Plankton tows this week revealled the presence of large amounts of copepods in the waters around Cape Cod. These animals are members of the zooplankton. What it means to be a planktonic animal is that you are capable of moving up and down in the water column but you are unable to swim against the current. In short, the current pushes you around wherever it is that it is going. The collective term, zooplankton, includes a wide variety of things that range in size from the microscopic to some animals that are truly large.
Lets start at the top and work down. By our definition (at the mercy of the current), at times things like sea-turtles and even ocean sunfish can be zooplankton. Sea-turtle species (and ocean sunfish) are all cold-blooded animals. What it means to be cold-blooded is that the animal’s body temperature is directly related to the temperature of the animal’s surroundings. It is quite different from you and I. Our bodies maintain a constant internal body temperature that does not change regardless of where we go. If we get an infection, we might develope a fever (a body temperature that has risen above normal to fight the infection). Above normal. That is the key thing about being warm-blooded. There is a normal. 98.6 degrees for human beings. Most whales, as it turns out, have similar normal body temperatures.
But when you are a cold-blooded animal, your surroundings and not your body chemistry dictate your internal body temperature. So animals like sea-turtles, that are very strong swimmers in sub-tropical and even temperate waters become slow and sluggish when the water temperature drops close to freezing. Such an even happened near the Cape this winter when the water temperature dropped, leaving more than eight hundred sea turtles “cold shocked.” In the cold waters, their body temperatures dropped so fast that they were no longer capable of swimming their way out of the cold water and began to propelled by the currents. (Back to our definition).
I consider myself fortunate to have found one stranded on the beach of Provincetown Harbor on Thanksgiving Day. And the truth is that I probably would not have stopped to look at it if it wasn’t for my dog. I saw the guy fifteen feet ahead of me kick it and move on when it did not respond. I would likely have passed it by if my dog had not pulled me over to it. I don’t know but I like to think it survivied. My dog and I drove it to the Audubon Center in Wellfleet (at the request of the stranding hotline–because all sea turtles are endangered species).
Back to my point, when the turtle was thrown upon the shore because it could no longer swim against the currents, it had become zooplankton.
The same is true for ocean sunfish that are quite strong swimmers in the sub-tropics but also get cold-shocked in the more northern waters in early spring and autumn. Ocean Sunfish can be as large as ten feet in diameter and weigh eight hundred pounds.
I included these examples to demonstrate that the term plankton is not reserved for microscopic things.
Most zooplankton, however, is really quite small. In our waters, it includes things like the larvae of fish, the larvae of crabs, the larvae of lobsters, and, of significant importance, copepods.
There are more species of copepods than I could list here. It is far easier to think of them as two groups; small copepods and larger copepods. Simply, the small ones eat the phytoplankton and the larger ones eat the small ones.
All copepods are closely related to lobsters and crabs. Under a microscope, they resemble lobsters in many ways. First, on either side of the head they have antennae that are longer than the rest of the body. On their undersides, they have numerous swimmeretts (like a lobster). They have distinctly segmented tails (by counting the number of segments in the last row you can key out species). At speed, they move very much like lobsters, flicking their tails while scrambling with their swimmeretts. In fact, at speed, if you measure speed in terms of body-lengths per unit of time (second or minute), these tiny animals are the fastest living species on the planet today. Just keep in mind that a body-length is about a quarter the size of a grain of white rice.
Other things you might notice under a microscope include that lack of eye-stalks and the presence of eyespots (a darker red-brown area on the head between the bases of the antennae). The eyespots cannot form images. They only can sense the presence of daylight, possibly a way that the copepod guages its proximity to the surface. The segmented carapace (hard body of a crustacean) also conceals another reddish-brown area just beyond the head referred to as the lipid sac. It is here that the protein-rich lipids (fats) are stored that make these tiny creatures an incredibly efficient food source for small fish like sand eel but also for huge creatures like the 60 to 80n ton Northern Right Whale.