No. 51–More on Ctenophores

In reviewing last month’s column in preparation for this one, I realize I may have piqued interest mentioning a surprising light show observed on my first night with M. leidyi but didn’t elaborate further. It turns out that many species of ctenophore are gifted with the power of bioluminescence.

Even after consulting numerous sources, I’ve not found an explanation for bioluminescence in ctenophores. I did observe, that first night, that the creatures flashed wildly when caught in the upward turbulent bubbling of the air stone. When I unplugged the pump the tank went dark almost immediately.

Some months later I learned from a DNR officer friend that the flashing when disturbed phenomena was an aid to law enforcement. Scofflaws illegally dragging the sounds for shrimp at night were easily spotted from the air because of their glimmering wake.

Most Ctenophores are planktonic swimmers and are divided into the two classes, Nuda and Tentaculata. Members of the class Tentaculata, as the name implies, have tentacles used for capturing prey while class Nuda, as their name implies, have no tentacles.

Unlike cnidarians, ctenophore tentacles are covered with cells called colloblasts rather than nematocysts for capturing a meal, hence, no sting. On contact the colloblasts burst releasing sticky threads to ensnare the prey. Lacking “fishing gear,” Class Nuda members engulf their prey as they move through the water propelled by eight rows of cilia. It is the rows of cilia that give rise to the common name “comb jelly”.

Most comb jellies feed almost exclusively on zooplankton to include small crustaceans copepods and amphipods and the larva of many other creatures. One species in particular, however, prefers larger prey.

A few days after collecting the original trio of M. leidyi, I scooped up a three inch long Beroe ovata, mentioned last month, and added it to their tank. Unfortunately for the trio, that led to one of those “oh – I won’t do that again moments” the next morning when I found the latest acquisition to be the only creature in the tank. The significance of this will become clear later.

A third class of ctenophores, Class Platyctenida, is made up of benthic (bottom dwelling) species bearing little resemblance to their free swimming cousins. Body shapes vary with most resembling flatworms or sea slugs. Inhabiting tropical coastal waters and relatively small, 5 to 15 cm, some are brightly colored to match sponges or corals that host them.

Ctenophores at first glance are elegant creatures in deed. The wonder and delight M. leidyi originally inspired in me has, however, been tempered in the research for this piece. There is, through no fault of its own, a dark side to this beauty.

Traveling from its home waters, Charleston Harbor perhaps, in the ballast tanks of a ship; M. leidyi was introduced into the Black Sea in the early ’80s bringing about an almost total collapse of the ecosystem’s fishery in less than ten years. Each year they increased in number and consumed more of the available zooplankton stocks till there little left for native larval fish.

It was only the eventual, and also incidental, release of Beroe ovata into those waters that ultimately turned the tide. B. ovata, as I learned many years ago, is a voracious predator of M. leidyi and has brought the beautiful plunderer under control in the Black Sea. It is hoped that the same will hold true for the Caspian which is thought to have been invaded via the canals connecting it and the Black Sea.

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