Sunday, January 25, 2015

On the Origin of Fish Face

Fish face. It's one of the first impressions we do of an animal, and the second you say those words almost everyone knows what you're talking about. Hands at the side of your head, cheeks sucked in, lips puckered, and moving your lips open and closed.

Yeah that's the one! Not really sure how the top hat fits in, but whatever.
Courtesy: Mark Norman Francis via Flickr

Opening and closing their mouth over and over is characteristic of many of the bony fish that we see most often, especially gold fish. Why are they doing that? Well they're breathing, and we're just seeing them getting water into their bodies. It's a bit like your stomach or chest going up and down as you breathe. When you make a fish face your hands on the side of your head moving with your breathing are mimicking an important part of the bony fish breathing system. It's not the fins (although I know that's what of lot of people are mimicking), and it's not technically the gills, it's the gill cover. It might seem like a silly distinction, but in fish the gill cover is as different from the gills as your nostrils are from your lungs. If it helps you can call the gill cover by its more scientific name, the operculum (oh-perk-you-lum).

To breathe, the opercula (plural) close and the fish creates negative pressure to pull water into its mouth. Then the mouth closes and the opercula open pushing water across the gills and out the fish's body. The gills themselves though, are the coolest part of this system.

This is either a fish gill or someone scalped Ronald McDonald.
Courtesy: Allan Reyes via Flickr

Gills are really amazing because they take huge amount of surface area and bundle it into a tiny package. One study found that Atlantic horse mackerel (Trachurus trachurus) can have 14 square meters of surface on their gills! That's a greater area than the floor plan of this guy's house, all packed into a 70cm long fish. How is that possible? Each of those red filaments coming off of that yellow arch are covered in microscopic sheets called lamellae (lam-ell-ay). There are a ton of filaments on each arch and even more lamellae on each filament. It's just another great example of how tiny things can really add up in surprising ways.

Scanning electron micrographs of filaments on the left and lamellae on the right
Courtesy: Evans et al. 2005

All of that surface area is very important to the fish's ability to breathe underwater. Fish don't actually breathe the water itself. It would take a huge amount of energy to break water molecules apart to get at the oxygen, so instead animals with gills breathe tiny bubbles of gas dissolved in the water. If you can't picture that, think about a soda bottle. None of the bubbles are visible until you take the cap off the bottle. Just like the horrifying amount of sugar in there; the bubbles are really small and surrounded by water. When the cap comes off the tiny bubbles expand, slam into each other, make bigger bubbles, and escape. 

The problem is that there's only a fraction of the oxygen available in air, in the water. So gills actually need to be more efficient than lungs at removing oxygen from their surroundings All of those lamellae are in contact with the water and have deoxygenated blood running just under their surface (that's why they're dark red). The blood has less oxygen in it than the water nearby so the oxygen moves from the water into the blood. Nature doesn't care for an uneven distribution. 

Communist implications of nature aside, it's a really efficient system for getting an essential chemical into the body. It's also great for getting waste out of the fish's body. Not only can CO2 leave through the gills, but other waste as well, nitrogen based wastes like ammonia especially. Even though fish do have kidneys that process waste; the gills are just so damn efficient at exchanging chemicals with the water that they actually excrete most of the fish's nitrogenous waste. In some species up to 80% of the ammonia and urea are excreted through the gills. Imagine if we only had to pee one fifth as often because we breathed our waste out. Netflix binges would be so much easier.

"Oh god! Is that what all these warm spots are?"
Courtesy: Alex Derr via Flickr

There's one more question I want to pose to you all this week. If fish breathe gas inside the water, why do they suffocate in dry air? It could be that their gills need to be wet to work. Which makes sense since our lungs are wet and protected from outside. But there's more to it than that. It could also be that it's harder for gasses and materials to move between two different media, in this case gas and liquid, which is also true. But one of the most significant reasons is that when a fish comes out of water the filaments and lamellae aren't buoyed up by the water and they lay flat on one another. When they bunch up, the surface area on the gills decreases, and the efficiency benefit is lost. So if you're doing catch and release fishing and you take your fish out of the water for a minute, slowly swish them back and forth a little in the water a little to help get their gills unstuck. That way he/she will stay healthy and have lots of babies so you can keep coming back every year.


Evans et al., "The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste.", Physiological Reviews, Vol. 85 No. 1, 2005, DOI: 10.1152/physrev.00050.2003

Hughes, G.M., "The Dimensions of Fish Gills in Relation to Their Function.", Journal of Experimental Biology, 45, 177-195, 1966

"Trachurus trachurus, Horse Mackerel" 
Encyclopedia of Life, Accessed via:

Sunday, January 11, 2015

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.

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 

Sunday, January 4, 2015

Stubby, Bobtail, Dumplings

Mollusks. Generally they're not a particularly cuddly bunch. For example, most people wouldn't call the inside of an oyster cute.

"D'aaaww who's a good boy?"
Courtesy: Larry Hoffman via Flickr

But there is one group among the mollusks that stands apart. The sepiolidae (pronounced seep-ee-oli-day) are an order of mollusks related to squid, octopus, and cuttlefish. More commonly these animals are called stubby, bobtail, or dumpling squid. None of the stubby squids get much bigger than around ten centimeters (about half the length of an unsharpened pencil), so they're round and small, which makes all of their common names very appropriate. The squid part of the name is a little misleading however because these animals show characteristics consistent with all of their cousins. They spend most of their lives on the bottom, like octopus; they have eight arms and two tentacles, like squid; and they have short rounded mantles, like cuttlefish. 

Who do you think you are? No, seriously I can't figure it out.
A Hummingbird bobtail (Euprymna berryi)
Courtesy: rtonyr via Flickr

Sepiolids are most closely related to cuttlefish which many people guess by the brilliant colors they can produce. Don't let the flashy get up fool you though; stubby squid are some of the best hiders out there. Not only can they change those colors to better match their surroundings, they're also master excavators.

Sepiolids are so cute that they even snuggle up in a blanket to sleep. They dig down by using their siphon (a tube coming from their body that they use to swim and breathe out.) to force water into the bottom and create a little depression. Then they settle down into it and use their arms to wrap themselves in a nice blanket of mud, leaving only their head sticking out.

Makes sense, you've got to wrap a dumpling 
to steam it properly
Courtesy: Nick Hobgood via Flikr

They spend most of the daylight hours tucked in deep water in their sandy beds and come out at night to hunt. Once they're on the prowl they'll often move to shallower water where they're sometimes encountered by lucky divers. 

So if they're this adorable as adults what do their babies look like? Well pretty much exactly like their parents. Dumpling squid have smaller egg clutches than many of their relatives. They lay up to about 50 eggs, which compared to a giant Pacific octopus' max of 100,000 is practically nothing. And unlike many of their relatives' babies, stubby squid don't spend time as a part of the plankton. The young sepiolids break out of their egg cases ready to roll, and walk away to find some food.

The eggs of the Pacific stubby squid (Rossia Pacifica
Courtesy: Chris Wilson via Flickr

You'd think that because these animals are so cute and charismatic that we'd know a lot about them. But because they live at the low end of recreational dive limits, are nocturnal, bury themselves, and tend to live on sandy bottoms which divers often ignore, we actually know very little about the lives of these cool little animals.


Rodrigues et al., "Burying Behavior in the Bobtail Squid Sepiola atlantica (Cephalopda: Sepiolidae)", Italian Journal of Zoology, Apr 06, 2010

Anderson, Roland C.,  "Rossia Pacifica, Stubby Squid", The Cephalopod Page,