them as such and the crabs are capable of escaping anyway.
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