21 — Skeleton shrimps, Caprellidea

Small rock pool critters, south Piha, 13/9/16.

Small green algae—the same rock pool.

Something about the benefits of looking closely… I was out at Piha today surfing, but while I was checking the surf from some rocks I took the opportunity to sticking my underwater camera into a rock pool and took a few pictures. Most of them didn’t really come out, as I was dodging the surges between photos and there was a lot of water moving around. I was mostly taking pictures of anemones, but I did take some of just general rock pool in the hope of seeing something interesting. It turns out that I did.

The first thing I noticed were the small structures sticking out of a small mussel shell (centre right). They poke up and have very small tentacle-like protrusions coming from their tips—each with a little blob on the end. These are tiny athecate hydroids (related to corals). They would be about 5 mm high. The golden-coloured structures at the bottom of the picture are also hydroids, but they're too blurry to identify with more certainty.

Then I noticed a blurry thing off to the left (I really wish this was in focus, but I was lucky to see it at all). This is a caprellid amphipod or skeleton shrimp, which is standing upright in the foreground attached to some green filamentous algae. I’ve photographed them before, but as they’re small and camouflaged, you don't see them until you blow the photo up.

Skeleton shrimps eat a variety of things, from algae to protists and diatoms. They have also been seen cleaning algal films of the stalks of hydroids, which would be beneficial for both parties; the hydroid gets it’s water resistance reduced and the skeleton shrimp gets a food supply and a somewhere to live.



More info:

http://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio07Tuat02-t1-body-d4.html

http://www.asnailsodyssey.com/LEARNABOUT/AMPHIPOD/amphCapr.php