Life History & Behaviour

Despite their name, hermit crabs do not live solitary lives. Population densities are often high as hermit crabs do not compete for food and do not defend a territory (Hazlett 1996). Encounters with conspecifics are therefore common. Aggressive behaviour is displayed in shell exchanges, when personal space is invaded and between males when attempting to secure a mate (Hazlett 1996, ). Injury is very rare in any of these situations however, instead hermit crabs communicate via various signals, sometimes in long bouts, before the lesser retreats into their shell or moves away (Hazlett 1996).

Hermit crab populations are often shell limited and several aspects of a shell, including relative size, internal volume, shape, species and density and type of epibiont coverings influence the quality of a shell as a shelter. Shell choice impacts upon a hermit crab’s survival and reproductive fitness. A stronger or more camouflaged shell presumably affords greater protection from certain predators. Conversely, heavier shells must reduce a hermit crab’s physical capabilities; most obviously speed and ability to self-right after being upturned. For females, the internal volume of the shell limits the number of eggs that can be carried (Terrosi et al. 2010). So a large, and therefore more fecund female, inhabiting a smaller than optimal shell, is not attaining its maximal reproductive potential (Terrosi et al. 2010). Shell quality also impacts upon male reproductive fitness and poor shell choice can prevent even large males from achieving copulations (Hazlett and Barron 1989)

Dardanus megistos is a very large species and as an individual grows the variety and availability of suitable shells decreases. Different size classes of D. megistos exploit different species of gastropod shell. Because the shell is so valuable and demand greater than supply, aggressive encounters between individuals for shells are not uncommon (Hazlett 1996). In these encounters the aggressor approaches, rotates the defender’s shell so the two apertures are roughly squared up and then begins rocking the defender and rapping the shell with its chelipeds (Hazlett 1996). The defender usually retreats into its shell as the aggressor approaches, but eventually is forced out by these rocking and rapping motions (Hazlett 1996). The defender fully emerges from its shell, often scuttling over the back of the aggressor, who then proceeds to make the shell change (Hazlett 1996). The defender is in a more vulnerable position and as soon as the aggressor changes shell it completes the exchange and the two go their separate ways (Hazlett 1996).

If the defender is comparable or larger in size it may defend its shell using signals. Raising the chelipeds and walking legs or extending the chelipeds clearly communicates size to an opponent in the hope of intimidation (Hazlett 1996). In this scenario the aggressor will often retreat, though perhaps after a long bout of persuasive signalling (Hazlett 1996).

Similar encounters occur between males guarding a female for the right to mate, although here the guarding male may need to defeat multiple competitors over as long as several days before the female signals that she is ready. After copulation the female broods the fertilised eggs against her abdomen, held in place by the pleopods. The hatching larvae are called zoea and become part of the zooplankton. After several zoeal molts, the larvae metamorphose into the juvenile stage called the megalopa, which unlike the adult possesses a symmetrical abdomen but must also find a shell to shelter in. The abdomen becomes coiled upon molting into an adult.

Photo: Storm Martin, Heron Island, 2012