Let's start this week off with a little quiz, shall we? What's in the picture below?
"Sand." Alright smart-ass, what else? "Shells."
Courtesy: Patrick Feller via Flickr
If you said mermaid's purse, or shark/skate egg then you're right. All of those animals are part of the chondrichthyes (pronounced: con-drick-thees) class of fish.These fish have skeletons made of cartilage rather than bone. We talked a lot about chondrichthyes diversity in the very first blog post if you'd like to get a refresher. The name mermaid's purse applies to the egg cases of all cartilaginous fish because they're leathery, and most are rounded squares like a purse. The leathery-ness is important too because it's tough but flexible. This means the case is tough enough to protect the embryo, but is flexible enough not to shatter. Instead of nesting; skates and egg-laying sharks just kind of drop their kids off somewhere sheltered on the bottom and get on with their lives. That might sound mean, but mom's body produces a yolk that's so rich and full of nutrients that the babies come out of the case fully formed and ready to survive. (Editor's Note: Depth and Taxa does not condone abandoning one's children with 18 year's worth of food and calling it good.) Laying eggs is a method of reproduction called oviparity, and it's only one of several different ways of developing your babies, all of which cartilaginous fish are capable of.
Oviparity is pretty familiar stuff, but let's go over it anyway because it's the root of the other types of reproduction in sharks and their relatives. Unlike many of their bony relatives, chondrichthyans all fertilize their eggs internally. You can actually distinguish males and females of these fish because males have what are called claspers on their pelvic fins (the fins closest to where hips would be.) Claspers are used to hold onto females during mating and deliver sperm into her vent. (the multi-purpose opening of many marine animals.)
The claspers are the little finger-like nubs on the inside of the fins.
Courtesy: eustatic via Flickr
Once the eggs are fertilized they develop the familiar embryo and yolk combination you might have seen shining a flashlight through a chicken egg. Then, like we talked about before, mom drops the eggs off and they develop until they hatch. While inside the mermaid's purse the embryo has only the yolk for nutrients, so when it runs out the baby starts to get hungry and that helps prompt hatching.
Plus the WiFi in the ocean is terrible, so there's no Netflix to
keep you perfectly still for weeks at a time.
Courtesy: Marian Gonzales via Flickr
Of the four types of chondrichthyes; sharks are less likely to lay eggs than some of their relatives. All skates lay eggs, as do all of the chimeras. None of the rays lay eggs, and only about a third of what we commonly call sharks don't give live birth.
The next type of fetal development is a weird combination of eggs and live birth. For a long time this was referred to as ovoviviparity (pronounced: oh-vo-viv-i-pair-itty), but this term is falling out of favor because it implies the fetuses aren't getting any nutrients from mom. Recent research suggests that many, but not all, sharks and rays with this means of development contribute at least some nutrients to their babies. This can be through secretions from the uterus that the unattached babies absorb through their skin or consume, or in the form of unfertilized eggs which the developing young eat after their yolks run out. In at least one species the first fetus to use up its yolk will actually eat its brothers and sisters before being born!
"Don't mess with me man, I have seen some s**t "
Courtesy: Justin Morgan via Flickr
What's consistent across these means of development is that the embryos are never physically attached to their mom. Most of the sharks that give live birth exhibit these strategies for developing their young. All of that is pretty weird and cool, but buckle up, 'cause we're about to take everything we just talked about and add another layer of bizarre.
The last means of fetal development is called placental viviparity. You read that right, placenta like in mammals. Placental development in sharks is a perfect example of one of my favorite concepts in biology: convergent evolution. Convergent evolution is when two very distantly related organisms develop similar traits or strategies completely independent of one another's genes. So even though some sharks have a similar fetal development strategy to mammals it doesn't mean we're related or that we got that trait from sharks.
Amazingly, in placental sharks, the embryos still start out with a yolk. Like other fish the baby shark starts off using up the yolk's nutrients, but late in this process mom's body supplies some of those nutritious secretions we talked about earlier. While her body does this, the lining of the yolk sac actually stretches out and fills with blood vessels. It reaches from the belly of the developing shark to the mom's uterine lining where it attaches and acts as the link between the two for gas exchange (getting oxygen in and CO2 out) and metabolism (getting nutrients in and waste out). This incredible strategy has developed in only a few species of sharks.
"Hey mammals, who's 'highly evolved' now huh!?"
Courtesy: Serena Epstein via Flickr
All of these strategies are spectacular means for getting chondrichthyan babies out into the world ready to survive from the second they emerge. By fully developing inside an egg or their mother; sharks, skates, rays, and chimeras have set themselves up as some of the most successful animals on the planet.
Hamlett, William C., "Evolution and Morphogensis of the Placenta in Sharks", Journal of Experimental Zoology, 1989, vol. 252(S2), pp. 35-52
Musick, J.A. and J.K. Ellis, "Reproductive Evolution of Chondrichtyes", pp. 45-79, In: "Reproductive Biology and Phylogeny of Chondricthyes: Sharks, Batoids and Chimeras", William C. Hamlett, ed., Science Publishers Inc., Plymouth U, 2005
Wourms et al., "The Maternal-Embryonic Relationship in Viviparous Fishes", pp. 5-10, In: "Fish Physiology: Volume XI: The Physiology of Developing Fish Part B: Viviparity and Posthatching Juveniles", W.S. Hoar and D.J. Randall, Academic Press Inc., 1988