WHAT MAKES AN ANIMAL A FISH?
Fishes evolved more than 500 million years ago, which makes them one the earliest vertebrates on our planet. They are a diverse group of animals that spend all their time in water.
Fishes are all vertebrates i.e. animals with backbones, and live in water, they have gills which aids in breathing inside water. Most have fins which are used for swimming, scales on their exterior which serves as a fortification and a streamlined body which is designed to ease their movement through water.
Based on the type of habitats they inhabit, fishes can be classified into two groups, which are; saltwater and freshwater. About an estimate of over 35% of fish species are known to live in freshwater, while the rest are mostly found in salt water, with a minute few other percentages inhabiting the brackish and fairly salt waters.
They all appear with varying combinations of shapes and sizes. While some species live out in the open ocean and don’t get close to the seafloor, others species however live in the depths and never get near the surface. Some fish species live in kelp forests or on the seafloor in so-called reef communities. There are herbivores, carnivores and omnivores among the population of various fish species. The total number of fish species on our planet is thought to be at over 27,000, with the largest fish in the world reaching lengths of about 15 meters, and weighs more than 13Kg.
Based on jaw types, fishes can be grouped under Jawless and Jawed fish. The hagfish and lampreys make up the jawless fish; the jawless species are known to have tongues and numerous teeth; while the jawedfish are grouped under bony (with rigid bone) and cartilaginous fish (with cartilage bones).
A fish’s ability to survive depends upon the urgency of its surrounding information gathering using its sensory organs. Fishes have a keen sense of smell, touch, feel and taste. They can feel out vibrations and possess a good sight; many fishes see in color while others are able to see in very low light levels. Their senses of taste and smell are well-developed as they possess a pair of nostrils on each side of their head and taste buds on their lips, tongue and mouth. Fishes also have ears but it isn’t visible as their ears are internal organs.
A number of species have a mate for life, and some spawn in large groups and leave their mate after mating. Some bear live young while others lay eggs. Some species care for their young, while other species do not. Every year, hundreds to thousands of fish aggregate at highly predictable locations to spawn. They produce a larva that will, after a required number of times in the plankton, go in to the reefs.
Scientists have proved that the survival of the larvae of a reef fish depends squarely on when they were spawned.
Climate change, invasive species and habitat destruction are the biggest threats to the world’s fish population.