Question

Which one of the following is a true fish?
A. Jellyfish
B. Starfish
C. Dogfish
D. Silverfish

Answer 1

Hint: The term fish is specifically used to describe any animal having backbone and gills throughout life and has limbs, if any, in the shape of fins. There are many types of aquatic animals which have common names ending in "fish" , that are not fish in this sense.

Complete answer:
You may already know, The Exocoetus is described as the 'flying fish'. They belong to the marine fishes’ family of class Actinopterygii. These are true fishes because they have streamlined bodies along with scales and they respire through gills.
Let’s look at Jellyfish which is a member of phylum Cnidaria, whereas silverfish is a member of phylum Arthopoda. Also remember, Starfish belongs to phylum Echinodermata. Dogfish belongs to class Chondrichthyes (super class Pisces).
It’s important that True fishes are considered as members of super class Pisces and have a streamlined body with scales. Dogfish possess a cartilaginous skeleton.
Dogfish is referred to as true fish, amongst Dogfish, Jellyfish, Starfish, and Silverfish. Jellyfish is not listed as a fish as it does not meet the characteristics of being a fish. Recall that, A fish should have gills, fins and a vertebra which is all lacking in a jellyfish. On the other hand, starfish belongs to the same criteria as a jellyfish, and is a star-shaped aquatic animal which leaves stuck to a surface. Whereas, Silverfish is like aquatic insects instead of fish.

Therefore, our correct answer is option C.

Additional information:
Seahorses are referred to as fish because they live in water, they breathe through gills, and also possess a swim bladder, but they do not have caudal fins instead have a long snake-like tail.

Note: Remember, there are various types of aquatic animals commonly called "fish”, but are not fish in the true sense. For example- starfish, shellfish, cuttlefish, crayfish and jellyfish. On the other hand, by definition all mammals (including cetaceans) like whales and dolphins, are not considered fish.