The American Horseshoe Crab. Carl N. Shuster Jr., Robert B. Barlow, and H. Jane Brockmann, eds. Harvard University Press, Cambridge, MA, 2004. 427 pp., illus. $95.00 (ISBN 0674011597 cloth).
Each spring, unnumbered thousands of American horseshoe crabs (Limulus polyphemus) approach the edge of the sea and emerge from quiet estuaries to lay eggs on intertidal beaches on the continent's Atlantic and gulf coasts, thus perpetuating an extraordinarily ancient life history. The American horseshoe crab, a “living fossil” that ranges from Maine to the Yucatán, is one of four extant representatives of the xiphosurans (horseshoe crabs and their extinct relatives), whose basic body plan has persisted for some 400 million years. The continued success of this marine species is remarkable in light of its risky intertidal sojourn, which makes it highly visible and vulnerable to terrestrial predators. On spawning beaches, shorebirds consume horseshoe crab eggs to fuel their tenuous migration, and watermen following a traditional way of life collect horseshoe crabs for bait. Fittingly, those competing pressures are where The American Horseshoe Crab begins and ends. In between, readers of this multiauthored book, edited by Carl N. Shuster Jr. (adjunct professor, Virginia Institute of Marine Science), Robert B. Barlow (director of vision research, State University of New York at Syracuse), and H. Jane Brockmann (professor of zoology, University of Florida), will find chapters on a wide range of topics by prominent researchers who share an abiding interest in this remarkable creature.
The authors' stated goals were to address “highlighted topics” and answer commonly asked questions on this most researched of all marine arthropods. The authors also set out to make the book accessible to a wide audience. The book's topical coverage is extensive, but the degree to which topics are accessible to a nontechnical audience is inconsistent. The language and information in different chapters range from the elementary to the technical. Glossaries are attached to the more technical chapters, and exhibits provide helpful background material. In contrast, the previous major edited volume on horseshoe crabs (Sekiguchi 1988) was organized under major headings, covered all four extant species of horseshoe crabs, and was written for a technical audience. Although The American Horseshoe Crab has a somewhat random organization typical of edited volumes, it has many accessible entry points, depending on the reader's interest and background.
The American Horseshoe Crab opens with a chapter by Mark Botton and Brian Harrington on the horseshoe crab–shorebird relationship. The chapter is devoted largely to migrant shorebirds and their brief stay in the Delaware Bay, where high densities of spawning horseshoe crabs cause initially buried eggs to surface and become available to the foraging birds. Increases in horseshoe crab harvest, coupled with declines in the spawning biomass of horseshoe crabs and in the abundance of shorebirds (the red knot [Calidris canutus] in particular), have prompted the development of a coastwide management plan, which is the topic of the book's last chapter.
In chapter 6, Botton, Shuster, and John A. Keinath continue the discussion of food webs and introduce the topic of epibionts. If you spend time watching horseshoe crabs on the beach, you'll notice a diverse assemblage of sessile organisms (epibionts) that use them as substrate. Allee (1922) summed it up by referring to the adult horseshoe crabs as a “walking museum.”
The two chapters by Brockmann on reproductive ecology and behavior hit the mark by targeting a broad audience exceptionally well. In chapters 2 and 3, Brockmann outlines for the reader in clear, straightforward prose what is known about horseshoe crab behavior and instructs the reader on the science behind the knowledge. She is careful to address the question, “How do scientists know that?” as she provides a narrative on horseshoe crab nesting behavior. For example, as Brockmann describes how females move through the sand to bury multiple clusters of eggs, she hints about how easy it is to track the progress of a nesting female using surveyor's flags. She makes it sound so easy that any reader can envision him- or herself participating in the scientific discovery.
No book on the horseshoe crab would be complete without a thorough discussion of limulid evolution. Shuster and Lyall I. Anderson coauthor two chapters that interweave evolution and natural history. In chapter 7, they use an analysis of the body plan to discuss how Limulus interacts with the physical environment and how it compares with other horseshoe crabs, both living and extinct. The horseshoe crab's persistence has been attributed to its generalist nature. However, Shuster and Anderson infer in chapter 8 that, with few exceptions, the distribution of horseshoe crabs has been tied to estuaries and associated spawning habitats through geologic time.
The book highlights the special contributions that studies of horseshoe crabs have made to human health. In chapter 4, Barlow and Maureen Powers engagingly summarize more than 70 years of vision research that has used Limulus as a model. Researchers have explored the horseshoe crab's remarkable ability to increase retinal sensitivity at night, described the role of vision in horseshoe crab mating behavior, and discovered mechanisms by which the brain interprets visual information. Work on horseshoe crab vision by H. Keffer Hartline, who shared the Nobel Prize for vision research with Ragnar Granit and George Wald, has led to understanding of basic mechanisms of vision common to all species.
Undoubtedly the best-known link between human health and horseshoe crabs is Limulus amebocyte lysate (LAL). LAL is the substance from horseshoe crab blood cells that causes clotting when exposed to bacterial endotoxin. The LAL-based bacterial endotoxin test (or gel-clot test) is the gold standard for detection of bacterial contamination in injectable drugs and implantable medical devices. In chapter 13, Jack Levin, H. Donald Hochstein, and Thomas J. Novitsky organize their thorough review of LAL development into three parts: discovery, development of standards and regulations, and commercialization. Chapters on the circulatory system (chapter 11 by Shuster) and the immune system (chapter 12 by Peter B. Armstrong) provide important background material for the LAL story. From the late 1970s to the early 1990s, research on LAL became a focal point for horseshoe crab researchers worldwide, contributing broadly to horseshoe crab biology. The chapter ends with a note that the LAL gel-clot test might someday be replaced by a genetically engineered version or other biochemical alternative.
Basic horseshoe crab biology is covered in several chapters on development and growth (chapter 5 by Shuster and Koichi Sekiguchi), physiology (chapter 9 by David W. Towle and Raymond P. Henry), and disease (chapter 10 by Louis Leibovitz and Gregory A. Lewbart). Overall, these chapters, along with the chapters on circulatory and immune systems, were more technical in nature and will serve as valuable references. The chapter on disease includes previously unpublished discoveries based on Leibovitz's research and veterinary practice at the Marine Biological Laboratory.
Sprinkled throughout the book are personal recollections on what stimulated researchers' interest in horseshoe crabs. In a couple of cases, accomplished scientists found their research subjects by literally happening across them. Shuster and Sekiguchi, who are recognized as fathers of modern horseshoe crab biology, share brief biographical sketches describing career paths that led them to lifelong studies of horseshoe crabs.
The book closes with a chapter on horseshoe crab conservation by Shuster, Botton, and Robert E. Loveland. Before 1999, conservation and management were inconsistent and highly localized. Concern over the possible effects on shorebirds of dramatic increases in the harvest of horsehoe crabs as bait for whelks (Busycon carica and Busycotypus canaliculatum), together with the loss of spawning habitat, has prompted fisheries managers to formulate a coastwide management plan (Atlantic States Marine Fisheries Commission 1998). Since the book's publication, harvest in the Delaware Bay region has been reduced to 25 percent of the peak harvest that occurred during the late 1990s. The authors appropriately acknowledge watermen's efforts to conserve horseshoe crabs by adopting bait-saving practices. One important conservation measure carries the name of the senior author. The Carl N. Shuster, Jr. Horseshoe Crab Reserve was established in 2000 to protect horseshoe crabs on the continental shelf off the mouth of the Delaware Bay. Hopefully, the implementation of the coastwide management plan and efforts to reverse the decline of shorebirds will be successful, and a reader 20 years from today will find it odd that a book on horseshoe crabs begins with a chapter on shorebirds.
The breadth of the topics covered and the thoroughness of the material presented in The American Horseshoe Crab ensure that the book's value, like its subject, will persist. In my opinion, the authors have achieved what they set out to do. A wide audience will turn to The American Horseshoe Crab to answer common and not-so-common questions about a truly remarkable creature.