Fish are the most diverse group among the vertebrates, with more species than all other vertebrates combined. Most fish species belong to the infraphylum Gnathostomata (jawed fish) which comprises the specimens in the classes Chondrichthyes, Sarcopterygii, and Actinopterygii (the latter two belonging to the superclass Osteichthyes). The class Actinopterygii includes the largest infraclass of vertebrates, the Teleostei (bony fish). Teleost fish are represented by approximately 35 000 described species of freshwater and saltwater fish (Facey et al. 2022). The class Chondrichthyes (cartilaginous fish) includes the subclasses Elasmobranchii and Holocephali. Elasmobranchs are represented by approximately 400 species of sharks and 500 species of rays and skates (Garner 2013). The anatomy and physiology across the many species of fish are incredibly variable. The first apparent difference is the shape of the bodies, generally classified as depressiform (vertically flat), compressiform (laterally compressed), fusiform (torpedo shaped), filiform (tube shape), and elongated (cylindrical). The body can be divided into a head, trunk, and tail, with the muscular portion at the base of the tail known as the peduncle. These divisions are sometimes barely discernible. The body is supported by a skeleton, made of either cartilage (cartilaginous fish) or bone (bony fish). The main element of the axial skeleton is the vertebral column, formed of articulating vertebrae. Other components of the axial skeleton are the cranium and ribs. Fins are bony or cartilaginous structures supported by the appendicular skeleton and are variably represented by the dorsal, pectoral, caudal, pelvic, anal, and ventral fins. The two major masses of skeletal muscle, the upper pair (epaxial) and lower pair (hypaxial) muscles, are located on each side of most of the fish. The integument of most fish species is covered by skin derivatives such as cycloid scales (present in the majority of bony fishes), denticles (placoid scales of sharks), scutes (bony plates), and cartilage, or can be leathery in scaleless fish. On the skin, sensory structures known as lateral lines are recognized as faint lines of pores running along each side of the body (Bleckmann and Zelick 2009). These pores are present in the head but are more prominent in the line extending from the mid-portion of the opercular region to the end of the peduncle. The opercular cavity and the gills of teleost are covered by the operculum, a movable, plate-like bony structure. Elasmobranchs do not have opercula and exhibit openings in the skin (gill slits) at the caudal portion of the head, cranial to the pectoral fins (sharks), or at the ventral side of the body (rays). Teleost fish generally have, per side, four gill arches, while elasmobranchs can have, per side, four to seven gill arches. The gill arch is a curved cartilaginous or bony structure, containing one or two rows of gill filaments on the one side, and small cartilaginous projections toward the oral cavity (gill rakers) on the other side. The eyes and nares are usually located at the sides of the head, with the exception of flat fish, which have eyes at one side of the body. Elasmobranchs exhibit pores in the head, either communicating the external surface to internal sensory organs (ampullae of Lorenzini or pores of the cephalic lateral line system) or to the ear canals and internal ear (endolymphatic pores and ducts). Other important external structures include the mouth, the barbels (sensory organs that look like whiskers near the mouth), and the vent (present in some fish as a common chamber of the alimentary, reproductive, and urinary tract) or anus and genital pore (when the reproductive and urinary tracts are separated from the alimentary tract). The trunk of fish contains the pericardial sac and the coelom. The heart lies within the pericardial sac. The blood flow in fishes relies on the passage of blood from the heart to the gills to the body and back to the heart. The fish heart consists of four chambers: the sinus venosus, atrium, ventricle, and conus or bulbus arteriosus. The liver is one of the largest organs in the coelom and is composed of lobes lying in the right and left cranial quadrants of the cavity and extending caudally over the intestinal loops. The gallbladder is a sac attached to the visceral surface of the liver, usually filled with green watery fluid. The spleen is an oval to flat organ attached to the mesentery and ligaments adjacent to the stomach. The swim bladder and the kidney are located at the dorsal region of the coelom. The swim bladder (gas bladder) is a gas-filled sac between the alimentary tract and the kidneys. This organ can be connected to the digestive tract by a pneumatic duct, in physostomous fishes, allowing the passage of air from the intestine to the sac. In physoclistous fishes, this connection is lost in adult fish, and the air enters the swim bladder via vascular rete, or “gas gland.” Kidneys can be divided into anterior kidney (primarily hematopoietic) and posterior kidney (primarily excretory). The kidneys exhibit different shapes and are generally paired retroperitoneal organs lying ventral to the vertebral column and dorsal to the swim bladder. The anterior kidney is a paired organ located in the dorsal cranial coelom. Of note, in elasmobranchs, hematopoiesis occurs primarily in the epigonal organ, paired structures adjacent to the gonads, or in Leydig’s organ, hematopoietic aggregates within the wall of the esophagus. The cranial part of the alimentary tract consists of the mouth, oral cavity, and pharynx. The caudal part is formed by the esophagus, stomach, small intestine, and large intestine. Some species exhibit a pyloric caecum, an extension of the intestine composed of fingerlike pouches connected to the intestine near the pylorus. The central nervous system is divided into the brain and the spinal cord. The brain of fish is usually divided into five parts, the most evident being the telencephalon, the optic lobe, and the cerebellum. The brain lies in the cranial cavity of the skull. Reproductive organs, ovaries and testes, are usually paired cylindrical elongated structures lying in the caudal coelomic cavity, lateral or ventral to the swim bladder. Given the poor development of reproductive organs in young fish, the identification and differentiation between the ovary and testis can be difficult. Of note, some species of teleost fish are sequential hermaphrodites, meaning they change sex during life and have ambiguous gonads. Given the wide variation of fish morphology, a species-specific anatomy review is always recommended before a gross necropsy examination. Important differences among jawed fish are highlighted in Table 18.1. Table 18.1 Anatomic features of jawed fish.
Chapter 18
Fish
18.1 Anatomy Review
18.2 Species Differences
Taxonomy
Common species
Special anatomic findings
Osteichthyes
Actinopterygii
Teleost (35 085 species)
Different shapes of scales or lack of scales
Bony skeleton
Presence of operculum
Presence of swim bladder
Several body shapes
Usually four gill arches
Cranial and caudal kidney with hematopoietic function (absent bone marrow)
Few species with accessory breathing structures
Sarcopterygii
Lungfish
Bony skeleton
Accessory breathing structures
Atrium and ventricle are partly divided by a partition (partial separation of oxygenated and deoxygenated blood)
Chondrichthyes
Elasmobranch
Sharks, rays, and skates
Cartilaginous skeleton
Four to seven gill arches
Absent swim bladder
Modification of the gills (corpus cavernosum)
Usually fusiform or dorsoventrally flattened bodies
Presence of endolymphatic pores
Epigonal organs and Leydig’s organ with hematopoietic function (absent bone marrow)
Presence of dermal denticles in sharks
Absence of cavitary adipose tissue
External gill slits (absence of operculum)
Holocephali
Chimaeras
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