I've got an aquarium. And now, I've also got two bowls of water on my desk. One holds the recovering flatworm (now busy digesting a barnacle) and the other is a maternity ward.
An invertebrate aquarium, especially one as biodiverse as mine is, requires regular maintenance. So my daily routine includes removing the pump and cleaning its filter. A simple job; 5 minutes is all it takes. Or took.
Recently, I've been finding amphipods inside the pump, glommed onto the filter. I guess there's a certain amount of algae in there, and the pump wall keeps out hungry crabs and hermits. Now my 5 minutes extends to 15, while I shake out the filter into clean water, then collect all the amphipods and return them to the tank.
I kept finding pairs, holding tight together, no matter how I swished and chased them. I finally decided to check them out. Were they mating? (In winter?) So three pairs have joined the flatworm on my desk.
Amphipods have seven pairs of walking legs, three behind them for swimming. The male holds the female's thorax with the first few (I see three) sets of legs, which still leaves him four for walking and grabbing. The idea is to put "dibs" on the female, keeping other males away. He may have to guard her for a week.
She will be holding her eggs under her thorax, but they cannot be fertilized as long as she's imprisoned in her hard carapace. The male grabs her and bides his time until she molts; then she will be flexible for a short while, until the new chitin hardens. During this time, he fertilizes her eggs.
I find times of incubation from 2 days to 59 days; I don't know what the timing is for this species. However, she will probably build herself a cocoon of kelp or sea lettuce stitched together, to brood her young until they are released into open water. They will look like adults, except much smaller.
This pair is probably the California beach hopper, going by the red antennae. She is as long as he is; they both are skinnier than the green ones. These are the ones you see if you pick up a mass of drying eelgrass at the high tide line; they hop all over.
The course of romance doesn't always run smoothly. Sometimes, the female objects to being held: I made a video which includes an amphipod argument.
I couldn't find much information, either in books or on the web, on the particulars of each species' life cycle. Maybe their very diversity makes it difficult to do more than generalize; around 6,000 separate species have been described. A few sites I found helpful were:
- Ehow: The Life Cycle of an Amphipod
- Encyclopedia Britannica
- Museum Victoria (Australia)
- And great photos of mating amphipods: Natural History of Orange County (wonderful photos of all sorts of animals!)