| Body temperature |
Cold-blooded (poikilothermic) |
Unable to regulate body temperature internally |
Save energy and adapt to different environments |
| Body shape |
Spindle-shaped or streamlined |
Reduce water resistance and drag |
Move faster and more efficiently in water |
| Body covering |
Water-proof dermal scales (placoid, cycloid, ctenoid or ganoid) |
Provide protection and support to the skin |
Prevent water loss and infection, and reduce friction |
| Fins |
Paired pectoral and pelvic fins and median dorsal and caudal fins with dermal fin-rays |
Provide thrust and balance for swimming |
Control direction, speed and stability in water |
| Respiration |
Gills with 5 to 7 pairs of gill-slits (naked or operculated) |
Allow gas exchange with dissolved oxygen in water |
Breathe underwater without surfacing |
| Circulation |
Two-chambered heart (one auricle and one ventricle) and venous system |
Pump deoxygenated blood to the gills and then to the rest of the body |
Maintain blood pressure and oxygen delivery |
| Skull and jaws |
Well-developed skull and visceral arches. First pair of arches forms upper and lower jaws |
Provide support and protection to the brain and sensory organs. Allow biting and chewing of food |
Enhance feeding efficiency and diversity |
| Endoskeleton |
Cartilaginous or bony. Notochord usually replaced by vertebrae |
Provide internal framework for attachment of muscles and organs. Allow flexibility and movement of the body |
Support body weight and shape, and enable locomotion |
| Brain and nerves |
Brain with five parts and ten pairs of cranial nerves. Lateral line system present |
Coordinate sensory input and motor output. Detect vibrations and pressure changes in water |
Process information and respond to stimuli. Navigate and communicate in water |
| Ear |
Internal ear with three semicircular canals. No tympanic cavity or ear ossicles |
Detect sound waves transmitted through water and bone conduction. Maintain balance and orientation |
Hear underwater sounds and avoid predators or prey. Keep equilibrium in water |
| Nostrils |
Paired nostrils that do not open into pharynx, except in Dipnoi |
Detect chemical signals in water (olfaction) |
Smell food, mates or danger in water |
| Digestion |
Alimentary canal with stomach and pancreas. Terminates into cloaca or anus |
Break down food into nutrients with the help of digestive enzymes. Eliminate waste materials from the body |
Absorb energy and materials for growth and maintenance. Regulate osmotic balance |
| Excretion |
Mesonephric kidneys and ureotelic mode |
Filter nitrogenous wastes from the blood and excrete them as urea. Regulate water and salt balance in the body fluids |
Remove toxic substances from the body. Adapt to different salinities |
| Blood cells |
Nucleated erythrocytes (red blood cells) |
Contain hemoglobin that binds oxygen in the blood. Have a nucleus that allows DNA repair and gene expression |
Transport oxygen to the tissues. Survive longer and cope with stress |
| Reproduction |
Separate sexes and paired gonads. Fertilization internal or external. Females oviparous, ovoviviparous or viviparous |
Produce gametes (sperm or eggs) that fuse to form zygotes. Give rise to offspring either by laying eggs or giving birth to live young |
Ensure genetic variation and continuity of the species. Adapt to different reproductive strategies |
| Eggs and cleavage |
Eggs with large amount of yolk and meroblastic cleavage (partial division of the egg) |
Provide nourishment to the developing embryo. Allow rapid cell division without increasing the size of the egg cytoplasm |
Support embryonic development until hatching or birth. Save time and energy for cell division |
| Extra-embryonic membranes |
Absent |
Not required for aquatic development |
Avoid complications of membrane formation |
| Development |
Direct without or with little metamorphosis |
Not required for drastic changes in body form or habitat |
Avoid larval stages that may be vulnerable to predators or competition |
