Conservation & Threats

Dardanus megistos, as with most hermit crabs, suffers few direct human impacts. Most species are too small to be considered food, and even those reaching larger sizes such as D. megistos are not consumed anywhere in significant numbers. Some species are commonly sold in the aquarium trade and being large and colourful D. megistos make attractive additions to a tank. The collection of animals for the aquarium trade from the wild can potentially threaten local populations. This is particularly so in regions of poor governance. The impact of such collection upon D. megistos is not well understood, neither at local or global scales, as is true for many marine invertebrates.

The dependence of hermit crabs on their gastropod hosts is a key consideration for interpreting their evolutionary history, ecological roles, physiology, morphology, behaviour, reproduction and life history, population structure and so too their conservation and threats. D. megistos is one of the largest hermit crabs, certainly the largest occurring on Heron Island. Consequently, they are greatly constrained in the species of gastropod in which they can shelter at larger sizes. Gastropods of this size, are however, usually rare. Such species intuitively take a longer time to reach a size suitable for use by a large D. megistos, making them more vulnerable to human impact, both direct and indirect. Compounded with this, the large size of these gastropods make them prized amongst shell collectors, even someone with little interest in shells is likely to carry home a large baler shell if they come across it on a morning walk. D. megistos supposedly reaches sexual maturity at much more modest sizes than those individuals encountered in the largest shells. The loss or reduction in the abundance of these larger gastropods may therefore be hypothesised to be of little consequence to D. megistos populations, but to what extent do these larger individuals contribute to the next generation, given their disproportionately high fecundity?

A further, perhaps more substantial and insidious threat, with the potential to seriously impact not just D. megistos but numerous other calcifying organisms including other crustaceans, the gastropods upon which hermit crabs rely and the corals providing the structure supporting the entire reef community, is ocean acidification. Ocean acidification is a topic of considerable concern; increasing levels of carbon dioxide released into the atmosphere result in increasing acidity when dissolved into the oceans. This in turn drives the calcium balance away from the crystal forms used by calcifying animals. This threat is global and cannot be directly managed by establishing marine parks or managing coastal developments and harvesting quotas. Instead, protection against acidification requires more large scale changes to industrial practices and our lifestyle.

D. megistos is widespread and common throughout its range. While in certain localities entire reef systems may be under threat due to overexploitation, pollution, coastal development, destructive harvesting practices or tourism, this species in particular is not likely to be under threat when considered across the entirety of its distribution. Stronger governance and the establishment and management of marine parks are the most substantial means of protecting not only D. megistos but countless species from these general, community level threats. An important step towards achieving such protection is the awareness and appreciation of coral reef species, not just the largest and most charismatic animals but also an understanding of the incredible underlying diversity and its collective importance to our existence.

Photo: Storm Martin, Heron Island 2012