Freshwater

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Part II: Fish and How to Care for Them
What Really Makes a Fish Go?
At one time or another, everyone has stood transfixed, watching aquarium
fish glide effortlessly through the water and wondering how they navigate
through their liquid environment with such ease. What makes a fish swim
better than we do? The answer is really quite simple.
Fish have a set of fins (six or seven of them, depending on the species) that
they use for locomotion. They also have a cool organ called a swim bladder
that helps them stay afloat (otherwise they would sink; see the section “The
swim bladder” near the end of this chapter for more). Fish have evolved to
conquer their watery environment with adaptations that have created the
perfect aquatic swimming machine. By contrast, even using rubber fins,
humans can achieve only a pale imitation of their aquatic friends.
To understand what makes a fish go, you must first understand each fin’s function.
Each individual fin has a specific job to do, and the combined effort of all
of a fish’s fins is what propels her through the water and helps her navigate
smoothly. Fin functions are an interaction of muscle power and sheer grace.
The dorsal fin
The dorsal fin is located along the back of the fish between the tail fin and the
head. This is the classic fin you see slicing through the water in the movie
Jaws. If you happen to see a Jaws-type fin while swimming in the ocean, you
may want to take up beach volleyball for a while. Fortunately for your peace
of mind, the dorsal fins on your aquarium fish generally remain underwater.
The dorsal fin provides lateral stability so that your fish can swim in a
straight line. Controlled swimming conserves energy. A fish that cannot swim
well doesn’t live very long because it can’t compete for food with its tankmates.
Each fin consists of a series of individual rays (fin segments, some soft
and some hard) loosely bound together by a membrane web.
A few aquatic pets, such as some goldfish and knifefish, do not have dorsal
fins. They have great difficulty swimming normally because they cannot keep
their movement in a straight line. Other species such as rainbowfish have
two dorsal fins.