Monday, April 12, 2010

Sharks, Stasis and Fake ID

You've heard it before: "Sharks are among Earth’s oldest life-forms," [Discovery Channel] having patrolled the seas, "essentially unchanged, for 400 million years" [National Geographic]. Among the most prevalent of sharky soundbites, gems like these are sensationalistic, misleading, and born of a strange sort of biased observation. Let's see if we can't challenge their grim persistence.

What of the first claim, that sharks are Very, Very Old? This one is repeated so often as to be clichéd and conceals an interesting and quite different reality. All living sharks (and rays, to which we'll return in a moment) are descended from a common ancestor dating to the Triassic Period, probably somewhere between 240 - 220 million years ago. This makes the set of all living sharks and rays about the same age as dinosaurs and, surprisingly, just a hair older than mammals! So how do they get away with the perception that they're almost twice as old as that: the paleontological equivalent of an eleven-year old bellying up to the bar and scoring a beer?

It turns out that living sharks and rays are the only survivors of a much older and previously much larger set of fishes. Collectively known as chondrichthyans, or fishes-with-cartilage-skeletons, this crew is at least 400 million years old and was both amazingly diverse and often bizarre in appearance. Calling a more recently derived subset (living sharks and rays) of this ancient group 400 million years old is like calling me 200 years old because that's how far back I can trace my lineage. It doesn't work.

Cladoselache, an early chondrichthyan. From Wikimedia Commons.

So when people speak of sharks as being twice as old as dinosaurs, they're hopefully not referring to the age of the living group (although I fear this is often the case), but rather to the broader idea of "the shark." This is a different concept, that of the body plan or general architecture of the beast. Torpedo-shaped, toothy, with the typically sharky complement of fins slicing through the water. In doing so, they are placing "the shark" in the same bin as "the shrimp": an artificial group of organisms that pretty much look the same. Critically, this ignores 400 million years of weird experiments in the shark body plan. Just a few examples of extinct chondrichthyans include forms that resemble lumpy rocks, or eels with a long spine behind their head, or strange little undersea birds, or one with a spiral of teeth that no one quite knows where to place on the fish. For some beautiful reconstructions of these oddities, check out this article in Dive Magazine.

And let's not forget about those living exceptions that happen to comprise the group I study: the rays. Rays, skates and their relatives (collectively called batoids) are the majority of living shark and ray species, and precious few of them look anything like sharks. One colleague has gone so far as to call her skates "charismatic slimy pancakes of wonder." The group also includes what is considered to be the most derived, non-sharklike chondrichthyan of all, the manta ray.

Manta ray. Photo credit Richard Harvey, from Wikimedia Commons.

With this background, we can finally address the second claim. Has the shark body plan remained "essentially unchanged" over deep time? [Some creationist websites claim zero change, which is ludicrious to anyone with eyes and Google - try it yourself.] The answer to this question is more complex than that of the first.

At first blush, it seems simple. Having defined "the shark" as a body plan rather than as a natural set of organisms, the comparison between ancient and modern sharks is revealed to be circular. We've designated starting and ending points based on their similarity and then raise our eyebrows when they turn out to be similar. Wow. Consider an analogy from finance. If a trader tracks a thousand stocks over the course of a year, most will finish either up or down from their starting price. A few will finish at, or close enough to, their initial value. Is it then meaningful for the trader to ignore all but those few static stocks, and in his bias marvel at how little they've changed over time?

But fortunately the natural world isn't the DJIA, and perhaps there is some kernel of wisdom that we can salvage from this debacle. Just how similar are the body plans of shark-like chondrichthyans today and those of 400 million years ago? There have been important modifications to jaw suspension, the internal girdles supporting the paired fins, and so on, but much beyond that we're going out of bounds of our artificial body plan playing field. Sharks' hydrodynamic, torpedo-shaped architecture is what some biologists cheekily call A Good Trick: a trait that a group retains or multiple groups independently stumble upon because it's highly adaptive for their environment and way of life.

In one last attempt to address the question, let's turn for perspective to sharks' sister group, the osteichthyans or fishes-with-bony-skeletons. There are two subsets of these: the ray-finned fishes, which are nearly all "typical" fishes (see last week's post); and the lobe-finned fishes, which include coelacanths, lungfishes, and limbed vertebrates like... you! Both ray- and lobe-finned fishes date to a common ancestor that lived about 420 million years ago, which had that torpedo-shaped, vaguely shark-like body plan (the Good Trick). Since they've been evolving for as long as have sharks, what have they done with that body plan since?

Like their chondrichthyan cousins, the ray-finned fishes have diversified into many different amazing forms, from millstone-like sunfish to gulper eels to sleek barracuda. And also like chondrichthyans, the basic shark-like body plan persists as a common "starting material" for most of these daring experiments in anatomy. Some ray-finned fishes alive today resemble the earliest members of this group, at least to the extent that some modern sharks resemble their ancestors.

And the lobe-finned fishes? Living coelacanths and lungfishes number only enough species to count on both hands, but like living sharks and rays they represent formerly much larger groups that also explored weird body plan variations. Still being aquatic, they've retained the same basic architecture of their ancestors. But what about the third member of this group, the tetrapods ("four-footed" beasts)? Some surviving members have changed relatively little over time despite their own extinct and experimental offshoots, and again these are typically aquatic or amphibious forms like salamanders. Where we see dramatic deviations from the ancestral body plan are in groups that have escaped the water and stumbled upon a new Good Trick. Among many examples are: flight, which evolved three separate times in tetrapods; new, faster ways of running in mammals and dinosaurs; and treetop leaping and swinging in many primates. Others came full circle and returned to the water, re-evolving shark-like body plans from very different starting material. The best example of these are not whales but rather the ichthyosaurs ("fish-lizards").

Ophthalmosaurus, a Jurassic ichthyosaur. Ichthyosaurs re-derived a shark-like body plan from a lizard-like ancestor. Image credit Nobu Tamura, from Wikimedia Commons.

Perhaps, someday in the far future, humble mice will be the only surviving members of what was once the most body-plan-diverse group of vertebrates, the mammals. Will some alien television narrator compare these mice to their tiny ancestors that once crept around Jurassic jungles? Might she remark, "My! How little they have changed...?"

Tuesday, April 6, 2010

Nested Sets of Sushi

This colorful opinion piece in Time by Josh Ozersky followed the 175-nation Convention on International Trade in Endangered Species (CITES); the outcome of which was, to be charitable, an unmitigated disaster for science-based fisheries management. At issue were US- and EU-backed proposals to ban trade in a number of marine species including sharks and bluefin tuna, which are being overfished vastly beyond the ability of these large, slow-growing fishes to replenish their populations.

It is difficult to deliberately fish a widespread, open-ocean species into extinction. The commercial inviability of continuing to target vanishing fishes will in most cases spare them from true extinction. Where these beasts can get in trouble is if they're hit by one or more extra liabilities: for example, going through a bottleneck in their life history where it's easy to find and fish them all up, or if they command exorbitant prices sufficient to keep fishing pressure intense to the point of complete stock exhaustion. The Atlantic bluefin tuna Thunnus thynnus bears both of these albatrosses. These voracious, silver leviathans can reach ten feet and 1400 pounds, taking decades to reach maturity. Populations are harvested wholesale when they gather in the Mediterranean, which is one of their exactly two spawning grounds. And to top it off, they are tasty. Very tasty.

Tuna being shaped at Tsukiji fish market, Tokyo.

The Japanese in particular are crazy about these things. A single, epic fish sold for $175,000 in Tokyo last year. It was no surprise that Japan spearheaded the effort to torpedo the proposed CITES ban on bluefin trade, along with a number of fishing nation allies including Canada, Indonesia, Venezuela, UAE and friends. The proposed ban was destroyed by a vote of 68 to 20, with 30 abstentions. That is a go-limping-home kind of whooping, crowned by a strident Libyan denunciation of sound fisheries science as "lies." Nice.

But this post isn't about conservation. It's about a curious twist in Mr Ozersky's mostly rational and impassioned call to boycott his once-beloved o-toro, bluefin sushi, in response to the shenanigans of Japan et al. Ozersky writes:

"It's been around for more than 400 million years, which means it is older than the trees, older than the Himalayas, older than the Atlantic Ocean itself. ... But either way, the loss of a creature that has been living here since before the continents formed won't be on my hands."

Wait. Huh?

Four hundred million years is old. Even in evolutionary time, this goes back to approaching the rise of large animal life on the planet. Paired fins and jaws had just made the scene. At this time near the beginning of the Devonian period, jawed fishes were finally getting their act together and beginning to diversify while their jawless cousins began a slow decline into obscurity. Most of these fishes were armored oddities called placoderms that have no living descendents. The first dinosaurs, lithe little things, wouldn't trod the earth for another 170 million years. So... is the Atlantic bluefin nearly twice as old as dinosaurs??

To answer this strange puzzle, let's consider the nature of life as a series of nested sets. This is the pattern of evolution by cladogenesis, in which an ancestral species gives rise over time to two distinct daughter species. This also has the nice consequence of producing a hierarchical arrangement of organisms. For example, all ferrets belong to a larger group called mustelids, along with weasels and sea otters and wolverines. All mustelids are placental mammals. All placental mammals are vertebrates, and all vertebrates are animals. Therefore, you don't predict to find weasels before mammals evolved, or mammals before the advent of animals.

Now how about tunas? Tunas are in a family of fishes called scombrids along with mackerels, bonitos, and other tasty things. They are nested well within the "set" of fishes called teleosts, which comprise over 99% of all living bony fishes, and the next larger group we'll worry about are the ray-finned fishes. So we can't have tunas without teleosts, we can't have teleosts without ray-finned fishes. Here's the problem.

Divergence times for groups containing bluefin tuna.

Teleosts, the crown group of bony fishes from marlins to mollies, date to the early Triassic period at the dawn of the dinosaurs, sometime around 240 million years ago (with some uncertainty). Right off the bat, we can see that there is no way that the bluefin can be older than that - you can't be your own grandpa, so to speak. What about ray-finned fishes? They crop up around 420 million years ago in the period right before the Devonian. Could this be what Ozersky meant? Even given that benefit of the doubt, does it matter? His heart is in the right place, but conservation may be legitimately justified by appealing to the maintenance of viable commercial markets, or ecosystems, or even an organism's aesthetic value (which I submit the bluefin has). Lemurs are worth protecting because they represent the entirety of a major offshoot of the primate family tree, not because they are vertebrates or animals or organisms with a discrete nucleus.

Species longevity is an interesting concept that gets at the heart of the pattern of "punctuated equilibrium" characterizing the fossil record of many groups. Punctuation is itself intriguing: why did this plant/animal/fungus' morphology change rapidly in a short period of time? But the equilibrium often gets lost in punctuation's spotlight: why do so many species not visibly change over long periods of time?

Across vertebrate taxa, species longevity tends to be on the order of only a few million years; perhaps 2-5. Even without knowing more about the timing of the scombrid fishes' radiation, we would not predict that the Atlantic bluefin is much older than that. The number of "famous" animals that buck the trend and appear to have changed very little over time can be counted on one hand, but are usually overblown. We'll return to the coelacanth, sharks, and similar cases in the next post, with a revelation or two that may surprise you.

Welcome to "This reView of Life"

The line with which Darwin closed On The Origin of Species captures much of why I am, why I always knew I would be, a biologist.

"There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

And as Darwin ended his transformative work, so begins this humble experiment in running commentary on science. I am inexhaustibly fascinated with and often appalled by the portrayal or synopses of science in the media, and hope to use this outlet to highlight interesting topics, redress wrongdoings, and expound upon loose ends in articles that come my way. I hope you will join me.