Many important insights in biology have been gained through the study of anomalous, or so-called ‘freak’, animals. An early example of the quest to reconcile such mutants is William Bateson's 1894 book Materials for the Study of Variation. In this imposing 600 page tome, Bateson argues that biological variation is not necessarily continuous, but can sometimes occur through sudden leaps. This argument was in contrast to Darwin's theory of gradualism, which posits that evolution acts through the incremental accumulation of minor random changes.
Bateson points out that small variations within development are more likely to have a drastic impact on adult traits, and thus large changes can occur even within a single generation. He includes hand-drawn illustrations and dense descriptions of many anatomical abnormalities, from a human female with a pair of ears on her neck to a trout with a ‘bull-dog head’. In addition to providing tattoo designs for future generations of hipsters, Bateson's extensive catalogue of eccentric creatures laid the foundation for the concept of the ‘hopeful monster’ – that very rarely, genetic mutations will produce freaks with a unique competitive advantage. The hypothetical image of a web-footed frog out-swimming its conventionally toed siblings provides an evocative, though perhaps improbable, example of Bateson's discontinuous evolution.
In the anecdotal tradition of William Bateson, a recent paper from Gerhard Scholtz and colleagues describes an anomalous crab with an immoderate number of eyes and antennae. Co-author Stephen Moore discovered the freak crab specimen, a member of the species Amarinus lacustris, in the Hoteo River of New Zealand's North Island. Amarinus lacustris are adorable little crabs, each about the size of a raisin, that live in freshwater and estuarine waters throughout New Zealand and in parts of Australia. This particular mutant crab possessed three eyes positioned in a horizontal row, a doubling of the carapace region in front of the eyes (the rostrum), and an aberrant antenna on top of its head.
The authors studied their unusual specimen through histology and 3D reconstruction of the crab's nervous system. They found that axons from the supernumerary eye successfully innervated the brain, suggesting that the third eye was functional. However, the extra antenna did not appear to connect with the brain, indicating that it was not operational. Finally, the neuroanatomical reconstruction revealed that the crab's brain was abnormally shaped, perhaps due to its juvenile state or possibly as a result of the same factors that produced its bizarre external morphology.
Anatomical malformations can be produced by environmental conditions that influence developmental processes, or through mutations in the genes that control development (or some combination of environmental and genetic factors). Scholtz and colleagues conjecture that the extra eye and antenna may have arisen through a duplication of the embryonic head, as occurs in conjoined twins. Crabs regenerate their eyes if they are damaged, and, under some circumstances, this regeneration can erroneously produce an antenna instead of an eye. Thus, the antenna may have originated from a regeneration event gone awry.
Was this three-eyed crab a hopeful monster? Because the crab was dissected before it could procreate in the lab, we will likely never know whether its unusual traits were heritable. But regardless of whether or not the three-eyed body plan had a promising future, such anomalous individuals continue to provide an important substrate for imaginative speculation about an animal's life history, development and evolution.