Welcome to Osteichthyes
Cephalochordata Urochordata Haikouella Myxiniformes Petromyzontiformes
Chondrichthyes Actinopterygii Sarcopterygii calcium salts in cartilaginous
proto-vertebrae, true gills, two eyes, olfactory lobes
many molecular characters
dorsal hollow nerve cord, notochord, post-anal tail
Surprisingly, the synapomorphy for Osteichthyes is ... the presence of a gas filled
structure called the gas bladder (the bladder looks silvery because it is covered with
guanine crystals, which makes it practically impermeable to gas leaks).
lymphocytes have unique antigen receptors
What is bone?
Another type of hard tissue composed of:
a matrix of (mainly) collagen fibers and
hydroxyapatite crystals 3(Ca3PO )4 2a(OH) (~ 20% of the weight, rigid,
water (~ 5%)
produced by specialized cells called osteoblasts. Blood vessels, nerve fibers run
throughout the bone in canals
Specialized cells called osteoclasts are responsible for breaking bone down. Osteoblasts
then produce new bone matrix to replace the digested area (in other words, bone can
The evolution of bone: it’s older than we thought
bone is found in a number of extinct fishes that existed before Osteichthyes originated.
This is why bone isn’t an autapomorphy for the “bony fishes”: †
Cephalo. Urochor. Haikouella Myxin.
[ -------- extinct jawless “fishes” --- ] [ -------- Gnathostomata --------------- ]
† † † †
† Chondrichthyes Osteichthyes
= loss of ability to make bone
(aktis [ray] + pteron [wing]) = ray-finned fishes
Autapomorphy for the Actinopterygii: The presence of ganoine in scales. Ganoine is
shiny, hypermineralized (~ 97% hydroxyapatite), acellular material laid down in thick,
successive layers producing “growth rings”
Actinopterygians, their big evolutionary innovations
......... nothing major to begin with ......... but
the ray-finned fishes are both incredibly
species-rich (at approx. 32,000 species, they are the largest vertebrate group) and
incredibly ecologically diverse.
WHAT HAPPENED TO MAKE THEM SO DIVERSE?
1. How not to sink like a rock in water
Density = mass/volume
Specific gravity = density of object/density of water
water has a specific gravity of 1.0, while salt water has a s.g. of ~ 1.026 so
specific gravity is > 1.0 you sink (in pure freshwater)
If your specific gravity is < 1.0 you float (in pure freshwater)
How to change your density: Chondrichthyes Solutions •cartilaginous endoskeleton (s.g. cartilage 1.1): all Chondrichthyes
•simply stay on the bottom and don’t worry about it (some skates and rays)
•liver full of lipids (s.g. 0.90 - 0.92; squalene: s.g. 0.86). For example, great white
sharks have a large liver that is ~ 90% oil (and this makes up ~ 30% of
their total body weight) = overall specific gravity close to 1.026 (almost neutrally
buoyant in salt water)
the benefit: oil does not compress like gas, so sharks can change depth very rapidly
(extensive vertical range in the water)
the cost: in order to change depth, you must swim, which is energetically costly
Osteichthyes has a new problem: the bony skeleton (s.g. of bone is ~ 2.0 so bony
fishes are much heavier than water. They will sink unless ....)
store lipids in skin, muscle, bones, liver
swim continuously (many big pelagic predators such as tuna, marlin) or stay on the
bottom and don’t worry about it (benthic fishes like sole, flounder)
the most common solution: the gas bladder (Good thing #1) But pressure
changes as you move up and down in the water, so how do you control the
volume of gas in your bladder?
There are three general types of gas bladders in ray-finned fishes:
1. Physostomous (physa [bladder] + stoma [mouth]) type 1 (the ancestral gas bladder
muscles around duct
esophagus gut pneumatic duct
To fill the bladder (the fish move upwards): come to surface and gulp air, which travels
from the mouth to the esophagus then through the pneumatic duct (red arrow points to it)
into the gas bladder.
To empty the bladder (the fish moves downwards): the muscles around the pneumatic
duct contract and open the passage between the pneumatic duct and the bladder. Gas
passes from the bladder into the esophagus and is spat out through the mouth (gass-
Found in the oldest ray-finned fishes, e.g., sturgeon. Doesn’t allow you to live in very
deep water because you need to come to the surface to fill you gas bladder. Not the most
effective gas bladder but provides some reduction in density.
2. Physostomous type 2