- Jaws
- Two sets of paired fins
- Several unpaired fins
- Streamlined body
- Gills or an accessory breathing organ
- Lays eggs that are fertilized internally or externally
Figure 13.5: The humphead or Napoleon wrasse (Cheilinus undulates), showing some of the
general traits of fish, including scales, fins and a streamlined body. ( 23 )
There are exceptions to many of these traits. For example, tuna, swordfish, and some species
of sharks showsome warm-blooded adaptations, and are able to raise their body temperature
significantly above that of the water around them. Some species of fish have a slower, but
more maneuverable, swimming style, like eels and rays (Figure13.6). Body shape and the
arrangement of fins are highly variable, and the surface of the skin may be naked, as in
moray eels, or covered with scales. Scales can be of a variety of different types.
Although most fish live in aquatic habitats, such as the ocean, lakes, and rivers, there are
some that spend considerable time out of water. Mudskippers, for example, feed and interact
with each other on mudflats for up to several days at a time and only go underwater when
occupying burrows (Figure13.7). They breathe by absorbing oxygen across the skin, similar
to what frogs do.
Agnatha: Jawless Fishes
Agnatha is a superclass of jawless fish belonging to the phylum Chordata, subphylum
Vertebrata (agnath meansjawless). There are two extant (living) groups of jawless fish, the
lampreys and the hagfish, with about 100 species in total. Although hagfish belong to the
subphylum Vertebrata, they do not technically have vertebrae.