Biodiversity and systematics in cephalopods: unresolved problems require an integrated approach

Abstract

Some problems of cephalopod biodiversity are discussed. Many squid species are represented by 2–4 intraspecies groupings that may be wholly or partly sympatric, but differ in spawning season and size at maturity. They may be genetically distinct stock units, but their taxonomic status remains unresolved. Discovery of a biochemical or molecular key to distinguish between intra- and interspecific differences may help to solve the problem of subspecific taxa in cephalopods, as stated by G. L. Voss in 1977. Electrophoretic study of allozyme differentiation is a good method for clearing up relationships between taxa within a family, but this method cannot be used in situations when the concept of subgenus or subfamily is necessary. The problem of suprafamilial taxa needs urgent attention. However, restructuring only one family or group of families leaving others unrevised may lead to skewing the entire system. Examples are splitting the Enoploteuthidae into three families (as proposed by M. R. Clarke in 1988) and raising the rank of the Sepiidae and the Sepiolidae to ordinal, a proposal by P. Fioroni in 1981. In such cases the method of common level should be applied: subdivisions in a large taxon shall be separated by approximately similar characters. Many attempts to select natural groups of families, for example in the Oegopsida, failed primarily because they were based on analysis of a single organ or system of organs when study of other organs/systems may lead to different natural groupings. The use of molecular techniques in cephalopod phylogeny may be profitable, but initial attempts have led to results that are not easily interpretable. The evolution of Recent Cephalopoda has probably proceeded with such large variations in rates among different clades that it is impossible to construct a non-contradictory system based on any single organ or system. No single organ/system-of-organs nor single methodology currently exists that will solve every problem in taxonomy. An integrated approach, based on analysis of as many organs and different taxa as possible, is necessary to construct an accurate picture and not a mosaic of dispersed random pieces.