Many years ago, my undergraduate thesis was on hermit crabs, and I’ve had a fondness for them ever since. Hermit crabs are the great survivors. They live in some of the harshest environments, pounded by surf, exposed to searing heat and desiccating winds, and somehow able to find sustenance on the most barren of beaches. Possibly the most common species seen around the shores of Phuket is the land hermit crab Coenobita rugosus.
How many species of hermit crab are there?
There are thought to be over 800 species of hermit crab. Fifty five different species of hermit crab have been recorded from Thailand’s shores and surrounding seas (McLaughlin, 2002). This can make indentification tricky. Hermit crabs fall in to three distinct families: the Dioginedae, the Paguridae and the Coenibitidae. The coenobitids (like this guy, pictured above) are the real land hermit crabs, often venturing some distance from the sea. So if you are beach-combing in the tropics, they are the ones you are most likely to see.
How do you identify Coenobita rugosa?
In the tropics (and I’m primarily writing this about Thailand), any hermit crab found high up on the shore, at or above the high tide mark, or inland of this, is likely to be a Coenibita species. However, to complicate matters there are actually three Coenibita species found on Phuket shores ((McLaughlin, 2002; Bundhitwongrut et al., 2014) all occurring in more or less the same habitat. Tricky! These are C. rugosus, C. violascens and C. brevimanus. The best way to identify Coenibita rugosus is by looking closely at the top of the outside edge of the end joint of its left claw. A series of small, linear tubercles can be seen on C. rugosus, but not the other two species. Incidentally, it is thought that these tubercles are involved in generating sound. Land hermit crabs will ‘chirp’ by stridulating, that is rubbing body parts together to produce a sound (as cicadas and grasshoppers do).
How do land hermit crabs breathe when out of the water?
A big problem for any animal primarily designed to live in the sea is how to breathe in air. The gills used by fish and most aquatic invertebrates to adsorb oxygen underwater don’t work very well on dry land. The gills of crabs are composed of many very thin plates (lamellae) each with a central shaft rather like a feather. Oxygen is absorbed and carbon dioxide expelled across the surface area of these plates. However, when out of water, these lamellae collapse directly on top of one another, greatly reducing the available surface area for gas exchange. Most coenobitids (such as C. rugosus) have adapted by having much smaller gills, they spend little time in the water, and with these gills somewhat adapted for breathing air. Around the gill is what is known as a branchiostegal lung. This is essentially gill tissue that is more adapted to absorb oxygen from air rather than water. Branchiostegal lungs are not unique to hermit crabs, other crabs that spend significant amounts of time out of the water, such as fiddler crabs, also possess them. Such are the wonders of evolution, each group has developed a different design of branchiostegal lung; there are many different ways to acheive the same end it seems. The coenobitid hermit crabs have also developed a second method for gas exchange. Protected inside their mollusc shell home, the upper surface of their thin-skinned abdomen has become highly vascularised, with gas exchange taking place directly through the skin. To further improve this the skin has become deeply wrinkled, increasing surface area. This has been termed an abdominal lung.
What do land hermit crabs eat?
What do they eat? Pretty much anything. They consume quite a lot of plant material, including fallen leaves such as acacia and beach cordia. They are also partial to a bit of carrion, crabs or fish washed up. Cannibalism too, is on the table if the opportunity presents. Smaller C. rugosus hermits need to be wary of getting too close to their bigger brothers.
The current evidence suggest that Coenobitid hermit crabs primarily use their sense of smell for food detection. The studies also suggest that their ability to detect smell was limited to water soluble molecules, and greatly enhanced in humid conditions. Here in Phuket a lack of humidity is rarely a problem!
Many hermit crabs also have pretty good eyesight. This is probably more related to defense and avoiding predators than feeding. Crabs have compound eyes; each eye is actually made up of a great number of hexagonal light sensors, each with their individual lens and cornea. These individual light sensors are known as ommatidia. Like fiddler crabs and ghost crabs, Coenobita rugosus and its relatives have eyes on stalks and eyes that are rather longer than they are wide. This vertical stretching of the eyes means that the angular change between each individual ommatidia is less in a vertical plane than in a horizontal, thus they have better optical resolution for objects, and changes in light, vertically than horizontally. This is probably an adaptation for predator detection; most predators likely to be bigger and casting a shadow or darker shape from above. If you look carefully at the eyes you will notice what appears to be a dark stripe near the centre of each eye. This is known as the pseudopupil. It represents the ommatidia that are more or less pointing straight towards you, and thus no light is being reflected back. As explained above, the angular change between ommatidia is less vertically than horizontally, so the pseudopupil has the shape of a vertical line. In humans, and other vertebrates, the pupil contracts or expands as light levels change, allowing more or less light into the eye. But crab eyes don’t have real pupils; this begs the question, how do they deal with changing light levels? The answer, in fiddler crabs at least, has been discovered only recently. When light travels through the lens of an ommatidiait is focussed on a column of dense, tiny finger-like projections (microvilli) containing photosensitive molecules. For crabs that are active both in bright sunlight and around dusk, these microvilli change size, growing or shrinking, and so dramatically altering the amount of light they capture.
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Farrelly, C. A., and Greenaway, P., 2005. The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (Coenobita and Birgus): gills, branchiostegal lungs and abdominal lungs. Arthropod structure and development, 34.
McLaughlin, P. A., 2002. A review of the hermit-crab (Decapoda: Anomura: Paguridae) fauna of Southern Thailand, with particular emphasis on the Andaman Sea, and descriptions of three new species. Phuket Marine Biological Center Special Publication 23(2): 385–460.
Bundhitwongrut, T., Thirakhupt, K. and Pradatsundarasar, A., 2014. Population ecology of the land hermit crab Coenobita rugosus (Anomura, Coenobitidae) at cape Panwa, Phuket Island, Andaman Coast of Thailand.