Red mangrove leaves improve with age like fine wine: The relationship between crab feeding, leaf age, and pre-existing herbivore damage

Background/Question/Methods

Field studies suggest that the mangrove tree crab (Aratus pisonii) feeds more on older versus younger leaves of the red mangrove (Rhizophora mangle). This pattern could be the result of crab damage accruing over time or be related to an active preference for older versus younger mangrove leaves, which may be associated with leaf chemistry or toughness. Thus, this study experimentally tested: 1) whether older mangrove leaves are fed upon more than younger ones by A. pisonii, and 2) why such preferences might exist. The study used: 1) multiple-choice feeding assays to assess feeding preferences, and 2) feeding assays with artificial food, where dried, ground leaves or chemical extracts were incorporated into agar-based food to assess the impact of chemistry. Leaf mass per area was used to estimate toughness of leaf material.   

Results/Conclusions

This study found that older leaves were preferred, however crude leaf extracts did not explain feeding preferences. Given that leaf age is confounded with the amount of pre-existing damage on leaves, experiments testing how damage influences feeding were conducted while controlling for age. They demonstrated that greater pre-existing damage resulted in greater crab feeding. Examination of leaf damage created during experiments revealed that new damage occurred on or adjacent to old damage for approximately 80% of all leaves examined. Damaged leaf material was less tough than fresh material. Artificial feeding assays, where crabs were offered undamaged, green versus damaged, brown leaf material, revealed that crabs were attracted to damaged leaf material. Thus, feeding preferences for older leaves are likely due to the presence of greater amounts of pre-existing damage, which is chemically and structurally attractive.