Self-Evident Victor of the Invert War

Invert war has been declared. Personally, I consider myself a lover, not a fighter. And all the inverts are worthy of love in my book. But, knowing that tempers may flare as biologists across the blogosphere come to the defense of their preferred spineless taxa, I thought it would be worth injecting a bit of perspective into the fray. To give credit where evolutionary credit is due. I am talking about a group of animals that, though not poisonous, tentacled, gigantic, smart, sexy, cute, or disgusting, is, hands down, the most successful in the ocean.

I refer, of course, to the copepods.

The copepods, for those who have not heard of them, are a subclass of the phylum Crustacea. They are small—most are less than one or two millimeters long—and abundant. Ridiculously abundant. The subclass Copepoda (under the class Maxillipoda) contains ten orders and something like 11,500 individual species. By some estimates, the copepods, taken together, are the single most massive group of animals on earth.

The other top contender is the Euphausiids—krill—but they have nowhere near the universal distribution of the copepods, which are found in nearly every marine and aquatic environment on earth. Copepods can be found burrowing in mud on the abyssal seafloor, swimming in fresh lakes and rivers, crawling under wet leaves, surfing the tide in estuaries, and parasitizing other animals in surprising and disgusting ways (one-half to one-third of all copepod species are parasites). They can even be found swimming down the throats of unsuspecting Orthodox Brooklynites. But the most emblematic copepod (at least for me) is a free-swimming pelagic zooplankter.

Most of the free-living copepods are found in the orders Calanoida, Harpacticoida, and Cyclopoida. They have a classic copepodite body: a bullet-shaped or ellipsoidal cephalothorax with a single eye in the middle of their head and long antennae sticking out to either side. The antennae are covered with setae, hair-like bristles that are used to sense water motion and slow the copepod’s rate of sinking. Behind is the abdomen, or urosome, with two feathery caudal rami. Many species actually use their antennae as swimming appendages. Underneath are legs, which may be used for swimming or feeding, by generating a current that pulls seawater filled with delicious pytoplankton and protists past their mouth.

They are formidable swimmers and feeders. Vertically migrating species may swim hundreds of meters every day, traveling up to the surface at dusk to feed on the phytoplankton that grows there and back down at dawn to escape predation. That’s a cool couple hundred thousand body lengths. At human scale, that’s like running 60 miles, both ways, just to eat a meal, with a total travel time of just two or three hours.

The experience of small animals in water is very different from our experience. When a human, whale, or submarine tries to move through the water, it feels resistance, from the friction of water on the surface of the moving body, and from the force required to push water out of the way. These forces are termed inertial drag. A small zooplankter—say, a protist or young crustacean larva—also feels resistance, but of a different sort. This is viscous drag, the force you feel when you drag a spoon through molasses. It is the actual resistance of the fluid to being sheared and deformed. To a tiny animal, swimming through water feels more like swimming through molasses. The ratio of inertial to viscous drag is called the Reynolds number, a very important ratio in fluid dynamics.

Why these unassuming little guys should dominate the oceanic zooplankton worldwide is a very interesting question. It may have something to do with their size—at around 1 mm, with a typical cruising speed of 1 mm/sec, a typical calanoid copepod’s Reynolds number works out to be approximately 1. This places them astride the boundary between the viscous, molasses-ey world inhabited by the smaller organisms they prey on, and the fast-moving inertial world inhabited by their own predators. The feeding current they generate by paddling their legs is thus highly efficient at dragging small prey to the gaping maw of the copepod. At the same time, a strong snap of the antennae can propel the copepod into the inertial world, accelerating it at 12 m/sec² to a top speed of over 0.5 m/sec, plenty fast enough to evade a marauding herring or anchovy.

This explanation for copepod dominance is, in Naganuma’s words, more of a “hypothetical conclusion” than a true explanation. It does not explain, for instance, why more other animals do not inhabit the viscous-inertial boundary. But it is interesting to ponder. At any rate, though other inverts may be prettier or have weirder sex lives, the copepods don’t need to front. They’ve already got it in the bank.

Naganuma, T. (1996). Calanoid copepods:linking lower-higher trophic levels by linking lower-higher Reynolds numbers Marine Ecology Progress Series, 136, 311-313 DOI: 10.3354/meps136311