Unit 3: Agnatha and Fishes

1. Agnatha (Jawless Fishes)

Agnatha is a superclass of jawless vertebrates. They represent the most ancient lineage of vertebrates. The only living members belong to the Class Cyclostomata.

1.1 General Characteristics and Classification of Cyclostomes

General Characteristics

• No Jaws: This is their defining feature. They have a circular, sucking mouth.
• No Paired Fins: They lack pectoral and pelvic fins, contributing to their eel-like movement.
• Skeleton: Entirely cartilaginous; no bone.
• Notochord: Persists throughout life.
• Gills: Gills are in the form of 5-16 pairs of gill pouches.
• Skin: Smooth, scaleless, and contains slime glands.

Classification (Class Cyclostomata)

Order	Common Name	Key Characteristics	Example
Myxiniformes	Hagfishes	Exclusively marine. Scavengers. Mouth with barbels. Produce enormous amounts of slime for defense.	Myxine
Petromyzontiformes	Lampreys	Found in marine and freshwater. Many are parasitic, attaching to other fish with their toothed, funnel-like mouth. Have a well-developed larval stage (ammocoete).	Petromyzon

2. Gnathostomata (Jawed Fishes - Pisces)

Gnathostomata includes all jawed vertebrates. The evolution of jaws was a revolutionary step, allowing for an active, predatory lifestyle. "Pisces" is the group containing all fishes.

2.1 Classification of Fishes (Up to Order)

Class 1: Chondrichthyes (Cartilaginous Fishes)

• Subclass Elasmobranchii (Sharks, Skates, Rays)
  • Order Carcharhiniformes: Ground sharks (e.g., Tiger shark, Hammerhead shark).
  • Order Rajiformes: Skates and Rays.
• Subclass Holocephali
  • Order Chimaeriformes: Chimaeras or "ratfish."

Class 2: Osteichthyes (Bony Fishes)

• Subclass Actinopterygii (Ray-finned fishes)
  • Order Cypriniformes: Carp, Minnows, Labeo (Rohu), Catla.
  • Order Siluriformes: Catfishes.
  • Order Perciformes: Largest vertebrate order. Perch, Tuna, Mackerel, Anabas.
  • Order Clupeiformes: Herrings, Sardines, Hilsa.
• Subclass Sarcopterygii (Lobe-finned fishes)
  • Order Coelacanthiformes: Coelacanths (e.g., Latimeria).
  • Order Dipnoi: Lungfishes (e.g., Protopterus).

2.2 General Characteristics of Chondrichthyes and Osteichthyes

This is a classic exam question. The key differences relate to the skeleton, gills, scales, and buoyancy.

Feature	Chondrichthyes (e.g., Shark)	Osteichthyes (e.g., Labeo)
Skeleton	Entirely cartilaginous.	Bony (ossified) endoskeleton.
Gills	5-7 pairs of gill slits, not covered by an operculum (except in Chimaera).	4 pairs of gills, protected by a hard, bony flap called the operculum.
Scales	Placoid scales (tooth-like structures).	Cycloid, ctenoid, or ganoid scales. (Placoid scales are absent).
Buoyancy	No swim bladder. Buoyancy is maintained by a large, oily liver and dynamic lift from fins.	Swim bladder (air sac) present for neutral buoyancy. (Absent in some bottom-dwellers).
Mouth Position	Ventral (on the underside).	Terminal (at the front).
Tail Fin	Heterocercal (asymmetrical, with the vertebral column extending into the upper lobe).	Homocercal (symmetrical).
Fertilization	Internal. Males have claspers.	Usually external.

3. Fish Biology

3.1 Migration in Fishes

Migration is the regular, mass movement of fish from one area to another, typically for feeding or reproduction. It is often triggered by environmental cues like temperature or day length.

• Diadromous Migration: Migration between freshwater and saltwater.
  • Anadromous: Live in saltwater, migrate to freshwater to breed. (e.g., Salmon, Hilsa).
  • Catadromous: Live in freshwater, migrate to saltwater to breed. (e.g., Eel).
• Potamodromous Migration: Migration that occurs entirely within freshwater (e.g., from a river to a lake).
• Oceanodromous Migration: Migration that occurs entirely within saltwater (e.g., Tuna).

3.2 Osmoregulation in Fishes

Osmoregulation is the control of water and salt balance. This is a major challenge for fish living in different salinities.

In Freshwater Fish (e.g., Labeo)

• Problem: The fish is hypertonic to its environment. Water constantly enters its body (by osmosis), and salts are lost.
• Solution:
  1. Does not drink water.
  2. Produces large amounts of dilute urine to excrete excess water.
  3. Actively absorbs salts from the water using salt-absorbing cells in the gills.

In Marine Fish (e.g., Shark)

• Problem: The fish is hypotonic to its environment. It constantly loses water to the sea.
• Solution (Bony Fish):
  1. Drinks large amounts of seawater.
  2. Excretes excess salts through salt-secreting cells in the gills.
  3. Produces very small amounts of concentrated urine.
• Solution (Cartilaginous Fish - Sharks):
  • They retain high levels of urea and TMAO (trimethylamine oxide) in their blood.
  • This makes their blood slightly hypertonic to seawater, so they actually absorb a small amount of water by osmosis and don't need to drink.

3.3 Parental Care in Fishes

While many fish simply release eggs and sperm (broadcast spawning), many species exhibit complex behaviors to increase the survival of their offspring.

• Nest Building: Many fish build nests, from simple pits in the gravel (e.g., Salmon) to complex bubble nests (e.g., Gourami, Betta).
• Guarding: The male or female (or both) will guard the eggs and fry from predators.
• Mouth Brooding: The parent carries the eggs, and sometimes the young fry, in their mouth. (e.g., Tilapia, some catfish).
• Pouch Brooding: The male Seahorse has a brood pouch where the female deposits her eggs. He fertilizes them and carries them until they hatch.
• Internal Bearers: Some sharks and guppies are ovoviviparous (eggs hatch inside the mother) or viviparous (young are nourished directly by the mother).