Elucidation of species interactions involving multiple processes is an important task in ecology, as different processes may operate in non-apparent, opposing directions. While herbivory is a major form of interspecific interaction affecting plants, calcareous algae are generally thought to be protected from herbivory due to CaCO3 deposition. However, field observations and experiments in temperate-subtropical intertidal rockpools showed that hermit crabs (Paguroidea, Decapoda) did not ingest Corallina but cut short its fronds. We tested the implications of this frond-cutting behaviour with field experiments where hermit crab density and Corallina frond length were artificially manipulated.
Hermit crab guts were fuller and contained more microscopic animals when Corallina fronds were shorter, suggesting that frond-cutting affects accessibility to food resources that exist in the basal part of algal mat. Corallina showed compensatory growth when fronds were artificially reduced in length/density, simulating the cutting by hermit crabs. Frond density increased, probably due to increased exposure to light of the basal part of Corallina coupled with a reduction in apical dominance.
The present study revealed a direct, non-trophic relationship between hermit crabs and Corallina in which the former affected the latter for gaining access to food resources. Overall, the combination of positive and negative effects of hermit crab led to a positive, facilitative effect on Corallina. In terms of the effect on algae or plants, this relationship is functionally analogous to herbivory, without involving direct consumption. We discuss the problem of extending the concept of ecological engineering to herbivory and herbivory-like phenomena and suggest the term ‘pseudo-herbivory’ to describe relations in which animals have non-trophic but herbivory-like effects on plants/algae, as seen in the hermit crab–Corallina relationship.