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Bilaterality is an important evolutionary principle, but hermit crabs seem to have abandoned the concept, at least partially. Hermit crabs are asymmetric with a curved, soft abdomen that fits nicely into the empty shells they inhabit. This asymmetry extends even to the neuromuscular system, although the first‐stage free‐swimming larvae, known as zoea, are symmetrical. Hermit crabs cannot excrete their own shells, so adults usually seek the abandoned shells of gastropods to inhabit. Although the asymmetric soft abdominal parts need protection, this muscular system has evolved to act as a hydrostatic support for each new shell. The abdomen has become decalcified to fit a range of shells, and the abdominal musculature can support and “grasp” the shell in its spirals. As the availability of inhabitable shells may be limited, a housing crisis often leads to pugilism among crabs seeking to acquire a new shell. Fights are fierce and may lead to the death of a crab over a new shell. Once a new shell has been selected, the crab will rather quickly exchange its old home for a new one with ritualised behaviour specific to the species. With worldwide distribution, hermit crabs are scavengers that live almost anywhere including on land, the littoral zone and shallow or deep ocean. Some species are even tool users after a fashion. They will adorn their shells with anemones to inhibit predators as anemones have stinging tentacles and will poison an octopus or other predator. Hermit crabs will even move the anemone to a new shell once they change shells.
Hermit crabs can be traced to the Jurassic (206–144 million years ago) and probably evolved from a lobster‐like ancestor, although the phylogeny is poorly understood. Crabs, in general, have exhibited an evolutionary medley of change that must have been imperceptible until after it occurred, but there are definitely different themes in the diverse order, Decapoda. The evolutionary change has included radiation into Brachyura or true crabs, and Anomura, the hermit crabs and others.
The infraorder Anomura has a considerable variety of compound eyes in the group. Terrestrial hermit crabs typically have apposition eyes, large species of marine hermits have refracting superposition eyes, smaller species have parabolic superposition, and the squat lobsters have reflecting superposition. It is as if Anomura is an evolutionary workshop for compound eyes.
Some hermit crabs possess an interesting compound eye that has the most complicated physiological optics on the planet. Externally, the eyes appear to have hexagonal ommatidia (individual ocular elements), much like insects, but quite in contrast to the reflecting superposition eyes and square ommatidia of shrimp (BJO cover essay, November 2006). Internally this hermit crab eye is quite different, and is known as a parabolic superposition eye. Each ommatidium has a corneal lens, which is shed with each molt. This corneal lens refracts incoming light rays bringing them into focus close to the proximal tip of the crystalline cone, a second optical element, in the ommatidium. The crystalline cone will manage incoming rays differently depending on the incident angle. Axial rays traverse the cone with little refraction and enter a light guide, which crosses the clear space (analogous to vitreous cavity) in the eye, and travel directly to the rhabdom or retina. Oblique incident rays first strike the corneal lens, are refracted and then strike the side of the crystalline cone, which has a parabolic profile and a reflective coating. After striking the mirrored surface, these rays are recollimated and emerge as a parallel bundle. These rays will cross the clear zone between the proximal tips of the crystalline cones of many ommatidia. This means that these rays will cross the light guide of these adjacent ommatidia without being deflected. The index of refraction of the light guides permits just that and brings the rays to focus on the retina (Nilsson, Nature 1988;332:76–8). These optics produce a real, erect image on the retina just like other forms of superposition eyes (BJO, November 2006 and September 2003). This eye is unusual because it uses two lenses and mirrors to create a single image.
The anomurans, and especially the hermit crabs, illustrate the evolutionary potential of the compound eye and its plasticity. The first compound eyes probably evolved in the early Cambrian and were of the apposition type. This basic form of a compound eye is rather inefficient. Evolutionary modification into superposition eyes would have improved the vision in dim conditions. But apposition and superposition eyes are optically very different, and it is hard to imagine how one can gradually evolve into the other. It is here the parabolic superposition eyes provide the link. The parabolic type makes less of an improvement compared with the refracting and reflecting superposition eyes. It also appears that this eye has evolved several times in different taxa of unrelated crustaceans and even at least once in insects. Interestingly, the parabolic superposition eye can act as a missing link in the evolution from apposition to refracting or reflecting superposition—a process, which appears to have occurred in parallel in several different groups of crustaceans.
In hermit crabs, this evolutionary transition is not something that happened just in ancient evolutionary history. The present mixture of apposition and different superposition types in these asymmetric crustaceans suggests that it is happening right before our eyes. Despite this intriguing status, hermit crabs are still strangers in their own homes.