Pisces: Locomotion | Zoology Optional Notes for UPSC PDF Download

Locomotion in Fishes

Methods of Swimming

1. Lateral Undulations:

   • Fish swim by the alternate contraction and relaxation of body muscles, known as myomeres.
   • This results in lateral undulations, causing the fish to oscillate from side to side during swimming.

2. Fin Movements:

   • Various movements of the fins contribute to swimming.
   • Fins play a crucial role in controlling direction, stability, and maneuverability.

3. Jet Propulsion:

   • Fish can achieve rapid movement by expelling water through the gill aperture in a sudden jet.

Chief Locomotory Organs

• The tail and caudal fin serve as the primary locomotory organs during rapid swimming.
• The tail lashes from side to side due to the alternate contraction and relaxation of myomeres along the vertebral column.
• Fins are utilized for slower movements.

Types of Locomotion (Breder, 1926)

1. Anguilliform:

   • Exhibited by highly flexible fishes with an elongated body, resembling eels.

2. Carangiform:

   • Undulations are mainly confined to the caudal region.
   • Seen in species like trout, rohu, mrigal, etc.

3. Ostraciiform:

   • Characterized by an inflexible body enclosed in a protective, box-like sheath.
   • Locomotion results from undulations of the caudal fin.
   • Examples include Ostracion, Tetraodon, Diodon, etc.

Muscles in Locomotion

• Coordinated action of body muscles and fin muscles drives fish locomotion.
• Connective tissue septa (myocommata) divide body muscles into segmental myomeres, aligning with the vertebrae.
• Fish with more myomeres exhibit greater flexibility (e.g., eels).
• Undulatory waves, originating from repeated contraction and relaxation of myomeres on opposite sides, generate thrust for forward propulsion.

Fin Muscles:

• Median fin movements, crucial for propulsion and steering, involve paired muscles at the fin-ray base.
• Key fin muscles include erector/protractor dorsalis, depressor/retractor muscles, and inclinator muscles.
• Caudal fin has additional muscles for expansion and contraction.
• Paired fins possess abductor and adductor muscles for outward and inward movements, respectively.

Dark and White Muscles:

• Myotomal muscles in teleosts comprise white (light) and dark (red) muscles.
• White muscles, predominant in body muscles, constitute 80-90% of myotomal muscles, resembling striated muscles in other vertebrates.
• Dark muscles, rich in myoglobin, form a superficial layer under the skin, thickening along the lateral line region.
• Fast-swimming species (e.g., herrings, mackerel) have a higher proportion of dark muscles.
• White muscles, known as 'fast muscles,' lack haemoglobin and are utilized for rapid, sudden movements.
• Dark muscles, termed 'slow muscles,' have abundant mitochondria and broad Z-lines, constituting 5-10% of muscle mass.
• Fin muscles, often dark muscles, play a crucial role in species relying on fin movement for swimming (e.g., Ostraction, Tetraodon, Diodon).
• Dark muscles require substantial oxygen during active swimming, facilitated by enhanced vascularization like rete mirabile.

Types of Locomotion in Fish

1. Carangiform Locomotion:

• Undulating body movements generate forward thrust, primarily driven by the tail against water pressure.
• Tail plays a significant role in propulsion.

2. Anguilliform Locomotion:

• Serpentine, snake-like movements characterize this type.
• Forward thrust results from the fish body's pressure against the water, with the tail playing a minimal role.
• Caudal fin is often reduced in eel-like fishes.

3. Ostraciform Locomotion (Trunk Fish):

• Head and body enclosed in a rigid bony case; tail with caudal fin projects behind.
• Slow movement involves dorsal and anal fins, while rapid swimming is facilitated by lashing movements of the tail.

4. Balistiform Locomotion:

• Fish seldom flex the body and move forward through undulating movements of the median fins.
• Complete waves are visible along the fins.

5. Labriform Swimming:

• Species like catfish, parrotfish, surgeonfish use pectoral fins for locomotion.
• Also known as 'labriform' swimming.

6. Dorsal and Anal Fin Swimming:

• Some species like globe fish (Tetraodon), porcupine fish (Diodon), and sea-horse (Hippocampus) swim with the help of dorsal and anal fins.
• Pectoral fins contribute to slow movement, and water expelled during respiration aids in progression.

Other Methods of Locomotion:

• Bottom Crawling: - Some fish, like the angler fish (Antennarius), use well-developed and muscular pectoral fins to crawl slowly on the water bottom.
• Undulatory Movement and Migration: - Eels crawl on the bottom using body undulations and can migrate between water bodies on land.
• Land Movement: - Air-breathing fish like Clarias, Heteropneustes, and Channa can cross short distances on land using pectoral fins and spines.
• Tree Climbing: - Climbing perch (Anabas) climbs trees with the help of pectoral fins and operculum.
• Flying and Gliding: - Flying fish (Exocoetus) jumps into the air using tail force and glides with enlarged pectoral fins. - Some species like Cypselurus use both pectoral and pelvic fins for gliding.
• Jumping Out of Water: - Species like Salmon, Mugil, Cyprinus, and Magalops jump out of water with force to escape predators or overcome obstacles.
• Suction Attachment: - Sucker fish attaches to large, fast-swimming fish or boats using a dorsal sucker, traveling long distances without exerting its energy.