Diversity of fishes week 1

Spines

• Usually hard and pointed

• Unsegmented

• Unbranched

• Solid

Rays

• Usually soft and not pointed

• Segmented

• Usually branched

• Bilateral, with left and right halves

Dorsal fins

The original function of the dorsal fin was a stabilizer during swimming, but it has been modified in many different ways, e.g.:

• Reduced or lost (in rays)

• Confluent with caudal and anal fin (in some eels)

• Converted into a suction disk (remora)

• Conversion of first dorsal spine into an ilicium (anglerfish)

Anal Fins

The anal fins shows the least, but still some, variation

among fishes:

• Reduced or lost (in ribbonfishes)

• Elongated and used for locomotion (knifefishes)

• Anterior part modified into a gonopodium for spermatophore transfer in livebearers (mollies)

Caudal fin

• Caudal fins of fishes vary in both external shape and internal anatomy

• The different types of caudal fins provide useful information about modes of swimming as well as phylogeny

Protocercal tail

The primitive undifferentiated caudal fin that extends around the posterior (e.g. lancelets)

Heterocercal tail

Unequal-lobed, vertebral column extends into the upper lobe (e.g. sharks)

Homocercal tail

Equal-lobed, caudal fin rays arranged symmetrically (most advanced bony fish)

Leptocercal tail

Resembles protocercal, but secundarily derived (e.g. lungfishes)

Isocercal tail

Last vertebra modified into small plate to which caudal fin rays attach (Gadidae)

Gephyrocercal tail

Lack of true tail, modified elements of dorsal and anal fin form tail (Molidae)

Branchiocranium

• That portion of the skull related to the gills, including the mandibular region, the hyal region (hyoid arch and branchiostegal series), and the branchial arches including their attached dermal plates, or the branchial skeleton proper.

• Teeth and gill rakers are part of the branchiocranium

Teeth

The oral jaws and many pharyngeal bones may bear teeth which can be divided in several types suitable for various feeding modes:

1. Canine

2. Villiform

3. Molariform

4. Cardiform

5. Incisor

6. Teeth fused into beaks

7. Flattened triangular cutting teeth

8. Pharyngeal teeth

Integumentary skeleton

• The integument is composed of the skin and skin derivatives, and includes scales in fishes and feathers and hair in birds and mammals

• The structure of the skin in fishes is similar to that of other vertebrates, with two main layers: and outer epidermis and an inner dermis

• Scales are the characteristic external covering of fishes

Placoid scales

• Characteristic of Chondrichthyes

• A flattened rectangular basal plate in the upper part of the dermis, from which a protruding spine projects

• Placoid scales do not increase in size with growth; instead new scales are added between older scales

• The teeth of elasmobranchs are evolutionary derivatives of placoid scales

Cosmoid scales

• Were present in fossil coelacanths and fossil lungfishes

• Probably arose form the fusion of placoid scales

• Growth is by addition of new lamellar bone underneath, not over the upper surface

Ganoid scales

• Found in Chondrostei (e.g. sturgeons, gars and paddlefish)

• Modified cosmoid scales

• Sturgeon scales are modified into large plates, with most of the rest of the body naked

Cycloid and ctenoid scales

• Present in the Teleostei, the vast majority of the bony fishes

• Evolved from ganoid scales and overlapping

• Cycloid scales lack ctenii (teeth on posterior border)

• Both used for studying taxonomy and life history; they grow with age, measurable by counting circuli (growth rings)

Soft Anatomy

• Muscles

• Cardiovascular system

• Alimentary canal

• Gas bladder

• Kidneys

• Gonads

• Nervous system

Muscles

Skeletal muscle

• Most of a fish's mass, other than skeleton

• White muscle: used anaerobically in short-duration burst swimming but fatigues quickly. Lack myoglobin

• Red muscle: forms a thin sheet under the skin; involved in sustained swimming and hard to fatigue. Abundant myoglobin

Smooth muscle

• Involuntary muscle that lines the walls of the digestive tract

Cardiac muscle

• Dark red involuntary heart muscle

Cardiovascular system

• Serves all bodily functions but is most closely associated with respiration, excretion, osmoregulation, and digestion

• Is a system of arteries, veins, and capillaries that carry respiratory gases, wastes, excretory metabolites, minerals, and nutrients

Blood flow in fishes involve a single-circuit system

Alimentary canal

Gas bladder

• Also called swim bladder

• Gas-filled sac located between the alimentary canal and the kidneys

• Functions mainly as hydrostatic organ to help control buoyancy

• Only bony fishes have gas bladders, and some species of teleost have lost their gas bladder (mostly deep-sea and benthic fishes)

Kidneys

• Kidneys are one of the primary organs involved in excretion and osmoregulation

• Marine and freshwater fishes differ considerably in kidney structure reflecting the different problems faced by living in very different solute concentrations

• Freshwater fishes produce copious amounts of highly dilute urine, while saltwater fishes drink seawater to correct dehydration, and excrete a low volume of highly concentrated urine

Gonads

• The sexes in fishes are usually separate

• The testes are internal, longitudinal and usually paired

• Can constitute as much as 12% of body mass

• The ovaries are internal, usually longitudinal, and paired or fused

• Can constitute as much as 70% of body mass

Nervous system

• Fish brains are on average only 1/15 the size of the brain of a bird or mammal of equal body size

• Sharks have much larger brains relative to body size than teleosts