August 01, 2004
Jellyfish are a very successful translucent invertebrate that inhabit all oceans of the world. Reproduction is sexual, with each jellyfish being male or female and releasing sperm or eggs from its gonads, which are usually visible through the clear substance of the organism. The gonads of the Atlantic Moon Jellys in this photo are visible as four round shapes. Fertilized eggs form a Planula that joins the zooplankton, moving with the currents and tides, until it gravitates to a fixed structure to which it attaches. Then begins often several years of life as a sessile polyp. These polyps can reproduce asexually by budding, forming other polyps. Eventually the polyps transform into stacks of tiny jellyfish-shaped creatures that separate from the stalk. This usually happens in the spring, but timing is heavily influenced by local temperature and nutrient availability. Jellyfish do not travel in schools (they have no brain), but they can be released nearly simultaneously from their sessile existence, as multiple polyps are stimulated to mature by local water temperatures. They're very weak swimmers and are carried by currents together, explaining the occasional occurrence of swarms of jellyfish, as demonstrated by these Moon Jellyfish in Boston Harbor, Massachusetts, photographed on June 13, 2004.