External Anatomy and Taxonomy

Ichthyology Lab 1: External Anatomy & Taxonomy Goals: 1. Learn the basic external anatomy of fishes. 2. Learn how to identify fish species. Tasks: 1. Choose a fish. 2. Identify all major external structures of your fish. Learn all the fins, mouthparts, etc. Learn the difference between spines (rigid, unsegmented) and rays (soft, segmented, branched at the tips) in the fins of your fish. 3. Identify the species of fish you are examining. 4. Compare the anatomy of other fishes in the lab. 5. List the species present in the laboratory. External body parts of a bony fish (Lutjanidae) - 1- Ichthyology Morphological Characters INTRODUCTION This lab is concerned with the external structures and organization of cartilaginous and bony fishes. As one might expect when examining a group with 25,000+ species, fish structures are quite diverse. This handout will introduce you to the general form of these structures. Compare their presence and appearances in the various fishes in the laboratory. Identification of these structures will be expected on the lab practical exams. You should also be familiar with the text of this handout. Structure and Organization FINS We see two basic types of organization of fins in fishes. In Chondrichthyes (sharks, skates, rays, and chimaeras) the fins are supported by radial cartilages that extend away from the body and are embedded in fleshy tissue. This condition is typical also of Sarcopterygian fishes (lungfishes and the coelocanth) and gives this group its name - the lobe-finned fishes. In Sarcopterygians the radial cartilages are replaced by bony supports. In Actinopterygian fishes (the ray-finned fishes) the fins are supported by bony fin rays and no fleshy tissue extends onto the fin itself. A fin membrane runs between the supports, making for a more fragile but more maneuverable fin. For all fishes the fins can be divided into two groups, the median fins (dorsal, anal, and caudal fins) and the paired fins (pectoral and pelvic fins). The function of each fin will vary from species to species, eg. propulsion, stability, turning, resting, walking, defense, etc. Observation of the position and shape of each fin provides clues to its function. Dorsal Fin The dorsal fin is located along the dorsal or upper margin of the body. The dorsal fins of Actinopterygian fishes can be of several types: rayed, spined, or adipose. Most fish possess a rayed dorsal fin in which soft rays support the fin membrane. In advanced fishes (Teleosts or spine-finned fishes) part of the dorsal fin is supported by stiff fin spines and part by soft fin rays. In these fishes, the dorsal fin may be divided into separate spiny and rayed sections (i.e., actually be or look like two fins) or may be continuous. Characteristic of salmoniform fishes (trout and salmon) is a fleshy adipose dorsal fin posterior to the rayed dorsal fin. Extreme modifications of the dorsal fin are found in remoras in which the dorsal fin has evolved into a suction device and in anglerfishes in which the first dorsal fin spine is modified into a fishing lure. Anal Fin The anal fin is located between the anus and the caudal fin. It may be supported by rays or rays and spines. It is the least specialized of the fins. However, in male Poeciliid fishes (guppies etc.) the anal fin has been modified into a copulatory organ. - 2- Ichthyology Caudal Fin The caudal fin has an internal structure that is quite complicated. It is supported by soft rays - never by spines. Modification of the most posterior vertebrae which attach the axial skeleton to the caudal fin rays are of great taxonomic and functional importance. The narrow area which, in typical fishes, separates the main muscle mass of the trunk and the caudal fin is known as the caudal peduncle. In some strongswimming fishes (e.g., Scombroids and many Lamnid sharks), the peduncle is very broad, but depressed (i.e., narrow when viewed from the side) in order to accommodate a powerful “tendon and pulley” system -- this is called a keel. The shape of the caudal fin reveals much of its specific functional importance. When the dorsal and ventral lobes of the caudal fin are of different sizes, the fin is referred to as heterocercal. In most sharks and in many primitive bony fishes the dorsal lobe is larger than the ventral lobe. This type of tail is called a positive heterocercal tail and generates lift as well as propulsion. In a few bony fishes the ventral lobe is larger than the dorsal lobe; this is a negative heterocercal tail. Examples include flying fish and freshwater butterfly fish. In most fishes the dorsal and ventral lobes of the caudal fin are equal in size, i.e., homocercal. There are several subdivisions in the general category homocercal tails: lunate, forked, rounded, emarginate, truncate. Pelvic Fins Pelvic fins (and pectoral fins) are paired. They are supported by either rays or rays and spines, but rarely just spines. Their position varies greatly between species, but in general they lie posterior and ventral to the pectoral fins. Functions of these fins include support and stability. Extreme modification of these fins to form ventral suckers has evolved independently in several fish groups (e.g., clingfish, lumpfish, and some gobies). Pectoral Fins The paired pectoral fins are always composed of rays. Position varies greatly in all fishes. In advanced fishes they are usually located in the midline of the body near the center of gravity and are used in turning and stability. HEAD STRUCTURES The significant external structures in the head region of almost all fishes consist of a pair of eyes, a pair of nares (nostrils, singular naris), a mouth, cephalic (head) lateral line canals, and some sort of gill opening. Eyes vary in placement. Fishes with eyes far anterior have broadly overlapping visual fields providing some degree of binocular vision and, therefore, depth perception. Eyes located more laterally or dorsally give different fields of view. The nares are located on the snout near the mouth, either ventrally in the case of sharks and skates or dorsally in most bony fishes. Each naris consists of an incurrent opening and an excurrent opening, and may possess flaps to close either opening. Unlike the nostrils of most other vertebrates, the nares of fishes are adapted for olfaction only; they do not communicate with the mouth - 3- Ichthyology cavity and cannot be used for breathing. Several lateral line canals are present on the head, but their names are not particularly relevant. In sharks, skates, and rays, the branchial openings consist of 5 - 7 pairs of lateral or ventral gill slits. Additionally, in skates, rays, and some sharks, there exist a pair of round openings that resemble and are often mistaken for ear-holes. These are spiracles. Like the gills they accommodate excurrent water flow, but can also be used for incurrent flow (“inhalation”) when the mouth is blocked. Three main cartilaginous structures make up the mouth proper. The Meckel’s cartilage forms the lower jaw and the palatoquadrate forms the upper jaw. The corners of the mouth are supported by labial cartilages. Unique to cartilaginous fishes is an ordered array of specialized sensory cells called ampullae of Lorenzini. These ampullae, located on the dorsal and ventral surfaces of the head, are receptors for the detection of electrical fields. Pores for the receptors are visible around the snout and may be confused with those of the lateral line. In bony fishes four pairs of gills are covered by a flat, bony cover - the operculum. The borders of the mouth are formed by three bones. The mandible forms the lower jaw. The premaxilla and maxilla form the lateral and dorsal portions of the mouth. Various spines (particularly from the preopercular bone) may be present on the head. Fleshy tufts of skin on the dorsal surface of the head called cirri are present in some teleosts (e.g., sculpins and greenlings). Long, thin fleshy protuberances called barbels are located near the corners of the mouth in several groups of fishes (e.g., cod, catfishes, nurse sharks). They are heavily invested with sensory cells and are used to detect prey. LATERAL LINE The lateral line system is used by fishes to detect low frequency vibrations (i.e., pressure waves) in the near field. In most fishes the main lateral line canal starts at the back of the skull and continues to the caudal fin at the level of the midline. Placement of the canal can vary, however. In flyingfishes which spend their time at the surface, the canals are positioned ventrally. The opposite is true for many demersal fishes. Openings for the canal are usually shielded by modified scales. In some fishes (e.g., greenlings) accessory lateral line canals run parallel to the main canal. SKIN Although some fishes are completely scaleless (many catfishes and eels), the skin of most species is partly or completely protected by a hard covering of scales. Bony plates, large modified scales, serve as armor for a variety of demersal fishes such as many South American catfishes, boxfishes, sturgeons, and sticklebacks. Dermal denticles (or placoid scales) are found in most cartilaginous fishes. Scales of bony - 4- Ichthyology fishes come in three types: ganoid, cycloid, and ctenoid. Ganoid scales have a thick inorganic surface layer and are found in primitive bony fishes like gars, Amia, and Polypterus. Cycloid and ctenoid scales are thin and translucent. Cycloid scales are generally flat and rounded (e.g., minnows, herrings and tarpon). Ctenoid scales, characteristic of the derived teleosts (Acanthopterygii), can be distinguished from otherwise similar cycloid scales by the presence of tiny tooth-like projections (ctenii) on the exposed, posterior edge of the scale. Cycloid and ctenoid scales are not always easy to distinguish. Instead, one can find a gradation of scale morphologies between the quintessential ctenoid and cycloid types. In general, soft-finned fishes have cycloid scales, and ray-finned fishes have ctenoid scales, but exceptions are numerous. The dorsal spines of some sharks and the caudal spines (stingers) of the sting rays are derived from scales. Similarly, the teeth of all fishes - most notably the sharks - originated from scale tissue. One aspect of scale morphology has proven to be of great use to fishery biologists. Like the rings of cross-cut trees, many scales possess annual growth rings. Each scale grows continuously by accretion along its periphery. During periods of active growth material can be added so rapidly as to produce daily growth rings. During periods of slow growth material is added slowly, producing a darker (more compacted) ring. This procedure is less useful for ageing older fish, however, as growth is often very slow and distinctions between years become obscured. - 5- Ichthyology IDENTIFICATION OF TAXA USING KEYS "Identifying" a fish specimen involves determining the correct taxa to which it belongs. Therefore, identification is often called determination. The process requires keying and checking. If the classification of the specimen is unknown, the fish should first be keyed out to family in a preliminary key to the orders and families of fishes. Preliminary keys like that in Miller and Lea (1972) for marine fishes of California, often include outline drawings of representative species as checks. Once its family is determined, the specimen is identified tentatively to species by using another key to the family's genera and species. Finally the tentative identification is checked by comparing the specimen with published illustrations, descriptions, and authentically-determined preserved specimens (voucher specimens). TAXONOMIC KEYS A taxonomic key is an algorithm: a set of rules or steps to be followed in sequence for solving a problem. The problem is to identify a fish specimen. Steps to be followed require observations of the specimen, with the result of each observation specifying which of two or more alternative paths leads to the next correct observation. The goal is to identify the specimen by following the correct path to its endpoint. Using a taxonomic key is akin to playing the old parlor game of "20 Questions" to identify some unknown object that someone being questioned is thinking of. With each "YES" or "NO" answer, the questioner eliminates members of an increasingly large set of what the object is not, until, by the process of elimination, only the object itself remains. In using a fish key, however, the "object" is an unknown fish specimen and the "questions" are ordered, contrasting statements about physical characteristics. For a particular specimen, each alternative must be ascertained as either true or false. The statement must be unambiguous. For example, "Anal soft rays many" is an unsatisfactory statement because it defies definite judgment. "Many," relative to what? On the other hand, "Anal soft rays more than 20" is unambiguous because it can be verified or denied by accurately counting fin rays. The character to be observed must be clearly determinable so that the decision made concerning it eliminates all (or just about all) individuals in the set of excluded taxa, leaving all individuals of the specimen's taxon together with others of the included set. If, for example, females of a species had twice as many anal rays as males, "Anal soft rays more than 20" becomes ambiguous unless the statement is qualified as to whether it refers to males or females. Keys may specify observations on more than one character per alternative. For example, "Anal soft rays more than 20; eye diameter less than 25% head length" provides further information as fail-safe. The order of characters implies that the count of soft anal rays is perhaps the better (more reliable) of the two characters; yet if anal rays are damaged or missing, a decision based on eye diameter will probably not lead you astray. - 6- Ichthyology For unequivocal pathfinding, a taxonomic key should be dichotomous. There should be no more than two alternatives per step or observation. Then if one alternative is false, the next step is specified automatically and the direction of the path is clear. Dichotomous keys usually fall into two categories: indented and bracketed. An indented key places the set of all included taxa under one alternative, and all excluded taxa under the other. For example: A. Anal soft rays more than 20. B. Total gill rakers on first arch fewer than 15; head length more than 30% SL. ........ Bassus macrocephalus BB. Total gill rakers on first arch more than 20; head length less than 25% SL. ........ Bassus microcephalus AA. Anal soft rays fewer than 18. B. Side of body with large black spots, no stripes. ........ Perchus nigromaculosus BB. Side of body with thin black stripes, no spots. ........ Perchus nigrofasciatus Indented keys have the advantage that similar taxa are set off together. But long keys are hard to follow because they have initial contrasts located far apart, and they waste space due to progressively longer indentations. Bracketed keys are more commonly used for identifying fish. Long keys are relatively easy to use and save space; alternatives are placed together as couplets and statements are not indented. For example; 1a. Anal soft rays more than 20....................................................................................2 1b. Anal soft rays fewer than 18...................................................................................3 2a(1a). Total gill rakers on first arch fewer than 15; head length more than 30% SL. ........ Bassus macrocephalus 2b. Total gill rakers on first arch more than 20; head length less than 25% SL. ........ Bassus microcephalus 3a(1b). Side of body with large black spots, no stripes. ........ Perchus nigromaculosus 3b. Side of body with thin black stripes, no spots. ........ Perchus nigrofasciatus Notice that you can backtrack if you take the wrong path. Alternative "a" of all couplets except the first refers back (in parentheses) to the preceding step. Unfortunately, many bracketed keys lack this provision. - 7- Ichthyology Most fish keys are made primarily for identifying specimens along the easiest and quickest path possible. Nonetheless, they may reflect phylogenetic classification by having closely-related species near each other wherever the utilitarian purpose is not compromised. For example, species in the same genus (congeners) are usually together because they are readily distinguishable as a group from non-congeners. Such keys that are based on easily observed external characters showing clear-cut differences, and that do not necessarily reflect phylogenetic order are called artificial. Many internal characters (e.g., skeletal characters) used to measure phylogenetic relationships are not easily observed. It follows that phylogenetic keys, which group taxa strictly by their putative phylogenetic relationships, are difficult to use since internal characters muddy the path. Phylogenetic keys are usually indented. - 8-