Rays of the World

Peter R. Last, William T. White, Marcelo R. de Carvalho, Bernard Séret, Matthias F.W. Stehmann, Gavin J.P. Naylor, Peter R. Last, William T. White, Marcelo R. de Carvalho, Bernard Séret, Matthias F.W. Stehmann, Gavin J.P. Naylor, Peter Last, Gavin Naylor, William White, Matthias Stehmann, Marcelo de Carvalho

• Publisher: CSIRO PUBLISHING
• Year: 2016
• Print ISBN: 9780643109131
• eBook ISBN: 9780643109155
• Topic: Biological Sciences
• Subtopic: Marine Biology
• Index: Biological Sciences

Rays are among the largest fishes and evolved from shark-like ancestors nearly 200 million years ago. They share with sharks many life history traits: all species are carnivores or scavengers; all reproduce by internal fertilisation; and all have similar morphological and anatomical characteristics, such as skeletons built of cartilage. Rays of the World is the first complete pictorial atlas of the world's ray fauna and includes information on many species only recently discovered by scientists while undertaking research for the book. It includes all 26 families and 633 valid named species of rays, but additional undescribed species exist for many groups.

Rays of the World features a unique collection of paintings of all living species by Australian natural history artist Lindsay Marshall, compiled as part of a multinational research initiative, the Chondrichthyan Tree of Life Project. Images sourced from around the planet were used by the artist to illustrate the fauna. This comprehensive overview of the world's ray fauna summarises information such as general identifying features and distributional information about these iconic, but surprisingly poorly known, fishes. It will enable readers to gain a better understanding of the rich diversity of rays and promote wider public interest in the group.

Rays of the World is an ideal reference for a wide range of readers, including conservationists, fishery managers, scientists, fishers, divers, students and book collectors.

1

INTRODUCTION

P.R. Last, M.R. de Carvalho, G.J.P. Naylor, B. Séret, M.F.W. Stehmann & W.T. White

SCOPE OF THIS BOOK

This book is the first complete pictorial atlas of the world’s ray fauna. It also provides general identifying features and distributional information about this iconic, but surprisingly poorly known, group of fishes. Our knowledge of many of the species is based on a small number of individuals, and few of them have been researched well enough to gain even a basic understanding of their biology and life history. A highly valuable collection of paintings of all living rays (as well as sharks) has been assembled as part of a multinational research initiative (Chondrichthyan Tree of Life Project) to gain a better understanding of the diversity and evolution of this fish group. Images sourced from around the planet have been used to illustrate all of the rays found in oceans and freshwaters of the world. In a strict sense, this book is an atlas rather than an identification guide. Its main aim is to document the world’s ray fauna and promote wider public interest in the group. New insights gained from molecular analyses of more than three-quarters of all living ray species, combined with reinvigorated morphological investigations, have led to many changes in both ray classification and the underpinning species diversity. The recognition of whole new families and genera of rays, and many newly described species, have resulted from this research.

In compiling this book, contributing authors gained an expanded appreciation of how little is known about members of this group. The published literature is riddled with incomplete or inaccurate information created by frequent misidentification of species and a limited research effort. In recent years, several regional guides to sharks and rays have attempted to redress this deficiency. Some 30 years ago a comprehensive guide to the world’s sharks was produced and upgraded global treatments have followed. However, a similar reference covering global ray diversity has been a long time in gestation. Unsurprisingly, research initiated for this book has raised more questions than answers. While the book constitutes a major milestone in the documentation of ray diversity, it should be viewed more as an overview rather than the definitive study of these fishes.

BIODIVERSITY OF RAYS

The iconic group of fishes known collectively as the ‘rays’ is familiar to most of us. Our association with rays, better known in scientific circles as ‘batoid fishes’, extends back to prehistoric times. But what really are the rays? The term ‘flat sharks’ has been applied affectionately to the group and this analogy is more appropriate than some might think! Rays are first cousins of sharks, having co-evolved from early Mesozoic or late Palaeozoic shark-like ancestors about 250 million years ago (mya). They also share many life history traits and a multitude of morphological and anatomical characters that define fishes as a whole, such as having a backbone, living in water, breathing with gills, mostly being cold blooded and skin often scaly, which distinguish them from the tetrapods (land-dwelling vertebrates).

Living fishes are comprised of two primary taxonomic groups. Rays, along with sharks and chimaeras (elephant fish, ghost sharks, rabbitfishes and spookfishes), collectively known as the ‘cartilaginous fishes’, form one of these two major groups of modern fishes, the class Chondrichthyes (having skeletons built of cartilage with superficial calcification rather than bone). Earliest members of the group possibly lived in the world’s seas during the late Silurian/early Devonian period more than 400 mya, or possibly even earlier in late Ordovician times according to evidence from their scales. Today, some 1250 or more species occupy a variety of aquatic ecosystems in all oceans, from adjacent continental and insular coastlines to the deep abyss, as well as inland in rivers and lakes. The other major extant group, the class Osteichthyes (Actinopterygii) or bony fishes, is much larger and makes up about 95% of modern fishes, including an impressive array of body forms, from eels to flounders, groupers and tunas.

Fig. 1.1. Aldrovandi’s 1640 monstrosity – the ‘ray-dragon’ was created from a carefully mutilated and dried skate. (BS)

Fig. 1.2. Fossilised stingray (Cyclobatis) from the Upper Cretaceous. (MC)

Rays are the largest subgroup of the chondrichthyan fishes and presently comprise 26 families and 633 valid named species. However, an additional 50 unrecognised or undescribed species are known to exist. In contrast, sharks consist of 34 families and ~516 valid species, and chimaeras 3 families and 51 species. Rays vary in dimension from ~25 cm or so to more than 6.5 m in total length or disc width. Rays differ from sharks and chimaeras in having gill slits located on the ventral (under) surface of the head. The body is mostly dorsoventrally flattened and usually modified into a disc formed by the complete (e.g. stingrays) or partial (e.g. guitarfishes) fusion of the pectoral fins with the head and trunk. Pectoral fins, which are often greatly enlarged, join the head forward of the gill slits. The disc may be circular, oval, triangular, heart-shaped or resembling a rhombus (rhombic). Five gill slits (six in one species) are located on the ventral surface of the body, with the eyes situated on the dorsal surface. All rays lack an anal fin, and the caudal and dorsal fin(s), which are variably developed across the group, are also sometimes absent. Scales are usually represented on the skin as fine denticles or, in many species, as heavy thorns and scutes; however, in some species, denticles are totally absent.

Most rays live in demersal habitats where the majority of species are either mainly benthic (living on the substrate) or benthopelagic (swimming close to bottom but not resting on the substrate). Their flattened body is perfectly adapted for life on the seafloor, either resting or lying concealed within the sediments. However, some members of the group have a powerful muscular disc with enlarged, angular pectoral fins for swimming actively in pelagic habitats well above the seafloor. These pelagic species are typically wide-ranging in the oceans, whereas demersal rays are usually more highly specialised, preferring specific habitat types and having narrower geographic ranges. Nevertheless, some demersal species are common at certain locations but remain poorly known because the habitats they occupy have been inadequately surveyed. Rarity of specimens in biological collections does not always equate to rarity in the wild. Nonetheless, our knowledge of most ray species is limited and characterising their habitat preferences remains a challenge.

Rays have successfully colonised a variety of niches over geological time, and the relative success of each group varies geographically. For example, marine ancestors of the Neotropical stingrays (Potamotrygonidae) colonised the freshwater habitats of tropical South America. Some skate genera occur only in particular ocean basins, often confined to shallow inshore habitats whereas others live on deep continental slopes down into the abyss. Ray faunas across the planet are the end products of long-term evolutionary processes influenced by plate tectonics, oceanographic factors and habitat availability. Their composition at regional scales differs significantly between geographic regions.

Our ability to define the structural elements and species compositions of regional faunas has been greatly assisted in recent years with the expanded use of DNA analysis. We are incrementally gaining a better understanding of regional linkages across ray groups and the associated biogeographical affinities of faunas. Underpinning these insights involves identifying and defining the basic currency of taxonomy and biodiversity – the species! The faunas of some regions are better known than others because both international research efforts and historically acquired knowledge of them vary greatly. For many ray groups and for some geographical regions, our knowledge is still in its infancy. The inadequacy of research specimens, images and baseline information for so many species was one of the greatest impediments during the compilation of this book.

Fig. 1.3. Numbfish (Narcinops tasmaniensis) resting on the seafloor. (PL)

Our technical knowledge of ray groups varies similarly. Some iconic groups, such as sawfishes (Pristidae) and devilrays (Mobulidae), have been well studied in some regions, but few ray groups have been the subject of taxonomic revision to gain insights on a global basis. Some rays are rarely caught and, because of their often large size, are subsequently hard to transport, and store for long periods in collections. Hence, the type specimens of many species described more than a century ago have been lost or adversely affected by long-term preservation. Old imagery of types is often poor, and most early taxonomic descriptions of rays are very brief, describing features of a particular ray family rather than of a species. Hence, most of these taxonomic descriptions are of limited value in resolving nomenclatural issues.

The first comprehensive faunal coverage of rays was prepared by Müller and Henle in 1841 (Systematische Beschreibung der Plagiostomen), in which family-level groups were established and 55 new ray species were described. Even then, ray species outnumbered those of sharks (115 rays out of 212 species in total). Later, Garman (1913) built on their work, and the work of Duméril (1865), in his Plagiostomia, a compendium of 507 species of sharks and rays (now with 30 more species of rays than sharks). Since then, regional ray assemblages have been described in various identification guides, but the world’s fauna had not been described in a single work for more than a century. The task of producing this guide was challenging in the absence of adequate material of many species in biological collections of the world. Authors needed to access all available data sources (existing literature, fresh and preserved material, photographs, etc.) to achieve this goal. Hence, the quality of species treatments provided in this book largely reflects the quality of information available for each species. The project identified glaring specimen and data gaps, and encountered other research impediments that need to be filled or overcome strategically in coming years.

ANATOMY AND SHAPE DIVERSITY

As a group, rays differ greatly in external features and more subtly in their internal anatomy and skeletal structure. Body shape in particular provides a strong indicator of their life history strategy and ecological role. The disc may be modified for life on the bottom or swimming in midwater. It can be rounded, strongly flattened, or soft and flexible enabling the ray to bury in silty substrates or move around freely inside caves and crevices. Pelagic rays are more active swimmers and usually have a firmer, more angular disc with heavier body musculature. Disc shape can also vary with developmental stages and between sexes in a species. Young skates usually have a more rounded disc that often becomes more angular with age. Adult male skates tend to have a more curvaceous disc with a longer snout than females. Such growth-related variations often make it difficult to characterise species precisely and have led to frequent misidentifications. Moreover, the extent of this variability has not been described for many species.

Fig. 1.4. Pectoral fins of most rays are joined to the head and trunk to form a flattened disc. (BS)

The shape and length of the tail also varies greatly in rays. In some groups, such as the giant guitarfishes (Glaucostegidae), the trunk and tail resemble sharks, whereas in stingrays (Dasyatidae) the tail can be very slender and whip-like. The attenuated tails of some whiprays, which can be several metres long, are among the longest of all fishes. In other ray families, such as the legskates (Anacanthobatidae), devilrays (Mobulidae) and butterfly rays (Gymnuridae), the tail is very short and filamentous. In these groups, the typical absence of dorsal and caudal fins results in the functional redundancy of the tail as a means of locomotion. Conversely, a strong muscular tail with large caudal and dorsal fins of some rays (e.g. shark rays) provide rapid propulsion and manoeuvrability. Tails vary from being extremely depressed to rounded in cross-sectional shape. Several groups have distinct skin folds on the mid-upper, mid-lower or lateral edges of the tail. The base length, height, shape and position of these soft structures are important in defining families, genera and species.

Fig. 1.5 Oronasal region showing the well-defined nasal curtain of the newly described maskray, Neotrygon australiae. (Carlie Devine)

The ray’s body is supported by a skeleton consisting mainly of cartilage that is covered (rather than substituted) by a thin layer of honeycomb-shaped or prism-like calcification, typical of chondrichthyan fishes in general. This calcification is usually most apparent where the skeleton needs to be strongest, such as where it endures more pressure or force (jaws, cranium, shoulder girdle, etc.). The cranium (skull) is a single structure without sutures that protects the anterior central nervous system and some sensory organs of the head; its shape varies significantly among ray groups. Some groups such as guitarfishes have an elongated, internal cartilaginous support for the snout (rostrum); the sawfishes take the jackpot with their long saw-like snout. The rostrum of other rays is almost completely reduced or absent (e.g. stingrays). In the cranium, nasal capsules surround the nasal organs and nostrils, orbits accommodate the eyes, and the otic capsules house the inner ears (labyrinth organs responsible for hearing and balance). The upper jaws of rays (palatoquadrates) do not articulate with the cranium, a feature exclusive to the group. The vertebral column is attached to the cranium and extends posteriorly all the way into the caudal fin when one is present; stingrays lack a caudal fin and have a long cartilaginous tube in place of vertebrae in a mostly filamentous or whip-like tail. Anterior vertebrae are fused into the tube-like synarcual cartilage, another unique feature of rays that provides support for the disc and shoulder girdle; stingrays have a second synarcual associated with vertebrae in the pelvic-fin region.

The pectoral and pelvic fins of rays are internally supported by slender, elongate cartilages (radials) associated with broader basal elements that articulate with the shoulder girdle (scapulocoracoid) and pelvic girdle (puboischiadic bar) respectively. The fin webs are also supported by ceratotrichia, slender fibre-like structures made of an elastic protein, unfortunately now more widely known as the core ingredient of shark-fin (or ray-fin) soup. In rays, the longitudinal elongation of the basal elements of the pectoral fin (directed toward the snout and tail) and radials (radiating outwardly from these basal elements) provide most of the internal support of the disc. The disc is also internally connected to the cranium by the antorbital cartilage, whose complete calcification is another unique feature of rays. The pelvic fins contain the claspers (male intromittent organs developed from pelvic-fin radials), which are instrumental in passing semen from male to female. Clasper skeletons may be simple (e.g. electric rays, stingrays) or very complex with many distinct overlapping elements (e.g. skates). Variation in regions of the skeleton of rays continues to be a major source of information concerning their systematics and evolutionary relationships.

Fig. 1.6. Large pelagic aggregation of cownose rays, Rhinoptera bonasus. (Sandra Critelli)

The head has undergone radical modification in rays. Its anterior profile can vary from short and broadly rounded, to long and pointed. A long angular snout is used by deepwater skates to bury and dig into soft sediments in search of prey. Skates living on soft muddy bottoms are less prone to damaging the delicate (often transparent) skin on their head and belly. Conversely, those living on rough, rocky bottoms often have thicker skin with a protective covering of granular denticles on their ventral surface.

Enlarged pectoral fins and a flattened disc enable rays to adopt several different methods of locomotion. They may use both the body and tail to ‘swim’ through the water like sharks, use only their pectoral fins, or use a combination of both methods. Those using the pectoral fin for propulsion either flap their fins to ‘fly’ (mantas), or undulate their fins (stingarees) producing a series of longitudinal waves. Frequency of fin movement largely depends on their size, and movements are usually more rapid when fins are shorter, and slower when the pectoral fins are disproportionately large. Some rays supplement tail swimming with short bursts of pectoral-fin undulation or flapping to accelerate and manoeuvre. Flexible rays (e.g. stingrays) are capable of spinning on a spot and their ability to change direction quickly comes from selective use of each pectoral fin. Tail swimmers often use the dorsal and caudal fins as rudders to make subtle shifts in direction.

In no other part of a ray, perhaps, has there been more radical modification than around the mouth and nostril (oronasal region), located on the ventral surface of almost all species. Groups have evolved many different feeding strategies for detecting and ingesting prey. Feeding requirem...

Table of contents

• Cover
• Title
• Copyright
• Foreword
• Acknowledgements
• Contents
• 1 Introduction
• 2 Phylogeny and classification of rays
• 3 Human interactions
• 4 Ray conservation
• 5 About this book
• 6 Glossary
• 7 Key to families of living rays
• 8 PRISTIDAE (Sawfishes)
• 9 RHINIDAE (Wedgefishes)
• 10 RHINOBATIDAE (Guitarfishes)
• 11 GLAUCOSTEGIDAE (Giant guitarfishes)
• 12 TRYGONORRHINIDAE (Banjo rays)
• 13 PLATYRHINIDAE (Fanrays)
• 14 ZANOBATIDAE (Panrays)
• 15 NARCINIDAE (Numbfishes)
• 16 NARKIDAE (Sleeper rays)
• 17 HYPNIDAE (Coffin rays)
• 18 TORPEDINIDAE (Torpedo rays)
• 19 RAJIDAE (Skates)
• 20 ARHYNCHOBATIDAE (Softnose skates)
• 21 GURGESIELLIDAE (Pygmy skates)
• 22 ANACANTHOBATIDAE (Legskates)
• 23 HEXATRYGONIDAE (Sixgill stingrays)
• 24 GYMNURIDAE (Butterfly rays)
• 25 DASYATIDAE (Stingrays)
• 26 POTAMOTRYGONIDAE (Neotropical stingrays)
• 27 UROTRYGONIDAE (Round rays)
• 28 PLESIOBATIDAE (Giant stingarees)
• 29 UROLOPHIDAE (Stingarees)
• 30 MYLIOBATIDAE (Eagle rays)
• 31 AETOBATIDAE (Pelagic eagle rays)
• 32 RHINOPTERIDAE (Cownose rays)
• 33 MOBULIDAE (Devilrays)
• Suggested reading
• Checklist of the world’s rays
• Scientific names index
• Common names index