D
- Deciduous: temporary, falling off
- Demersal: living on or near the sea bed
- Dendritic: resembling a tree or shrub
- Denature: the "unfolding" of a protein resulting in a lessening of its biological properties. In the case of some fish toxins, denaturing with hot water can lessen painful symptoms.
- Dentate: with tooth-like projections
- Denticle, denticulate: a little tooth, having an edge with small projecting teeth, the placoid scales of cartilaginous fishes
- Depressed: flattened from top to bottom
- Dermal: pertaining to the skin
- Dewlap: a fold of loose skin
- Disc: the flattened head and body of various fishes such as stingrays, which also commonly includes the pectoral and ventral fins
- Distal: remote from the point of attachment
- Dorsal: pertaining to the back
- Dorsum: the upper (dorsal) surface of the head or body
- Dorsal fin: the fin on a fish's back
- Dorsal fin origin: the most anterior point of the dorsal fin base
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