We have been seeing high levels of plankton in our water lately. Many Aquarium exhibits were a bit cloudy last week from all the algae and other planktonic organisms that are becoming more numerous as the season progresses.
Suspension-feeding animals, meaning animals that feed on material that’s suspended in the water, really shine during plankton blooms. Many of our sea cucumbers, hydroids, bivalves (clams, scallops, and mussels) and barnacles have been very visible during this time. When white, salt and pepper, burying or creeping pedal sea cucumbers sense high levels of plankton, they unfurl their feeding tentacles and start pulling in suspended material from the water. Barnacles extend their cirri (modified feet) into the water and literally comb out the plankton, pulling it into their test (shell) down to the mouth inside.
Other animals, such as pink-lipped hydroids, are seasonal—only appearing in our exhibits from about mid-April through September. The hydroids are only about half an inch high at present, but will soon expand to about one and a half inches in length. These colonial animals have stinging cells, or nematocysts, that capture tiny zooplankton from the water. They, in turn, will attract their own predators, small nudibranchs that come in with the water from about May through August to feed on the hydroids.