Olive shells: the genus and the species problem

On first seeing this book my initial thought was to wonder whether another book on Olive shells was really necessary. It did not take long using the book to convince me that not only was a new book on the subject justified, but that this work was by far the most comprehensive revision of the genus to date.

The authors have spent some twenty years studying Olives in depth, including extensive field work, examination of all available type material, examination of some 30,000 specimens and over 100,000 measurements in 3,000 specimens, together with aquarium observations and experimentation - I found the method of measurement of radula growth and the shell colour change resulting from transferring animal from a dark substrate to a light substrate particularly interesting. The extent of this background research may be gauged by the fact that the authors have published some 36 papers on Olives during this period of study. This thorough preparation ensures that this book is both comprehensive and authoritative.

Olive shells are highly variable in colouration and it has long been a vexed question among students of the group exactly what constitutes a valid species. The authors set out to resolve this 'species problem' and look at different methods of classification before their more detailed study of the group. This preliminary section of the work could be usefully studied by anyone interested in Molluscan taxonomy - irrespective of whether that interest extends to Olives or is limited to any other group.

Following the section on the species problem the authors look in detail at, among other things, the anatomy of Olives, the structure of the radula, reproduction, feeding methods and preferences, ecology, collecting methods - including a simple shell trap which could easily be adopted by other collectors, shell structure and micro structure and shell colour and patterns - particularly looking at the polymorphisms which result from the cryptic colouration of both shell and animal depending on the colour of the substrate. This colour variation has led to much confusion through great intraspecific variation and interspecific convergencce where different species living on the same substrates develop similar colouring that the authors term a 'local colour palette'.

At the core of the book each species recognised as valid is discussed over two or three pages which includes a detailed description of the species emphasising those characteristics most important for identification, comments on the animal, habitat preference and distribution. Each species account incorporates a distribution map and an illustration of the species annotated to point out important characteristics. Similar species are grouped together for ease of comparison, and there are some 15 tables to help differentiate the species in each group, although no taxonomic significance is placed on these groups.

Nomenclature receives detailed coverage in the section 'Ie tombeau des Olives' - confusingly, if poetically, titled in French when the text is in English. Here for each taxon the authors give full-page references to the original description, location of type specimens, an indication of whether the taxon is considered a valid species, subspecies or colour form or should be considered a synonym, nomen nudem or nomen dubium. For valid taxa a list of synonyms is given, and helpfully the authors also indicate how each taxon has been treated by previous works on Olive shells making it easier to reconcile earlier studies with the present work.

The text is written in a relaxed, almost conversational style making what could have been a dry study easy to follow. The book is profusely illustrated throughout with diagrams and photographs, while 29 black and white plates illustrate some 296 type specimens, almost all in both dorsal and ventral view, which on their own make this an important taxonomic work. In addition some 48-colour plates illustrate some 625 specimens, again nearly always in both dorsal and ventral view, together with a number of living Olives. This illustration of so many multiple specimens showing variation within each species is extremely helpful for identification. A detailed twenty page bibliography is included and there is both a subject and a systematic index.

At £80, this book may not find its way on to every conchologist's shelf, although it will be absolutely essential to anyone specialising in the genus, but even for the non-specialist it would be well worth borrowing a library copy. It can be highly recommended.