Fish Fins: Anatomy and Types

• Fins are fish's most distinctive anatomical features
• Fins are specialized organs of locomotion adapted for swimming in water.
• Fins consist of bony spines or rays that protrude from the body
• Skin covers and joins the bony spines or rays together
• Fins can be webbed or similar to a flipper
• Fins are located in different places on the fish and serve different purposes
• Fins help with moving forward, turning, keeping upright, and stopping

Types of Fins

• Unpaired or median fins
• Paired lateral fins

Unpaired/Median Fins

• These include one or two dorsal fins along the mid-dorsal line
• A ventral anal fin is located behind the anus or vent (cloaca)
• A tail or caudal fin is located around the tip of the tail
• Dorsal fins may appear in a series or be reduced or absent
• Anal fins may be absent, especially in bottom dwellers

Paired Lateral Fins

• Paired fins are the pectorals and pelvic fins
• Paired fins correspond to the fore- and hind-limbs of terrestrial vertebrates
• Pectoral fins are located anteriorly and pelvic fins posteriorly
• Both unpaired median and paired fins are supported by skeletal rods called radials and dermal fin rays
• Fin diversity in fish is due to their adaptive responsiveness

Fin Use

• Fish swim mainly by lateral movement of tail and tail fin
• Paired lateral fins maintain equilibrium when the body is at rest
• Lung-fishes use them as legs in walking
• Flying fish glide using their large and extended pectoral fins
• Pelvic fins in some male Chondrichthyes are modified as claspers
• Remoras use their anterior dorsal fin as an adhesive disc or sucker on their head

Caudal Fins (Tail Fins)

• Key for forward propulsion during swimming
• Caudal fin reduces in bottom dwelling rays
• Sometimes absent in sting rays
• Caudal fins have different shapes correlated with their habits

Types of Caudal Fins

• Diphycercal
• Heterocercal
• Homocercal

Diphycercal Fins

• Considered the most primitive type of caudal fin, not exhibited by many living fish
• Vertebral column extends to the tip of the tail
• Upper and lower portions of the tail are symmetrical or nearly identical
• Vertebral column extends horizontally to the end of the tail
• The fin has a single, large uniform lobe that lashes from side to side
• The tail is primitively symmetrical and pointed
• Occurs in modern cyclostomes, primitive sharks, Chimaera, lung-fishes

Heterocercal Fins

• Intermediate type where the vertebral column bends upwards
• Vertebral column reaches up to the tip of the more prominent dorsal lobe
• Makes the caudal fin strongly asymmetrical
• Characteristic of bottom feeders with a ventral mouth and no swim-bladder (air-bladder)
• Strokes of the larger dorsal lobe in swimming direct fish towards the bottom

Homocercal Fins

• Characteristic of higher bony fishes (teleosts)
• Symmetrical externally but asymmetrical internally
• Posterior end of the vertebral column is turned upwards and becomes greatly reduced
• Tip of the vertebral column does not reach the posterior limit of the fin
• There is no apparent dorsal lobe
• Ventral lobe is greatly enlarged and divided into two equal superficial lobes
• Caudal fin splits into two equal lobes in Labeo rohita

Caudal Fin Evolution

• Diphycercal fins are considered the most primitive
• Heterocercal fins are an intermediate stage
• Homocercal fins represent the advanced stage