Nervous and Sensory Systems

The central nervous system of crustaceans differs from that of vertebrates. Instead of the single control point brain, crustaceans have a small brain located behind the eyes and then a series of ganglia, large nerve bundles, one fused pair per segment, which act as mini-brains and so control is distributed across the system (Ruppert et al. 2004). The ganglia are connected by a pair of ventral nerve chords (Umbach and Lang 1981).  The brain is termed the supraesophageal ganglion as it is preoral, that is, before the oesophagus (Ruppert et al. 2004). The brain is composed of three parts, originating as the three anterior most pairs of ganglia, those of the acron and two cephalic segments (Ruppert et al. (2004). Anterior to posterior these are termed the protocerebrum, deutocerebrum and tritocerebrum, being associated with the eyes, antennules and antennae respectively (Stephens, 1985, Ruppert et al. 2004). The tritocerebrum does not appear entirely preoral, in fact it wraps around the oesophagus, a structure known as the tritocerebral commissure (Ruppert et al. 2004). The ganglia of the next six body segments, the three posterior most of the cephalon and the three anterior most of the thorax, are also all fused together in a large structure called the subesophageal ganglion, which is associated with the external mouthparts; the mandibles, maxillules, maxillae and the three pairs of maxillipeds (Stephens 1985). Following this, the next three thoracic segments, associated with the three most anterior pairs of pereopods, are fused together into the thoracic ganglion (Stephens 1985). Nervous control of the reduced forth and fifth pereopods of hermit crabs along with the absent first pleopod are bundled into the thoracic-abdominal ganglion (Stephens 1985). The remaining paired ganglia of the abdomen, associated with the pleopods and uropods, are asymmetrical, the right being reduced (Chapple and Hearney 1974). Many crustaceans possess giant motor neurons and giant interneurons which allow a very rapid escape response (Umbach and Lang 1981). This is most commonly observed in shrimp, which flick their abdomens to rapidly propel themselves backwards when threatened. In hermit crabs this escape response has been adapted and is responsible for the rapid retreat into the shell (Umbach and Lang 1981), making them almost impossible to be caught out of their shell.

Central nervous system of a generalised hermit crab. The ancestral arthropod is hypothesised to have a pair of ganglia per body segment. Here, each pair has fused together and further, many pairs have fused with those of other body segments. The two central nerve chords run ventrally. Illustration by Storm Martin 2012, with reference to Stephens 1985, Ruppert et al. 2004.

Most arthropods, including hermit crabs, have compound eyes. Compound eyes are composed of many, hundreds to thousands, of singular individual eye units called ommatidia, each of which faces a slightly different angle because of convex shape of the eye. Compound eyes are divided into superposition or apposition eyes, the latter of which typifies hermit crabs (Nilsson 1990). However, both Dardanus megistos and D. lagopodes have recently been demonstrated to possess superposition eyes (Nilsson 1990). Superposition eyes form a single erect image and have the advantage of producing a brighter superimposed image, while apposition eyes produce multiple inverted images (Nilsson 1990). There are three forms of superposition eye, those of Dardanus posses the refracting form (Nilsson 1990). Superposition eyes are largely considered superior to apposition eyes as they take in more light and so have greater vision in darker situations (Nilsson 1990). The evolution of superposition eyes in Dardanus has been hypothesised to stem from nocturnal and crepuscular activity (Nilsson 1990).

The statocyst at the base of the antennule allows orientation within a gravitational field, much like our own inner ear labyrinth. Illustration by Storm Martin 2012, adapted with modification from Ruppert et al. 2004

Though hermit crabs have good vision which is important for avoiding predators, they rely on strong chemosense (smell/taste) and mechanosense (touch) to find food. The setae covering the appendages and much of the body, giving Dardanus megistos its hairy appearance, are the sensors providing this information. The antennae and the aesthetascs of the antennules are also important sensors. In addition to chemosensory and mechanosensory setae, other setae of the body respond to vibration and these are also important for predator avoidance (Ruppert et al. 2004). As well as holding the chemosensory aesthetascs, the base of the antennules carries a statocyst each. This structure is analogous to the inner ear of vertebrates and allow orientation with gravity (Ruppert et al. 2004). The statocyst contains a heavy particle called the statolith, which is surrounded by mechanosensory setae (Ruppert et al. 2004). These setae detect the displacement of the statolith within the statocyst cavity (Ruppert et al. 2004). Orientation within the gravitational field is important for locomotion and navigation of the environment, for crustaceans as with ourselves.