While we often think of docks and piers covered in attached organisms like mussels and algae, there are also plenty of freely moving organisms like crustaceans and fish that benefit from the habitat provided by attached critters. Last month, I noticed that the submerged portions of the docks in Bodega Harbor were covered in a waving mess of tiny animals. These creatures were none other than caprellid amphipods of the species Caprella mutica, which is non-native in California but commonly found in harbors and estuaries. However, I had never seen such a dense collection of the animals in my life. Check out this video to see what I mean.
Most of the caprellids are engaged in body-waving motion, which is an active method for capturing food from the water. Their long antennae are covered in fine hair-like projections resembling a comb that can trap plankton and detritus. Many of the caprellids are hanging on to an arborescent (tree-like or branching) bryozoan called Bugula neritina (also non-native). That is the bushy purple stuff, which is actually a colony of many interconnected animals. Caprellids like to hang on to something, so it’s easy to see how the non-native bryozoan with all of its branches might benefit the caprellids by providing many “perches.” I wonder if the caprellid offers any benefit to the bryozoan, like preventing other animals from overgrowing it, or if they disrupt feeding of the colony’s tiny animals (“zooids”).
Here are a couple of other things to watch for in the video:
at 1:15, you can see that two caprellids at the bottom of the screen appear to be engaging in crustacean fisticuffs.
at 1:25, you will see a group of small caprellids filter feeding around the inhalant siphon of a native sea squirt (Ascidia ceratodes).