Читать книгу Fishes: A Guide to Their Diversity - Philip A. Hastings - Страница 9

ANATOMY OF FISHES

While their anatomy varies greatly, all fishes have several features in common. In this section, we briefly review and illustrate the major features of fish anatomy, focusing on those that are most important for distinguishing among lineages and groups.

External Anatomy

Several external regions of fishes have specific names.

SNOUT The area of the head between the tip of the upper jaw and the anterior margin of the orbit.

CHEEK The area of the head below and posterior to the eye, anterior to the posterior margin of the preopercle.

NAPE Dorsal area just posterior to the head.

OPERCULUM Plate-like structure covering the branchial chamber and consisting of four bones: the opercle, preopercle, subopercle, and interopercle.

BRANCHIOSTEGALS Slender, bony elements in the gill membrane, slightly ventral and posterior to the operculum.

ISTHMUS Area of the throat ventral to the gill openings.

LATERAL LINE Sensory system consisting of pores and canals along the head and body for the detection of vibrations and water movement, often associated with perforated scales along the body.

CAUDAL PEDUNCLE Area of the body between the insertions of the dorsal and anal fins and the base of the caudal fin.

ANUS (VENT) Terminal opening of the alimentary canal.

Body Shapes

Many fishes are somewhat elongate, laterally compressed, and oval in cross section. Several specialized shapes are recognized, including the following primary examples:

COMPRESSED Flattened laterally, sometimes strongly so, and often deep-bodied.

DEPRESSED Flattened dorsoventrally.

GLOBIFORM Rounded, often spherical.

ANGUILLIFORM Greatly elongate and usually tubular.

FUSIFORM Roughly bullet-shaped, often tapering both anteriorly and posteriorly.

Fins

The fins of fishes are either unpaired or paired. The unpaired fins, also called median fins, include the dorsal, anal, and caudal fins, as well as the adipose fin in some fishes. The paired fins include the pectoral and pelvic fins.

Fin-ray Elements and Dorsal-fin Configurations

The fins of actinopterygian fishes are composed of two types of rays: soft rays, which have evident segments, are bilaterally divided, are often branched, are typically flexible, and are usually connected by a fleshy membrane; and spines, which lack segments, are not bilaterally divided, are never branched, and are usually stiff and sometimes pungent. These fin-ray elements are derived from dermal tissues and are collectively called lepidotrichia. The dorsal fin of actinopterygians may be composed of soft rays only or of both spines and soft rays. In the latter case, the two parts of the fin may be continuous, separated by a notch, or completely separate. The fin rays of chondrichthyan fishes are flexible, unsegmented, and derived from epidermal tissues; they are called ceratotrichia.

Pelvic-fin Positions

The pelvic fins of fishes vary considerably in their position on the body, a feature useful in distinguishing many groups.

ABDOMINAL Inserted well posterior to the pectoral fins.

THORACIC Inserted slightly posterior to or directly under the pectoral fins.

JUGULAR Inserted slightly anterior to the pectoral fins.

MENTAL Inserted far forward, often near the symphysis of the lower jaw.

Caudal-fin Shapes

The caudal fins of fishes come in a variety of shapes that are roughly related to a species’ swimming behavior. Slow moving fishes often have rounded caudal fins, while fast swimming fishes have deeply forked fins with stiff upper and lower lobes. Most sharks and the early lineages of ray-finned fishes have a heterocercal caudal fin in which the vertebral column is deflected dorsally and extends along the upper, larger, caudal-fin lobe. Most ray-finned fishes have a homocercal caudal fin, which is externally symmetrical and supported by a series of laterally flattened bones. A few specialized groups such as the flyingfishes have a hypocercal caudal fin in which the lower lobe is larger than the upper lobe. Shapes of caudal fins include the following examples:

ROUNDED No sharp or straight edges, convex posteriorly.

TRUNCATE Posterior profile vertical.

EMARGINATE Upper and lower rays slightly longer than central rays.

FORKED Separate upper and lower lobes that join at a sharp angle.

LUNATE Crescent-shaped posteriorly, with extremely large upper and lower lobes.

HETEROCERCAL Vertebral column is deflected dorsally and extends along the upper, larger caudal-fin lobe.

Mouth Positions

In addition to the size of the gape and the size and type of teeth, the position of a fish’s mouth provides clues to its feeding habits. These include the following:

TERMINAL Mouth located at the tip of the snout.

SUBTERMINAL Mouth located below the tip of the snout.

INFERIOR Mouth opens ventrally, well posterior to the snout.

SUPERIOR Mouth opens dorsally.

Oral and Pharyngeal Jaw Diversity

In the chondrichthyan fishes, the upper jaw is formed by the palatoquadrate cartilage, while in the ray-finned fishes, it is formed by two bones, the maxilla and the premaxilla. In early lineages, both of these bones bear teeth and are included in the gape, while in more derived ray-finned fishes, only the premaxilla bears teeth and the toothless maxilla is excluded from the gape. In addition to these “oral jaws,” ray-finned fishes have a second set of jaws, the “pharyngeal jaws,” located anterior to the esophagus, comprising bones associated with the upper and lower gill arches. In many of these fishes, the oral jaws function to grasp and/ or ingest prey, while the pharyngeal jaws are often specialized for processing prey.

Standard Meaurements

Several standard measurements are used to document the size and shape of fishes (Hubbs and Lagler, 1958; Strauss and Bond, 1990). These include the following:

TOTAL LENGTH (TL) Horizontal distance from the most anterior point on the head to the tip of the longest lobe of the caudal fin. The most anterior point is often the tip of the snout, but may be the tip of the lower jaw in some species.

FORK LENGTH (FL) Horizontal distance from the most anterior point on the head to the end of the central caudal-fin rays.

STANDARD LENGTH (SL) Horizontal distance from the tip of the snout to the central base of the caudal fin (i.e., the end of the hypural plate). The latter can often be located as a crease formed when the caudal fin is slightly bent.

HEAD LENGTH Horizontal distance from the tip of the snout to the posterior margin of the operculum.

SNOUT LENGTH Horizontal distance from the tip of the snout to the anterior margin of the orbit.

BODY DEPTH Maximum vertical distance between the dorsal and ventral outlines of the body.

SNOUT-VENT LENGTH Distance from the tip of the snout to the anterior margin of the vent.

DISK WIDTH In batoid fishes (rays), the maximum distance between the lateral margins of the left and right pectoral fins.

Sensory Systems

Fishes have the full array of sensory systems common to all vertebrates (olfaction, taste, vision, and hearing), as well as some unusual ones such as the lateral line and electroreception. Details of these systems are often useful in diagnosing various lineages of fishes. Numerous reviews of the sensory biology of fishes are available, including several chapters in volume 1 of the Encyclopedia of Fish Physiology, edited by Farrell (2011).

OLFACTION Fishes have left and right olfactory organs (paired in most fishes, unpaired in agnathans) that are chemoreceptive. Each side includes incurrent and excurrent nostrils (or nares) that may have a divided single opening or paired openings.

TASTE Fishes have chemoreceptive taste buds located inside the mouth, and in many groups also on the gill arches, barbels, fin rays, and the skin.

VISION The eyes of fishes come in a variety of sizes and forms and frequently are reflective of a species’ habitat and habits. Eyes are often large in nocturnal species, upwardly directed in mesopelagic fishes, and small or sometimes absent in fishes from dark habitats including the deep sea and cave environments.

HEARING AND BALANCE Fishes have an inner ear with one (hagfishes), two (lampreys), or three (all other fishes) semicircular canals that function in maintaining balance and orientation. The main organs of hearing are the paired otolith organs, each of which consists of a sensory epithelium with an overlying calcium carbonate otolith (bony fishes) or otoconia (cartilaginous fishes). Sound waves are propagated from the water, through the tissues of the head, to the otoliths or otoconia, whose vibrations are detected by the sensory epithelium. A variety of so-called “otophysic connections” between the inner ear and the gas bladder serve to amplify sound reception in some fishes. These include anterior projections of the gas bladder that extend close to or, in some cases, into the otic capsule, and the Weberian apparatus, a mechanical linkage formed from modified anterior vertebrae, stretching between the gas bladder and inner ear of otophysans (Braun and Grande, 2008).

LATERAL LINE The mechanosensory lateral-line system of fishes detects water flow and vibrations made by movements of other organisms. Its sensory organs, called neuromasts, are located in pored lateral-line canals on the head (cephalic lateral line) and body (trunk lateral line), as well as on the skin (superficial neuromasts). Their expression in fishes varies greatly, but their configuration provides clues to the habits of many species (Webb, 1989, 2013).

ELECTRORECEPTION Receptors that detect weak electrical fields produced by other organisms are present in lampreys, all cartilaginous fishes, and some bony fishes (Kramer, 1996). In the cartilaginous fishes they are called ampullae of Lorenzeni, and involve sensory cells located at the base of canals filled with conductive jelly and open to the surface. They are especially common on the ventral side of the head, where they facilitate detection and capture of prey items. In teleosts the electroreceptive sense detects electrical fields in the environment, including those generated by conspecifics, as well as potential prey.

Skeletal Anatomy

The skeletal structure of fishes has been studied extensively for clues to both phylogeny and function. The skeletal structure of cartilaginous fishes was recently reviewed by Claeson and Dean (2011). Several excellent guides to the osteology of ray-finned fishes are available, including the classic text by Gregory (1933) and a recent overview by Hilton (2011). For ray-finned fishes, several superficial bones of the head are especially useful in identifying various groups of fishes (illustrated below). The major components of the caudal fin and posterior vertebral column are also illustrated below.