If you’ve been anywhere near the National Aquarium, Baltimore, in the last few days, you’ve probably noticed the invasion of jellies! And no, we aren’t referring to the jellies on exhibit that arrived last summer during Baltimore’s Waterfront Invasion. We’re talking about jellies that are popping up in the Baltimore harbor water right now.
The jellies we are seeing in the harbor are Atlantic sea nettles (Chrysaora quinquecirrha), and we typically see them in the Inner Harbor in their largest numbers in the early fall season. Jellies go with the flow of the water, so currents often play a role in their sudden appearance.
There are multiple reasons jellies invade the water this time of year. Usually the late summer months (July/August) are our driest, so the salinity of the water is a bit higher due to the lack of input of freshwater drain-off. And for jellies, the saltier the water, the better.
Their population size also grows through the summer due to warm water temperatures and abundance of food. A single mature Atlantic sea nettle can release as many as 45,000 eggs every day. Fertile eggs settle on the bottom to form polyps that resemble tiny sea anemones. These eggs lay dormant through the cold winter months, and when water temperatures rise in the spring and early summer, each polyp starts producing and releasing hundreds of tiny free-swimming sea nettles.
This stage of life is called the medusa phase. In the warm summer water they grow rapidly into adults that start producing even more sea nettles for the cycle to continue.
This boom of the harbor sea nettles will come to an end as the water temperatures start to drop. By the end of October or early November, the medusa phase will die off completely, as this species is unable to digest food in cold water. The polyp stage of the Atlantic sea nettles will survive through the winter to start next year’s crop of sea nettles.
Other species of jellies (e.g., lion’s mane jellies) thrive in cold water. Locally, we may see Leidy’s comb jellies and moon jellies this fall, so be on the lookout!
And if you really want to see an invasion of all types of jellies, stop by our exhibit or check out our jellies website.