Superclass : Pisces : General Characters

Character	Description	Reason	Advantage
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