My roommates and I keep fish. We recently acquired a pair of balloon mollies (one of which has a swim bladder infection).
Anyway, one of the roommates asserted that mollies give live birth, rather than laying eggs. JuanitaTech says guppies do too.
My question: assuming he’s correct, why are they classified as Osteichthyes? As I understand it, an animal’s classification usually rests heavily on whether they lay eggs or give live birth.
I think that guppies (and possibly mollies too) are classified as Actinopterygii, which is sometimes considered a subclass of Osteichthyes, but it is also sometimes considered a distinct class of its own.
Its been awhile since biology class, though, so that may not be the most accurate information.
Guppies and mollies are both live-bearers, I’m sure of that.
There are a number of live-bearing tropical fish. I’ve had platy fry last for a while after birth, but I never had any success with swordtails.
This web page says that live-bearers belong to the family Poecilliidae. Osteichthyes refers to the group of bony fish as a whole, as opposed to cartilaginous fish like sharks. So, no reason why mollies can’t be Osteichthyes and live-bearing at the same time, AFAIK.
They’re Osteichthyes (no italics as its not a genus or species name) because they’re still bony fish (which is what Osteichthyes means), regardless of how they give birth because they have gills, have fins, are cold blooded (body temperature is dependent upon the environment) and have skeletons made of true bone and not cartilage. At the level of Osteichthyes on the taxonomic tree, which I believe is a “subclass” of Vertebrata, the other options are to be a primitive fish (Agnatha), cartilaginous fish (Chondrichthyes), amphibian, reptile, bird, or mammal. They’re obviously not any of these.
Animals are classified based on a number of different criteria. Live birth, while one of the characteristics of mammals, is not the defining characteristic of mammals. Several bony and cartilaginous species also give live birth as do several species of snakes. It’s the gills (as adults and not just juveniles, like amphibians) and fins that set fish (both bony and cartilaginous) apart from other vertebrates. The defining characteristic of mammals iare having live birth, having hair at some stage of their life (for dolphins it’s a few hairs on the nose that fall off shortly after birth), and producing milk to nurse their young. All three are necessary for something to classified as a mammal.
Dr. One-L
While your scientific knowledge is superior to my own, I have to ask the following:
If the primary characteristic of mammals is live birth, what then is a platypous?
[slight hijack][only because I felt that the OP was already answered]
Thanks, guys.
Very, very weird. 
I once heard or read about someone saying that if they get the chance to ask God questions in the afterlife, the first one would be “Why the platypus?”
Actually, you’re right. I’m mistaken. (You caught me-- my doctorate is really in molecular genetics and not zoology, so while I’ve picked up a lot via osmosis here at work, sometimes I misremember). Now that I’m home I’ve pulled out my National Geographic Book of Mammals, which reminds me that the 3 essential characteristics of mammals are actually that they are warm-blooded (as are birds), have hair, and nurse their young with milk. Which is true even for those wacky monotremes.
Dr. One-L, since the OP seems to be satisfied that his question has been answered, would it be considered hijacking if I asked you to tell me a little bit about molecular genetics? It sounds interesting. I find myself constantly regretting that I took a doctorate (JD not Ph.D or MD, but I am considering an LL.M) in a “soft” science.
Also note that while these livebearing tropical fish do give birth to live young, they are not placental animals–it’s just that they keep the eggs inside themselves until the eggs hatch. Mammals (well, most of them…) have placentas and all the mess that entails.
Indeed. There are some snakes that do this, too, so bearing live young is less important as a classifier than they sometimes make it sound in junior high.