The Systematics and Evolution of Littorina

The Ray Society has a history back to 1844 of publishing fine works on natural history with a special emphasis on the British Isles. This latest production is by David Reid of the Natural History Museum and is the latest (but I hope not the last) result of his studies in the Littorinidae which began in the early 1980s. It deals with 19 recent and 4 fossil species which he places in 4 subgenera of Littorina - Liralittorina, Planilittorina, Littorina ss, and Neritrema - together with 6 fossil species whose attribution is doubtful. The contents cover Material and Methods, Morphology, Systematic Descriptions, Phylogeny, and Macroevolutionary History.

Littorina as defined in this publication is a fascinating genus, some members of which have received intensive study - and it still has secrets to unravel. Part of the reason for the interest taken in it must lie in Littorina species' accessibility as intertidal animals. But they have also proved to be very complex. There are different developmental strategies - planktotrophic and non-planktotrophic, the latter comprising the subgenus Neritrema and embracing oviparity and, in L. (N.) saxatilis only, ovoviviparity. Many species, especially in Neritrema develop ecophenotypes and geographical variants; some of the latter, this work strongly suggests, have their distribution controlled by crab predation or the lack of it. These features are overlaid by shell colour polymorphism and sometimes by the effects of sexual dimorphism and ontogeny. Populations within a species may show significant genetic separation which is sometimes observable within very short distances on the shore - again there are examples within L. (N.) saxatilis.

David Reid lucidly and painstakingly takes the reader through the maze of factors relevant to the study of Littorina and produces an account that makes sense, is comprehensive and critical in its overview of the mass of past work (up to 1996) on the genus, adds numerous new observations, and thoroughly revises the taxonomy and phylogeny of the genus, using new cladistic, biochemical, evolutionary and ecological analyses. At the same time he makes it clear what is not known and where further study is needed. I could not think of any questions that he had at the least not considered.

The story that emerges, particularly from the chapters on Phylogeny and Macroevolutionary history, but also from the Systematic Descriptions is of a genus which originated possibly in the Paleocene, certainly before the Oligocene, in, probably, the north Pacific. About 10 million years ago, at a time of climatic cooling, it appears to have adapted to exploiting cool water shores dominated by macrophytic algae which resulted in a burst of speciation leading to its current diversity and importance in inter-tidal biotas. L. (Littorina) and L. (Neritrema) arrived in the Atlantic as a result of the trans-Arctic interchange of mollusca in the Pliocene. There is a loose end here, namely if the genus arose in the Pacific, why is L. (Liralittorina) striata regarded as the least developed living species, distributed only among Atlantic islands? David Reid offers two possibilities on pages 384-5.

Production of this book is excellent; the type is clear and easy to read and one cannot fault the well-placed and numerous photographs, text figures and tables. After much effort I found one typographic error (for collectors of these it is on page 120). The only thing I did wonder about was the lack of any sort of subject index. It would be nice for instance to be able rapidly to find where illustrations of protoconchs are located or all the sections on development. But the book is very carefully structured and production of an index would have added to the cost, and to the time needed to produce it. It is quite clear that this is going to be a standard work for a long time to come. David Reid is to be congratulated for producing it and for so deservedly winning the Zoological Society of London's Gold medal for a scientist under 40.

The Systematics and Evolution of Littorina is throughly recommended for acquisition by anyone or any institution concerned not only with marine mollusca, but also with the broader fields of speciation and adaptation in animals as a whole.

Finally, may I claim a very small and indirect personal involvement in this book. Among the fossil Littorina examined were those in my brother's collection which were collected by both of us in the 1950s. Figure 37D shows a specimen of L. littorea from Stratton Hall Levington. I have a notebook dating from [1954] which has a list of fossils from that site which includes L. littorea