Eyes and classification of malacostracan crustaceans

Abstract

During the past 100 yr various methods of classifying mysids, krill, prawns, shrimps, crabs and lobsters have been proposed, including the use of behaviour, gill type and the relationship of the carapace with the head and the thoracic segments. At least two factors have been ignored in all these schemes—larval characters and optical mechanisms. A survey of malacostracan crustaceans reveals three types of optical mechanism. The simplest uses apposition optics, often with hexagonal facets, and is found in all decapod larvae and most adult brachyuran crabs. Refracting, superposition eyes with hexagonal facets occur in mysids and euphausids, and reflecting, superposition eyes with square facets using mirror optics are found in adult prawns, shrimps, macrurans such as lobsters, and dromiid ‘crabs’. Artificial relationships have been imposed on malacostracan crustaceans by a failure to incorporate eye structure in schemes of classification. Once evolved, eye structure is a stable character and provides evidence for the continued grouping together of penaeid, caridean and stenopid prawns and shrimps; it demonstrates their affinity with lobsters and other Macrura, and confirms the unity of a re-defined Brachyura and the weakness of the Anomura as a coherent taxon.