Sharks: The Animal Answer Guide

1 Introducing Sharks, Skates, Rays, and Chimaeras

What is this book about? This book is about chondrichthyan fishes (chondros + ichthys, literally “cartilaginous fishes”), a term that includes the subgroups of elasmobranchs (sharks, skates, and rays) and holocephalans (chimaeras), and is organized around common and not-so-common questions about these spectacular animals. As a shorthand device and to avoid continually saying “sharks, skates, rays, and chimaeras,” we refer to the overall group of chondrichthyans as “sharklike fishes,” at least where all subgroups share a trait. Where differences occur or unique traits deserve mention, we are more specific. To avoid confusion, we use the technical term “selachians” (or “true sharks”) when we mean sharks in the sense that most people think of them (the torpedo-shaped predators with prominent fins and big teeth that appear on the Discovery Channel). The term “batoids” refers to the more flattened skates, rays, and sawfishes (think of bat rays if that helps), and “chimaeras” is the general term for the strange-looking holocephalans. Also, ichthyologists (fish researchers) have agreed that common names of species are capitalized , so we talk about “Whale Sharks” and “White Sharks,” but we don’t capitalize “reef sharks” and “dogfish sharks” because those names refer to groups, not just one species. The questions posed in this book often seem simple and elementary (for example, “Do sharks have to keep swimming to breathe?”). The answers , however, are often anything but simple because of the diversity and complexity of body types, species, ecology, anatomy, behavior, and physiology and the special solutions that sharks (and skates and rays and chimaeChapter 1 Introducing Sharks, Skates, Rays, and Chimaeras 1 2 Sharks: The Animal Answer Guide ras) have evolved during their 400-million-year history. It is also worth remembering that these answers are the result of careful research conducted by shark biologists over the decades. The chondrichthyan fishes include several groups whose names can sometimes be confusing at first. The table provides a quick listing of terms applied to the different groups of these related but diverse fishes and the traits they share. Glossary of terms used with chondrichthyan (sharklike) fishes Batoids Skates, rays, guitarfishes, and sawfishes Cartilage Non-bony but calcified skeletal material Claspers Paired cartilage-supported male reproductive organs derived from pelvic fins Chimaeras Odd relatives (Holocephali) of the elasmobranchs; also known as ratfishes, rabbitfishes, and ghost sharks Chondrichthyans All sharklike fishes, including selachian sharks, batoid skates and rays, and chimaeras Denticles The scale type in sharklike fishes; technically, they are enamelcovered structures and are referred to as placoid scales Elasmobranchs Sharks, skates, and rays (but not chimaeras) with a slightly calcified cartilaginous skeleton, internal fertilization via modified pelvic fins, rows of replaceable teeth embedded in the connective tissue of jaws rather than in the jaw “bones” themselves, and scales (denticles) modified from tooth forerunners Gill slits External gill openings, typically five but as many as seven in elasmobranchs, one in chimaeras Guitarfishes A family of elongate rays with pronounced, pointed snouts Rajiforms The order of batoids made up of guitarfishes and skates Rays Several families of batoids that bear live young and have a long, whiplike tail, often with poisonous barbs Sawfishes A family of large, elongate rays with very pronounced, toothstudded snouts Sawsharks A family of small, elongate sharks with very pronounced, toothstudded snouts Selachians The group of elasmobranchs that includes sharks and sawsharks but not the batoid skates and rays Skates A family of rajiforms that lay eggs in cases (“mermaid purses”) and have a thick tail with two small dorsal fins and a tail fin 3 Introducing Sharks, Skates, Rays, and Chimaeras What are sharks and how are they classified? What then is a shark? The taxonomic class Chondrichthyes includes sharks, skates, rays, and chimaeras because they share a number of common traits. They are all fishes (aquatic animals with a vertebral column that breathe using gills and have scales and fins). All Chondrichthyes have a cartilaginous skeleton, which is a skeletal support system softer than bone because it is not as hardened with calcium as is true bone, although the skeleton is moderately hardened with calcium crystals. All groups also breathe via gills by pushing or pulling water in through their mouths and out their gill slits (with some minor exceptions). Because of anatomical differences, sharks, skates, and rays are grouped in one subclass, the Elasmobranchii (“plate gills”), and chimaeras are grouped in a different subclass, the Holocephali (“whole heads”). (The taxonomic names...