Biology and Ecology of Anguillid Eels, edited by Takaomi Arai

Biological and ecological studies of the eel genus Anguilla have fascinated scientists around the world for more than a century. After Tesch (1977) published his book on eel biology that focused on two Atlantic eel species, most publications were based on international symposia, including Eel Biology (Aida et al. 2003) and Eels at the Edge: Science, Status, and Conservation Concerns (Casselman & Cairns 2009). Since 2003 no scientific textbook on eels has been published, and consequently Biology and Ecology of Anguillid Eels, based on the compilation of new scientific research and covering the biology (taxonomy, phylogeny and evolution) and ecology (spawning, migration and feeding) of the eels in its 15 chapters, is a welcome and long overdue update.

Chapter 1 describes the taxonomy and distribution of the eel genus Anguilla. After the first systematic description of freshwater eels by Ege (1939), who identified 16 species distributed worldwide including northern temperate, tropical and southern temperate eels, this chapter highlights that – although morphological characteristics of freshwater eels are important as conventional identification tools – these characteristics are not necessarily useful in identifying eel species and molecular genetic analysis has been proposed as a useful method to assist in the taxonomy of the genus Anguilla.

Chapter 2 is an overview of current trends in studies on the evolution and phylogeny of the genus Anguilla. Beginning with the work of Johnson et al. (2012) on Protanguilla palau Ida & Sakaue, 2012, a living fossil from Palau Island, one of the most primitive and independent lineages of true eels with its evolutionary history dating back to the Triassic 251–199.6 million years (Mya) ago, it highlights that the origin of Anguilliformes should be about 200–250 Mya. Several studies relate to the dispersal scenario of the freshwater eel based on phylogenetic approaches including the Tethys Sea Route, and multiple radiation events plus the Central American Isthmus route. New theories come from the phylogenetic studies on multidirectional dispersion and multiple radiation events plus the central American Isthmus route hypothesis.

Chapter 3 discusses the evolutionary genomics of North Atlantic eels. Teamwork by European scientists encompasses several studies relating to population genetics, the detection of selection footprints, speciation and demographic history, hybridization, spatio-temporal modelling, environmental transcriptomics, and eel genomes in the Atlantic eels. The authors argue that these studies provide a rich source of data to better characterize gene expression variation in natural or experimental populations, besides providing more power to discover and identify new genes involved in important pathways that might help our fundamental understanding of eels, and ultimately result in better measures for the management and conservation of the species. This idea and its science and technology approaches will be interesting when applied to the tropical eels.

Chapters 4, 5 and 6, all by the same author (Takaomi Arai), discuss the spawning grounds and the larval segregation of Atlantic, Japanese and Tropical eels beginning with historical scientific oceanographic expeditions to discover the spawning grounds of Atlantic eels and the Japanese eel. The book indicates important findings on several oceanographic factors that may play an important role as clues about the segregation and spawning grounds of the Atlantic and Pacific eels.

In Chapter 7 the authors present information on the early life history and recruitment of freshwater eels, including external morphological characters of glass eels, recruitment time and the association with the recruitment environmental area, age at recruitment and recruitment season, recruitment behaviour, glass eel invasion and abundance, and upstream migration. Worldwide trends and concerns about eel stocks are discussed in this chapter, together with the conservation and management of the species after the decline of both Atlantic and North Pacific populations. The authors emphasize the importance of conservation and management of the tropical eels by not over-exploiting or over-importing glass eels, especially as there is currently little information on their basic life history and their geographically limited distributions.

Adaptation to varying salinities based on physiological approaches is the theme of Chapter 10. It highlights that the eel can survive in habitats with varying salinity because of the specific mechanisms of osmoregulation involving ion and water movements across specialized epithelia at the gill, kidney, oesophagus and intestinal level. Chapter 11 concerns migration, gamete biology and the spawning of eels. The author highlights eel migration including an historical perspective on the first finding of leptocephali, spawning ecology, history of both Atlantic and Japanese eels as already described in Chapters 4, 5 and 6, and their finally reaching the spawning site by seamount landmarks and both temperature and salinity fronts. Furthermore, physiological preadaptation for migration is also discussed in this chapter, as well as coordination and timing of the silvering transformation, energetics and navigation.

Ecological aspects are discussed in Chapters 8, 9 and 12. The theme of Chapter 8 is upstream migration of juvenile eels and habitat use, the seasons of upstream migration, size and ages, duration of migration season, swimming and climbing ability, behaviour and environmental migration triggers, diversity and shifts in habitat use, segregation and distribution, ontogenetic habitat shifts, other factors influencing habitat choice, and seasonal shifts in habitat. The author also discusses management and fish passes, barriers to migration, transfer and stocking. It is concluded that the upstream migration in Anguilla eels is a very important process in migration ecology. Any disturbance to this process may influence the successful recruitment of juvenile eels, their growth habitat and even the downstream migration period of the silver eels.

In Chapter 9 feeding ecology of the freshwater eels is discussed based on developmental life stages including leptocephali and elver, but not for glass eels. The different habitats, feeding and temperature effects, feeding habit and body size of the yellow eel and its interaction with other species is also mentioned. The authors support the conclusion that Particulate Organic Matter (POM) or ‘marine snow’ in the form of faecal pellets and other food sources from larvacean houses or zooplankton play an important role in leptocephalus feeding. The authors conclude that during metamorphosis the leptocephali do not ingest food and the newly formed glass-eels will at first eat items similar to the leptocephali but upon reaching estuaries or coastal waters switch to benthic feeding. However, the glass eel period lasts for several months before they return to the estuary as benthic feeders and become fully pigmented benthic glass eels.

The remaining chapters deal with conservation and management strategies for the eel. In Chapter 12 contaminants in eels and their role in the collapse of the eel stocks is discussed, including bioaccumulation, current pollution levels in the North Atlantic eels, effects of pollutants, lethal and sublethal effects, evolutionary and genotoxic effects, and evidence from ecotoxicogenomic and transcriptomic studies. Furthermore, it is also suggested that habitat restoration and protection and the potential effects of climate change be integrated into the management planning of anguillid eel resources. Chapter 13 relates to the American eel (Anguilla rostrata) stock status in Canada and the USA and includes species ecology and geographic distribution, trends in commercial landings and abundance indices, habitat requirements and availability, threats and limiting factors, population modelling, threat mitigation and recovery targets. Chapter 14 reviews the management and fisheries of Australasian eels, including the importance of eels, current status, species ecology and geographic distribution, trends in commercial landings and abundance indices, habitat modifications and threats, population modelling, stock enhancements, and future prospects of the Australasian eels. In the case of the New Zealand eel, the concerns are mainly associated with habitat protection, provision of up- and downstream access in rivers, and obtaining a better understanding of its status. Fisheries, stock decline and conservation of anguillid eels, including the basic science of evolution and geographic distribution, plasticity of habitat use, to the fisheries and fishing gear including glass eel, yellow eel and silver eel fisheries, are considered in Chapter 15. Aquaculture is also discussed, together with conservation topics in terms of the population decline of temperate eels, possible causes of population decline, eels on the IUCN Red List, and stock assessment models.

It is important to note the comment from the author of the last chapter that the tropical eels in Indonesia and the Philippines are now targeted as the next candidates for aquaculture to compensate for the shortage of Japanese glass eels in East Asia. Therefore it is imperative, particularly in Indonesia, the Philippines, Malaysia and Vietnam, to carry out comprehensive research into the biology and ecology of tropical eels.

Within its 340 pages, this book written by 20 eel biologists and ecologists from the Atlantic and Pacific regions is powerful enough to be used as a scientific textbook. The editor of this book has intelligently compiled all the chapters, written by recognized authors from around the world. This book contributes much new information, new science and new strategies on ways to conserve and to manage the remarkable species of Anguilla eels in the world. Some of the colour figures in the book are of poor quality and size, but overall this is an outstanding book and recommended as an essential read by scientists and students alike.