Sea Snakes

SEA SNAKES. Harold Heatwole. 1999. 2d ed. Krieger Publishing Co., Malabar, Florida. ISBN 1-57524-116-1. vi + 148 p., 49 figs., 9 tables. $29.50 (softcover).—Sea snakes are of great interest to biologists and to the public at large because of their unusual adaptations for a marine life and because of their toxic venom. Unfortunately for science, sea snakes and herpetologists are generally not sympatric, and even when one is in an area where sea snakes occur, they are rarely easily located or studied. As a result, there have been relatively few studies of sea snakes by a rather small number of scientists. These studies tend to be highly specialized treatments of some particular aspect of sea snake biology. Summaries of the widely scattered literature on marine/estuarine snakes are rare, especially by an author with a wide breadth of professional and personal knowledge. Thus the publication of this second edition of Heatwole's book is a very welcome occasion because he is certainly the one scientist with the greatest experience with this group worldwide. The book is the most up-to-date treatise on marine snakes and provides an unequalled entry to the literature. However it does have some significant limitations as a guide to the field for the serious scholar, which reflect the author's lack of experience with North American natricines and his choice of areas to emphasize. These problem areas do not detract significantly from the overall excellence of the book, and herpetologists owe Heatwole a debt of gratitude for providing a major revision of his first edition in this Australian book series.

The book is divided into 10 chapters, followed by an appendix containing a worldwide species list and a suggested reading list by chapter topic. The chapter topics and the number of pages devoted to them are as follows: What are sea snakes? (8 pages); Distribution and biodiversity (17 pages); Natural history (16 pages); Food and feeding (7 pages); Enemies (4 pages); Population and community ecology (9 pages); Adaptation to life in salt water (6 pages); Diving adaptations (23 pages); Venom (13 pages); and Sea snakes and humans (22 pages). The question arises regarding how an author should decide the amount of coverage to devote to a particular topic. I suggest that it should be a compromise between the intrinsic importance of the topic and the amount of cogent literature available. Of course, it is tempting or perhaps inevitable that authors will tend to emphasize topics within their own specialty or even studies that they themselves have conducted. This would be acceptable if the author in question had contributed a significant proportion of the literature in the field, which is certainly true of Heatwole in aspects of diving physiology and field ecology of Australian species but not so in other realms such as salt and water balance. For example on pages 67–68 and Figure 7.1, we are given a peculiar story about how Heatwole was working on the salt excretion of the file snake Acrochordus granulatus in 1968 and was “scooped” by my work on other marine snakes. Strangely enough Heatwole and Taylor did not publish these findings until 1987 (note that the figure legend is incorrect), and they deserve hardly a footnote today, not a featured place in this book. Indeed the entire story of how sea snakes and other marine/estuarine snakes maintain water and salt balance is a major feature of their evolutionary pathway into saline waters (the intrinsic importance argument). There is a considerable and sophisticated literature available, yet this book devotes only six pages to the topic (compared to 23 pages on diving). In addition, the limited treatment in this book ignores important concepts such as “incidental drinking” (Dunson and Mazzotti, 1989), which are critical to an understanding of issues such as the huge variation in salt gland size, and thus secretion rate, among fully marine species of sea snakes. For example on page 52, Heatwole points out that egg-eating snakes “accidentally ingest considerable amounts of sand along with their meal of eggs”; the same also is true of incidental ingestion of sea water, which is greater for egg eaters than for those snakes that ingest single large fish meals.

The only similar major lapse in adequate coverage of an important topic that I detected deals with the coastal natricine snakes found in eastern North America. On page 8, Heatwole notes that he has taken the taxonomy for this group from an extremely ancient 1957 reference. On page 9, he reports that none of the saltwater forms is taxonomically distinct. This fallacy is repeated on pages 15, 31, and 133. There also is a failure to recognize that one form is found in mangroves, not just in salt marshes. Of course, many if not most herpetologists will be aware that the Gulf and Atlantic coast natricines of the Nerodia fasciata lineage were placed in a separate species, Nerodia clarkii, by Lawson et al. (1991), based on allozyme differences.

A less important issue deals with the very limited knowledge of sensory perception in sea snakes. Heatwole may be unaware of an obscure, but very interesting, paper by Hibbard and Lavergne (1972), which reported on morphological grounds that the vision of the most widespread sea snake, Pelamis platurus, probably is quite poor. I suspect that this species, which feeds while floating passively at the surface with rapid sideways sweeps of the open mouth, may have a movement detector type of eye similar to that of frogs.

I found the section (p. 10–12) on giant mythical sea serpents out of place in a scientific book and not a very insightful approach to the topic. For example, no mention is made of the most famous such creature (the Loch Ness monster) and the actual research relative to its food requirements and the amazing fact that it has been given a scientific name.

In conclusion, I repeat my earlier statement that I find this book to be an extremely useful addition to the herpetological literature. In spite of its deficiencies, it is by far the most up-to-date and accurate single summary of the entire literature on sea snakes. I would definitely recommend it to colleagues and students as the place to begin their reading on sea snakes.