bony-fish belong to the Actinopterygii; there are only eight living species of lobe finned fish,
including the lungfish (Figure13.10) and coelacanths (Figure13.11).
The vast majority of fish are osteichthyes, and this group is the most various of vertebrates,
making them the largest group of vertebrates in existence today. They are characterized
by a relatively stable pattern of cranial bones, and the head and pectoral girdles (arches
supporting the forelimbs) are covered with large dermal bones (bones derived from the skin).
They have a lung or swim bladder, which helps the body create a neutral balance between
sinking and floating, by either filling up with or emitting such gases as oxygen; have bone fin
rays (jointed, segmented rods) supporting the fins; have an operculum (a cover over the gill),
which helps them to breathe without having to swim; and are able to see in color, unlike
most other fish.
One of the best-known innovations of this group is the ability to produce endochondral or
“replacement” bone, by replacing cartilage from within, with bone. This is in addition to the
production of perichondral or “spongy bone.” The effect is to create a relatively lightweight,
flexible, “spongy” bone interior, surrounded by an outline of dense bone. This is still much
heavier and less flexible than cartilage.
The ocean sunfish is the most massive bony fish in the world, up to 3.33 m (11 ft) in length
and weighing up to 2,300 kg (5,070 lb) (Figure13.12). Other very large bony fish include
the Atlantic blue marlin, the black marlin, some sturgeon species, the giant grouper and the
goliath grouper. In contrast, the dwarf pygmy goby measures only 1.5 cm (0.6 in).
Figure 13.12: An ocean sunfish, the most massive bony fish in the world, up to 11 ft in
length and 5,070 lb in weight! ( 16 )