#7. Comb jellies – beautiful swimmers or deadly invaders?
In which the E@L discovers a treasure trove of overlooked gems
A Surprising Discovery
It was a beautiful Sunday afternoon at Roaring Point Park on the Nanticoke River, a large tributary of Chesapeake Bay. The sandy beach was covered with bathers enjoying the warm weather and water. Most of them probably didn’t even notice the glassy little glistening globules beside them on the beach. As I walked along the beach, though, my eyes were drawn to them because they sparkled like diamonds in the sun.
At some locations, hundreds collected in depressions as the tide receded. I thought it remarkable to find so many at the same time and place, but then again, maybe not. As I waded out to chest deep water to cool off, I could feel little bumps against my hands and fingers as I moved them through the water. Back on the beach, I realized that I had been wading through a soup of comb jellies, or ctenophores, most only the size of my fingernails.
Ctenophores on the beach (B. Stevens)
The Un-Jellyfish
Ctenophores are clear gelatinous marine organisms, but they are not closely related to other jellyfish. Unlike most jellyfish, comb jellies do not sting, and do not have obvious tentacles. Instead, they are covered with eight rows of overlapping comb-like plates. The name ctenophore comes from the ancient Greek “cteno” (or kteno) meaning comb, and “phora”, meaning bearer, thus “comb-bearer” or comb jelly.
Ctenophores swim by raking their combs (ctenes) up and down, which provide them with a means of locomotion, and produce flashing displays of iridescent color. Some ctenophores are bioluminescent, but you would only see that at night.
The most common ctenophore in the mid- to upper Chesapeake Bay is known as Mnemiopsis leidyi. Also known as the sea walnut, it is about the size and shape of a walnut and can reach 5 inches in diameter. Most ctenophores are small and transparent, and thus inconspicuous to the casual observer. They are also extremely fragile (being mostly water), so are difficult to capture and handle alive, and for this reason are not well studied by scientists.
Mnemiopsis leidyi, flashing its comb rows (M. Faasse, World Register of Marine Species, licensed by Creative Commons)
Ecosystem Exploiters
Comb jellies are voracious predators of plankton, especially small crustaceans known as copepods, as well as fish eggs and larvae, and some even eat other ctenophores. When comb jellies are abundant, they can totally eliminate fish from an ecosystem.
This actually happened in the Black Sea, where comb jellies are invasive. The American ctenophore Mnemiopsis leidyi was introduced to the Black Sea in the 1990s by ship ballast water and soon became abundant. Like many invasive or introduced species, it found a niche in the ecosystem and exploited it, devouring not only fish eggs and larvae but most of their food as well. This caused a massive collapse of fisheries in the region, especially for the Black Sea Anchovy. Ctenophores soon spread to the Caspian Sea and Azov Sea, collapsing fisheries there as well. In the early 2000’s they were found in the Aegean and Adriatic seas as well, raising concerns about fish populations there.
Surprisingly, after eating themselves out of house and home, lack of prey didn’t have any adverse impact on Mnemiopsis populations, which kept reproducing rapidly. It turns out that they just eat their own offspring when food is scarce. When the going gets tough, the tough become cannibals.
From Predator to Prey
Fortunately, comb jellies have their own predators, including another ctenophore called Beroe. It also became introduced to the Black Sea, and soon Mnemiopsis populations began to decline. Along with the presence of hypersaline conditions, this has helped prevent Mnemiopsis from having a significant impact on Mediterranean ecosystems. It remains to be seen whether Beroe will control the ctenophore invasion, as invasive species often have many unforeseen consequences.
Another predator of Mnemiopsis is the sea nettle Chrysaora quinquecirrha, and its recently renamed cousin the bay nettle, Chrysaora chesapeakei, a jellyfish with stinging tentacles common in Chesapeake bay. Ctenophore populations bloom in spring and early summer in Chesapeake Bay, but are largely consumed by sea nettles in July-August, which reduce the comb jelly populations significantly.
Bay nettles are such a nuisance to swimmers that NOAA has established a website for predicting their presence. I dread encountering them while scuba diving, as their stinging tentacles always seem to find the only exposed skin - my face and lips. Bay nettles also commonly get wrapped up in fishing gear, eliciting many colorful epithets from affected fishermen. But in reality, bay nettles are a great benefit to fishermen because they keep the fish-larvae-eating ctenophores under control.
The sea nettle Chrysaora quinquecirrha (barnegatbaypartnership.org)
A Gelatinous Enigma
On that afternoon, I was amazed and puzzled to see so many small (half-inch) ctenophores washed up on the beach. On a lark, I returned the next day to see whether they were still there. Same time of day, same stage of tide (high), but no ctenophores. I walked the entire beach without seeing any, then decided to cool off in the water and wait for a bit.
An hour later, the tide started to turn, and a few little diamonds sparkled on the beach. As the tide receded, more and more of them appeared, stranded by the waves. There weren’t nearly as many as the day before, however, and I never felt them bouncing off my hands in the water. I’m left with a conundrum.
On one day I witnessed an amazing spectacle – thousands of juvenile comb jellies washed up on the shoreline among oblivious beach-goers – and the next day, only a hint of a similar event. But why? My guess is that the combination of wind (southerly) and tide was just right to concentrate thousands (millions?) of ctenophores onto the beach that Sunday. As for Monday, perhaps they had been blown farther upriver, or maybe I just didn’t wait around long enough for the tide to recede. It’s possible this happens regularly, but normally goes unnoticed.
On the other hand, maybe it was an unusual event, possibly signalling a massive takeover of the Chesapeake Bay Ecosytem by a normally mundane resident. It’s not inconceivable that the combination of warming waters and nutrient overloads in the Bay led to an explosion of ctenophore populations. If so, it could threaten resident fish species, which are already suffering from these impacts and other invaders (e.g. blue catfish and snakeheads, that eat other small fish).
Nature is full of wonders. Too often, we are so busy with our own concerns that we are oblivious to her charms. The opportunity to witness such a massive stranding event was like peering through a magical window into nature’s secrets. In the big scheme of things, an event of this magnitude probably has little impact on overall comb jelly populations. But nonetheless, it’s not something we get to see every day.
So next time you are out in nature, take a moment to look around for anything unusual. You might be witnessing some amazing natural phenomenon, and not even know it.
A fascinating read - worth the time of anybody who is interested in nature