Predators

While their gastropod shell affords them some protection, hermit crabs are not without predators. Smaller intertidal species are preyed upon by birds, though these are unlikely to be important predators for larger, deeper water species like D. megistos. The most common predators of D. megistos are likely to be octopus, mantis shrimps (stomatopods) and a variety of sharks and rays (elasmobranchs). Amongst the bony fishes, the puffers (Tetraodontidae) are important coral reef predators of hermit crabs and other hard bodied invertebrates (Waikiki aquarium 2009). On Heron Island, the most common of these fish is perhaps the Valentinni’s sharpnose puffer (Canthigaster valentini), a small species incapable of preying on any but the smallest D. megistos. Other, larger tetraodontids found on Heron and across the Great Barrier Reef include the dog-faced puffer (Arothron nigropunctatus) and the stars and stripes puffer (Arothron hispidus).

Dog-faced puffer (Arothron nigropunctatus), a likely predator of D. megistos. Photo: Storm Martin, Fiji, 2011	Stars and stripes puffer (Arothron hispidus). Puffers have strong crushing teeth, preying on hard invertebrates. Photo: Storm Martin, Ningaloo Reef, WA, 2012

Stomatopods, or mantis shrimps, are colourful, intelligent and charismatic predators whose hunting behaviour is well studied. Those species with club-like appendages specialise in smashing gastropod shells and the exoskeletons of crabs and other crustaceans (Caldwell and Dingle 1976). Though some attain sizes conceivably capable of taking larger D. megistos, the species of stomatopod present at Heron Island are smaller. Here, stomatopods certainly remain important predators of smaller D. megistos as well as other medium sized hermit crabs like D. lagopodes and Calcinus gaimardii.

This colourful mantis shrimp is an intelligent and powerful predator of hard bodied invertebrates such as gastropods and crabs, smashing through defences with a club like appendage. Photo: Yolana Kailichova, Heron Island, 2012

Octopuses are also highly intelligent predators; their ability to solve problems such as removing a stubborn hermit crab from its shell is well documented. Octopus, though cryptic and elusive, are not uncommon at Heron Island and almost certainly are amongst the most important predators of D. megistos and D. lagopodes. Octopuses may extend a tentacle into the aperture of a hermit crab shell and will pull strongly for hours or even days to extract their prey (Ross 1971). An observational experiment using two Mediterranean Dardanus species found that octopus quickly learn to avoid hermit crabs that housed stinging epibiotic anemones, whereas those without this symbiotic protection were always successfully extracted from their shell and consumed (Ross 1971). It is unclear as to whether D. megistos houses any anemones or other cnidarians that afford such protection from octopus, no anemones were apparent from any of the samples collected from Heron Island as part of this project.

Unlike the large well known sharks with ‘razor sharp’ teeth designed for shearing the soft flesh of fish and marine mammals, many species of elasmobranch have a strong upper jaw firmly attached to the skull and hard plate-like dentition (Michael 1993). This arrangement is well adapted to crushing the exoskeleton of crustaceans and the shell of molluscs and is most notably common amongst the rays (Michael 1993). In the shallows of the sandy inner reef zone, stingrays move across the fine sediment, searching for buried invertebrates with electroreception (Michael 1993). At Heron Island the most common stingrays which can be readily observed in the shallows include the large pink whiprays (Himantura fai), bluespotted-ribbontail rays (Taeniura lymma) and shovelnose rays (Rhinobatos typus and Rhynchobatis djiddensis). These species will certainly take hermit crabs when available (Michael 1993). However, at Heron Island, D. megistos was only found from the reef crest and flat, never from the sandy inner lagoon (see Habitat) and so while an important predator of other hermit crabs, these rays will rarely encounter D. megistos in this zone. Larger individuals of D. megistos are reportedly more commonly observed at greater depths, the lower reef slope. Here they may potentially fall prey to these rays but records are lacking, though H. fai can be found hunting to at least 20 metres (Michael 1993).

Stingrays like this bluespotted-ribbontail ray (Taeniura lymma) common around most of the Great Barrier Reef, are important predators of hermit crabs. Photo: Storm Martin, South Mole Island Whitsundays group GBR, 2011

Eagle rays (Myliobatidae), including the white-spotted eagle ray (Aetobatus narinari) which frequents Heron Island, also specialise in crushing mollusc shells and in doing so often ingest hermit crabs (Michael 1993, Schluessel et al.2010). The mollusc (or hermit crab) is ingested whole, masticated, ejected and then the soft parts are reconsumed (Michael 1993). In this way diogenid hermit crabs have been recorded to comprise 20% of the eagle ray’s diet from the Great Barrier Reef (Schluessel et al. 2010). Tiger sharks (Galeocerdo cuvier) have also been known to eat large molluscs whole, particularly conch, and so sometimes take hermit crabs (Michael 1993). These large sharks are generalists (Michael 1993) and are unlikely to feed upon hermit crabs in significant numbers. However, the hermit crabs taken by tiger sharks are likely to be the larger, deeper dwelling individuals, those at less risk from the smaller but more typical predators. In this way tiger sharks potentially play a significant regulatory role on D. megistos populations, though the importance of such a role is completely unstudied.

Several species of shark are also invertebrate specialists. The medium sized zebra shark (Stegostoma varium), which can be seen while diving at Heron Island, rests on the bottom during the day and hunts mainly molluscs and crabs by night (Michael 1993). It is therefore likely to be an important predator of D. megistos. Similar to the zebra shark in size and also hunting by night, the tawny nurse shark (Nebrius ferrugineus), uses rapid pharyngeal pumping to suck molluscs and hermit crabs from their shell (Michael 1993).

The catsharks (Scyliorhinidae) are a large group of small bottom-dwellers of both reefs and deeper waters and also feed largely on invertebrates (Michael 1993). Some species have been documented to have developed a specialised strategy for dispatching hermit crabs, flipping the gastropod shell over before quickly snatching at the retreating crab and tearing it from its shell with thrashing movements of the body (Michael 1993). While likely to be predators of D. megistos across their South-East Asian range (see Distribution), catsharks are not well represented on the Great Barrier Reef (Michael 1993). Here they can perhaps be considered to be ecologically replaced by the epaulette shark (Hemiscyllium ocellatum), another small, sluggish bottom dweller which is abundant at Heron Island (Heupel and Bennet 2007) and can easily be come across on the reef flat at low tide. Epaulette sharks are flexible and eel-like, forcing their bodies into crevices or burrowing their heads into the sediment to reach prey (Michael 1993, Heupel and Bennet 1998 (1)). A diet analysis study conducted at the Heron Island research station did not find hermit crabs in the guts of epaulette sharks, though other decapods were important diet components (Heupel and Bennet 1998 (1)). However a parasitological study, also conducted at Heron Island, recorded a particular species of nematode worm from epaulette sharks whose larvae was only found in diogenid hermit crabs, a certain brachyuran crab and pistol shrimp (Lester and Sewell 1989). The larvae of these parasites are transmitted to the final host when it consumes the infected larval host (Heupel and Bennet 1998 (2)). Importantly, the adult parasite was only found in epaulette sharks (Lester and Sewell 1989), providing some evidence that these sharks may prey upon hermit crabs (Heupel and Bennet 1998 (2)).

Finally, it is worth noting that the most common sharks observed at Heron Island; black-tip reef sharks (Carcharhinus melanopterus), white-tip reef sharks (Triaenodon obesus) and sicklefin lemon sharks (Negaprion acutidens), all hunt mostly soft bodied prey such as fish and cephalopods and are unlikely to prey on D. megistos.

Several sharks are likely to prey substantially upon Dardanus megistos. Though not confirmed, evidence from research conducted at Heron Island suggests the epaulette shark (Hemiscyllium ocellatum) to be one such species, a common sight around the reef flat of the island on the incoming tide. Photo: Storm Martin, Heron Island, 2012