Small fry.

I went fishing yesterday afternoon. The weather was no problem; I was sitting at my kitchen table, fishing for copepods in the detritus I washed out of the aquarium filter.

I caught two, and umpteen baby amphipods, all very active. I didn't find a mate to the one I found the other day, however; I'll try another location next time.

Here's the latest catch:

Looks like a shrimpy thing, again. But from above, the identifying feature stands out:

Only one eye, in the middle of the head. All copepods have just this one eye, some only in the larval stage.

This little guy* (0.8 mm, nose to tail, excluding antennae and tail streamers) was zipping about like a nuclear-powered bumper car; to pin him down so he could have his photo taken, I had to trap him in a drop of water and then "vacuum" up most of the liquid with the corner of a paper towel, until he could barely spin in a slow circle. Which he proceeded to do, without a pause.

While he swam around, I could see the yellowish center material pulsing; he flipped his tail up, then straightened it with a snap, seeming to want to use it for locomotion. (Didn't work; he was in that water-drop jail.)

I knew next to nothing about copepods, and my home library wasn't of much use; a couple of photos of Daphnia, and a mention of the one eye or an over-generalized discussion of habitats (anywhere there's water) was about it. The web was a bit more productive. Here's what I learned, with links to the sources:

• Copepods are small crustaceans, mostly under 1 cm. long) Their name, "Copepod", means "oar feet", because they "row" along, with pairs of legs acting in unison. One branch is called the "Cyclopoidea", referring to the single eye, and referencing the Greek and Roman mythological one-eyed giants, the Cyclops.
• Identification is difficult without the aid of a good microscope. (And the knowledge to go with it.)
• They are the most important source of protein in the ocean; along with the worms, they are the most abundant multi-cellular animals on earth. Everything eats them; from mussels and fish to birds and even whales. (In these days of a threatened marine environment, it's folly to ignore their needs.) My old textbook, Living Invertebrates (1987) makes an interesting point: the prey of carnivourous copepods, fish eggs and larvae, larval forms of other invertebrates, etc., at later stages turn around and feed on the copepods. Turn about's fair play.
• "Trillions of little copepod guts produce countless fecal pellets ..."; what a picture!
• Some are free-swimming, some bottom feeders (The Harpaticoida; mine are Harps, I think.); some have taken to a parasitic lifestyle. Food supplies may be diatoms, algae, bacteria, mosquito larvae (Good on them!), even small fishes. The parasites may suck blood or eat skin particles.
• Like other crustaceans, they shed their exoskeleton as they grow. They go through three general stages on their way to adulthood; first, the nauplius (plural nauplii). In this stage, they look like the one I found last week. Oldenburg University has a good photo; scroll down to the bottom. They moult in this stage a half-dozen or so times, then progress to the copepodid stage, which looks more like the adult. These moult five times, at each moult adding another segment to the tail**, (French; use Google translate.) with the appropriate appendages.
• In the adult stage, they mate. Like they say on Facebook, "It's complicated." Afterwards, the female carries two eggsacs around on either side of her tail. UVic, Australia, has a simple diagram. Here's an animation (Dutch). They hatch after a few days.

But I was searching for a match to last week's copepod:

I found a couple of similar photos; the nauplii, and an unidentified aquarium resident. Museum Victoria has a diagram, also unidentified. So I am still uncertain, but I think what I had is probably a junior copepod,  a nauplius.

For comparison, here is a mini-amphipod from the fishing expediton; it had two eyes, and kept the tail curved under; the various appendages at the tail end flutter constantly, keeping the water circulating around it.

This one was about 1.5 mm long, nose to tail. Unlike most of the other amphipods I saw, it was grey rather than green.

*In the absence of egg-sacs, I decided he could be male.
**Razouls C., de Bovée F., Kouwenberg J. et Desreumaux N., 2005-2009. - Diversity and Geographic Distribution of Marine Planktonic Copepods. Available at http://copepodes.obs-banyuls.fr/en
[Accessed December 21, 2009]