Classifying Fishes

Many fishes have unique characteristics that make their relationship to other fishes difficult to determine. A phylogenetic tree is often used to show the hypothesized relationship between organisms. A phylogenetic tree is made up of branches, which are lines that represent a particular group. Nodes are the points where the branches intersect. Fishes that are close to one another on the tree have more shared similarities than fishes that are farther apart on the tree. Similarly, fishes that branch from the same node, have more shared similarities than fishes that branch from different nodes.

The tree in Fig. 4.10 describes relationships between fishes. The node descriptions represent shared characteristics. For example, the node at the base of the tree indicates that all fishes have a chambered heart, neurocranium, and liver. In contrast, the node at the top of the tree indicates that only lobe-finned and ray-finned fishes have three earbones.

The most primative organisms are those that evolved first. They have the least branching on a phylogenetic tree (hagfish in Fig. 4.10). The most derived, or advanced, organisms are those that evolved more recently. They have the most branching on a phylogenetic tree (ray-finned fishes in Fig. 4.10).

Jawless Fishes: Hagfish and Lampreys

Hagfish and lampreys are some of the most primitive fishes. They are jawless and scaleless (Fig. 4.11). They also have reduced skeletons. Hagfish and lampreys are classified as agnathans.

Hagfish do not have true fins. They have barbels around the mouth and a single nostril. Instead of jaws, they have structures with tooth-like projections for pulling off food and two pairs of comb-shaped teeth on a cartilaginous plate that protracts and retracts (Fig. 4.12 A). Hagfish are famous for their abilities to tie themselves in knots to escape predators. Hagfish also produce lots of slime—a single hagfish can produce twenty liters of slime in minutes. Hagfish are normally found in deep water, where they are scavengers.

Lamprey have gill arches and more complete skulls than hagfish. Many lamprey are parasitic on other vertebrates. Parasitic means that one organism benefits to the detriment of the second organism. Parasitic lampreys use their mouth parts (Fig. 4.12 B) to attach to the host. The lamprey then use their teeth to cut through tissue and feed on blood and other bodily fluid.

Cartilaginous Fishes: Sharks and Rays

Cartilaginous fishes, classified as Chondrichthyes, have skeletons made of flexible cartilage rather than hard bone. Cartilaginous fish include chimeras, which have a hard gill covering called an operculum. Sharks and rays are also cartilaginous, but they have naked gills (no operculum). The term elasmobranch means naked gill. Sharks and rays are elasmobranchs (Fig. 4.13). They have heterocercal tails, which means the tail is uneven, with the top lobe generally longer than the bottom lobe. They also have sensory organs called ampullae of Lorenzini. The ampullae of Lorenzini detect electrical fields and help sharks find their prey.

Cartilaginous fishes have a broad range of characteristics and life styles. For example, some cartilaginous fishes give live birth, whereas others lay eggs. And, contrary to many stereotypes, many cartilaginous fishes have good eyesight. This group also includes a surprising number of colorful fishes.

Lobe-finned Fishes: Lungfishes and Coelocanths

The lobe-finned fishes, classified as Sarcopterygii, include lungfish and coelocanths. The lobe-finned fishes have paired, lobed fins. Lobed fins are thick and fleshy and represent a body form that is in-between the walking limbs of land animals and the swimming fins of many species of fishes (see Fig. 4.14). The lobe-finned fishes also have other unique features. For example, coelocanths are able to detect electric fields, and lungfish are able to breath air using modified lung structures.

Primitive Ray-finned Fishes: Sturgeons, Paddlefish, and Gars

The primitive ray-finned fishes, classified as Actinopterygii and Neopterygii, are very diverse (see Fig. 4.15). The relationship between the fishes in this group is not fully understood, but they are grouped together because they share characteristics like scale shape, heterocercal tail shape, and vertebrae structure.

Many fishes in this group have cartilaginous skeletons, but the cellular origin of their cartilage is different than in sharks and rays. Fishes in this group can also have barbels (feelers below their mouths) and bony scutes (big plates) along their bodies. Some fishes in this group, like the paddlefish, can also detect electrical fields. Others, like the beluga sturgeon, are long-lived (over 110 years). Sturgeon are also famous as a source of caviar, or raw fish eggs, which is a delicacy.

Modern Bony Fishes: The Teleosts

The modern bony fishes are classified in the Division Teleostei. The word teleost means perfect bone. Teleosts make up the majority of living fishes.

Within the group of teleosts, there is an evolutionary trend toward specialization for specific environments. Fig. 4.16 shows examples of teleost species organized from primitive (Fig. 4.16A) to more derived (Fig. 4.16C). As teleosts evolved over time, some trends in body-type emerged:

1. Shorter, deeper bodies
   
   1. Exceptions are specialized, long-bodied fishes. 
       
2. Reduction in bony elements
   
   1. This allows for lighter, quicker movements for catching prey and avoiding predators.
       
3. Repositioning and specialization of the dorsal fin
   
   1. Positioning becomes more forward on the body, more elongate and diversified (usually into two fins, one that is spiny and one that is soft-rayed).
   2. Specialization appears for protection, mobility (knifefish and triggersfish), venom delivery (scorpionfish), attachement (sharksucker fish), ornamented lure (anglerfish), or camoflauge (sargassumfish).
       
4. Change in placement and function of the paired fins (pelvic and pectoral)
   
   1. Paired fins become higher and further forward with more vertical orientation.
   2. Placement is advantageous for sculling and fine movement (wrasses).
       
5. Modifications of the caudal fin to an outwardly symmetrical fin
   
   1. This allows for more high-power caudal swimming (jacks and tunas).
       
6. Added control over the gas bladder
   
   1. Adaptation moves toward internal control of gas (as opposed to gulping air).
       
7. Improvements to the jaw
   
   1. Adaptation moves toward better sucking ability.

Advanced teleosts also have some internal body adaptations. For example, the gas bladder, which is used for floatation regulation, is further forward and higher up in advanced fishes. This helps increase balance. Many advanced teleost fishes also have a muscular crest, or ridge, on their head. The most advanced teleosts can be generalized as fast, maneuverable, spiny fish with sucking mouth structures (Fig. 4.17). These adaptations have allowed teleosts to specialize and dominate in the shallow, food rich areas of the ocean and freshwater environments.