Question

Which one is a true fish?
A) Jellyfish
B) Starfish
C) Dogfish
D) Silverfish

Answer 1

Hint: A true belongs to the phylum Chordata and superclass Pisces under the class Tetrapoda.

Complete solution:
A true fish belongs to the phylum Chordata and vertebrata, so they have a backbone. They have had notochord during the embryonic development which is now replaced by the backbone. They belong to class pisces and have a single chambered heart. They breathe through gills and have fins for propulsion.
Let us analyse one by one the different types of fishes mentioned in the options in order to know which one is an actual or true fish.
Jellyfish belong to the invertebrate phylum Coelenterata. Starfish belongs to the invertebrate phylum Echinodermata. Silverfish belong to invertebrate phylum Arthropoda, so it is an insect. All three of these are not true fishes. They are invertebrates and do not possess any feature of true fish that we have studied earlier.
Dogfish is a common name for Scoliodon. It is a kind of shark, that is a cartilaginous fish, classified under subclass chondrichthyes in class Pisces. It is a true fish that falls under phylum chordata.

Therefore, the correct answer is option C.

Note: Chondrichthyes are cartilaginous fish, that is, their skeleton is made up of cartilage, rather than bone. All sharks, skates, and rays (e.g., the southern stingray) are cartilaginous fish. They lack swim bladders and buoyancy is maintained with oil stored in livers. Members primarily inhabit marine ecosystems. However, some exceptions are present. They also lack operculum. The tail is heterocercal in nature.