Mommy, Where do Baby Sharks Come from?

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.

References:

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

Accessed via: http://www.vims.edu/people/musick_ja/pubs/Repro_Evol_of_Chondrichthyans.pdf

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 

They're in the Trees Man!

It's autumn here in the Northern hemisphere, and in the Pacific Northwest many of our salmon species are making their return to the rivers they were born in. This amazing phenomenon has been well documented on TV, but there is an incredibly cool piece to the story that's often missing. One that weaves the ocean, the river, and the land together and shows us that nothing is alone in the environment.

Pacific salmon are a pretty cool group of fish, but honestly it can be really hard to agree on just what the heck a salmon is. This confusion comes from old terms for the same fish doing different things. Ever noticed how salmon and trout look almost exactly the same on the outside? Well that's because they pretty much are. All trout, salmon, char, freshwater whitefish, and graylings are part of the salmonid family. Amazingly many of these fish can spend their entire lives in freshwater, or they can spend part of it in fresh and part out at sea. Fish that have a life cycle which takes them back and forth between salt and fresh water are called anadromous (pronounced an-ad-row-muss) fish. Weirdly enough some species have a freshwater exclusive and an ocean going form, and they get different common names because of it. For example a rainbow trout (Oncorhynchus mykiss) lives in freshwater exclusively, but a steelhead (also Oncorhynchus mykiss) goes from fresh to salt and back again. It's genetically the exact same fish, but because steelhead fill up on tasty ocean plankton they get much bigger and their meat turns a lot pinker.  

"I haven't decided which I want to be yet. I'm taking classes in both and seeing which I like more."

Courtesy Ingrid Taylar via Flickr

Honestly the rest of this post could be about what is and what isn't a salmon, but that can get tedious and there's other things to get excited about this week. In general when people talk about Pacific salmon they're referring to one of five different species, which are all in the genus Oncorhynchus which means hooked nose. These are the coho (O. kisutch), pink (O. gorbuscha), chum (O. keta), chinook (O. tsawytscha), and sockeye (O. nerka). Aside from being many species instead of just one, Pacific salmon differ from Atlantic salmon (Salmo salar) by being terminal spawners. After they reproduce all five of the species listed above die. When I first learned this it seemed so sad and pointless to me. After all Atlantic salmon don't die after spawning, but it turns out the deaths of the adult Pacifics bring enormous amounts of nutrients into inland environments. 

"It's cool birds, I wasn't using my eyes anyway."

Courtesy Lewis Kelly via Flickr

When thousands of salmon flood a stream and die there, their bodies begin to decay in the water, but look at that picture above. Where's the shore? That fish is lying out in the middle of the woods. Even if the shore is just off camera a few feet I guarantee that fish didn't have "walk on land" as part of his bucket list. So how'd he get there? Well the answer is probably a bear. 

Bears are good swimmers, they love the fattiness of salmon, and they don't mind scavenging on rotting food. Bears and other animals drag salmon away from the streams to munch in peace and the parts they don't eat mix into the soil. Then plants in the area pick up those nutrients and use them to grow. One study found that trees without salmon nutrients grew about 2/3rds as fast as those with them. So trees are, through salmon, taking nutrients from the ocean and using them to grow; and there are salmon streams that are as far East as Idaho (That's 450 miles in a straight line from the mouth of the Columbia River.) where oceanic nutrients can be detected in the trees. It's not just the trees either; studies have found oceanic nutrients in the shrubs, ferns, insects, birds, amphibians, fish, and mammals of these environments. 

No wonder we call him the King

Courtesy spappy.joneS via Flickr

The way we know know this is pretty cool too. Scientists use isotope analysis to see how much of a certain type of Nitrogen is inside the trees. You can kind of think of isotopes as sub-species of atoms. They're not all unique enough to warrant calling them something else, but they often behave a little bit differently. Different environments favor the production and preservation of different types of each atom. The ocean, as it happens, is very favorable to the form of Nitrogen that has an extra neutron. So researchers are able to burn samples from the trees and use a cool device called a mass spectrometer to figure out how much of their chemical composition came from the ocean. At one site in Canada they found that in some years up to 80% of the Nitrogen available for Sitka spruce (Picea stichensis) came from those years' salmon runs.

It's become increasingly clear that salmon are important for the health of Pacific forests. And the implication is astonishing. If we want healthy trees, that grow more rapidly, create more diverse habitat, scrub carbon from the atmosphere, and produce more lumber, then we want healthy salmon. Amazing large scale projects with that goal in mind are already happening, and keeping salmon streams healthy is as easy as making sure you pick up after yourself when you visit a river. It may be a long time before we see anything close to historic runs again, but so much is being done on every level of the community that I'm confident we can make a difference.

If that seems hard to believe, remember all of these trees are partly made of fish.
The world is way weirder and cooler than we ever expect it to be.
Courtesy ArkanGL via Flickr

References:

"Family Salmonidae: Salmons and Trouts", The Burke Museum online

accessed via: http://www.burkemuseum.org/static/FishKey/salmon.html

Reimchen, Tom, "Salmon nutrients, nitrogen isotopes and coastal forests", Ecoforestry, Fall 2001.

Accessed via: http://web.uvic.ca/~reimlab/reimchen_ecoforestry.pdf 

Reimchen et al. "Isotopic Evidence for Enrichment of Salmon-Derived Nutrients in Vegetation, Soil, and Insects in Riparian Zones in Coastal British Columbia.", American Fisheries Society Symposium, XX: 000-000, 2002

Moore, J, & Schindler, D, (2004) "Nutrient export from freshwater ecosystems by anadromous sockeye salmon (Oncorhyunchus nerka) Canadian Journal of Fisheries and Aquatic Sciences, Vol. 61, 2004