Anatomy of Fish Body Shapes

Fish have a wide variety of body shapes, playing a role in swimming and survival.
The body shapes of fish vary greatly compared to other invertebrates.
Different body shapes have different impacts on swimming behavior and survival.

Fusiform (Torpedo-shaped): Rapid swimmers like tuna, salmon, and mackerel.
Compressiform (Laterally compressed): Cichlids and Nile perch (Oreochromis niloticus).
Depressiform (Dorso-ventrally compressed): Catfish (Clarias gariepinus).
Anguilliform (eel-shaped): Eel fish.
Globi-form: Pufferfish (Tetraodon fahaka).

Fish body is divided into head, trunk, and tail regions.
Head: The anterior part of the body, extending from the mouth to the gill cover (operculum). It houses the mouth, digestive system, respiratory system (gills and gill cover), nervous system (brain and spinal cord), and sensory organs like eyes, nostrils and barbels (in some species).
- Subdivisions of the head region include the snout, occiput, and isthmus.
- Mouth position: Terminal (both jaws same length) in Cichlids (mid-water feeders), Sub-terminal (lower jaw shorter than upper jaw) in Catfish(bottom feeders), Inferior (lower jaw shorter than upper jaw) in Sharks (bottom feeders), and Superior (lower jaw longer than upper jaw) in Pike (surface feeders).
- Teeth are of different types: Maxillary, Mandibular, Palatine, Canine, Needle-like; Saw-like, permanent and temporary.
Trunk: The middle section of the body, between the operculum and the caudal peduncle.
- Important topographical areas include: Nuchal region (the dorsal surface behind the occiput), Breast (the ventral area behind the isthmus), Belly (the area behind the breast). Caudal peduncle (constricted part separating the trunk from the tail) and lateral line.
Tail: The posterior part of the body, extending from the caudal peduncle to the caudal fin. The caudal fin is a major part of the tail region, essential for propulsion.

Skin: The protective outer layer, composed of the cuticle, epidermis, and dermis. It has defense mechanisms against diseases. Skin types are scaleless or scaly.
Cuticle Layer: Contains mucopolysaccharides, specific immunoglobulins, lysozymes, and free fatty acids for defense.
Epidermis Layer: Composed of stratified squamous epithelium, mucus secreting cells, club cells (alarm cells), macrophages, and lymphocytes.
Dermis Layer: Contains pigment cells (chromatophores), mast cells, blood vessels, nerves, and sensory nerve endings. These layers work as a protective barrier against infections.
Scales: Translucent plates of dermal origin into the epidermis, providing a protective exterior and reducing water friction. Types of scales include:
- Cycloid: Oval with smooth edges (e.g., cichlids).
- Ctenoid: Oval with comb-like teeth on the edges (e.g., some rock fish).
- Ganoid: Diamond-shaped, often overlapping (e.g., some bony fish).
- Placoid: Tooth-like (e.g., sharks).

Protection: Acts as a physical barrier against bacteria, fungus, viruses, and parasites.
Classification: Scale type and distribution patterns help to identify and classify species.
Osmoregulation, Excretory, and Respiratory Activity: Skin and lining systems are involved in these essential functions.
Coloration: Sensory structures and pigment cells contribute to fish color.
Aging: Determining fish age from scale rings based on seasonal growth variations.
Spawning: Spawning Marks (e.g., in Salmon) indicate reproductive stages and habits.

Paired fins: Pectoral and pelvic fins.
- Pectoral fins: Used in swimming, orientation, and braking. They often contain sensory structures.
- Pelvic fins: Stabilizing and braking of fish during swimming. They contain sensory structures and are used as support for some bottom feeders.
Unpaired fins: Dorsal, anal, and caudal fins.
Dorsal fin: Stabilizing during swimming and in achieving quick changes of direction. Three types based on structure (single/dual and different structure combinations).
Anal fin: Helps in braking in conjunction with caudal and dorsal fins. It contains sensory structures and in some species is modified for reproduction.
Caudal fin (Tail fin): Shape and size of the caudal fin are linked to swimming habits. It is a major organ for propulsion. Shapes like lunate, forked are related to different habits.

The lateral line is a series of sense organs in the form of open tubes that run along the body side of fish down to the tail.
It helps to detect minute disturbances and vibrations in water, including chemical changes and electrical currents.
The system consists of canals with pores that allow water to enter. Nerves connect to the brain to process these signals, enabling the fish to sense its surroundings. Blind fish use the lateral line for survival.