Ctenophora Life Cycle
Ctenophora are a phylum of water animals commonly known as comb jellies. While they appear quite similar to the cnidarian jelly fish, being boneless, largely liquid and often translucent with faint bio-luminescence, they are definitely a distinct group of creatures with their own attributes. Their life cycle is simple compared to many, consisting of only three stages and sometimes, arguably, only two stages.
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Identification
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Ctenophora are identified as a group by basic structural details, rather than size, color or other superficial aspects. All are structured as an external, balloon-like mantle layer, with a mouth at one end and a central core of intestines and organs contained within the mantle. Where cnidarian jellies may have stings and propel themselves through a pulsing action by filling their bell shaped mantles with water and forcing it out in a jet, ctenophora have no stingers and all of the group move through the sculling action of hundreds of cilia: thin, whip shaped tendrils arranged on plates, with the plates arranged in long, thin combs of cilia and the combs arranged in rows from the leading tip of the jelly to the base. These combs of cilia give the group their common name of "comb jellies." The cilia beat the water in a rippling effect, as though they were performing "the wave" from the top of the jelly to the bottom. A peculiarity of comb jellies is the rainbow shimmer of diffracted light caused by the movement of the cilia as light shines through the fine, transparent membrane.
Egg
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The first stage of the life cycle of the ctenophora is the fertilized egg. All but a few Ctenophora species are hermaphroditic--each individual jelly being capable of producing both sperm and egg. Ctenophora broadcast sperm and eggs into the water they swim through, and when sperm and egg meet each other a fertile egg is formed. In some cases the eggs will be fertilized by sperm from the same original parent comb jelly, in others it will encounter sperm from a separate comb jelly. Ctenophora themselves do not actively mate with each other, however, they simply sow sperm and eggs and let chance accomplish the rest.
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Larva
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The egg floats on ocean currents until hatching into the larval form. For many ctenophora the larva is distinguished from the adult only by size. In others the larva has not completely assumed the structure of the adult, and beginning with a simple tubular form develops the layering and internal features of an adult as it grows. It is common for many forms of ctenophora to begin producing eggs and sperm before reaching adult size, allowing the species to reproduce quickly. In hostile environments this increases the chances of survival. In safe circumstances it allows species to multiply at a hectic rate. The larva, like the adults, are predatory carnivores, surviving on microscopic denizens of plankton colonies.
Adult
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The adult forms of ctenophora range in size and structure to a remarkable degree. While most swim, at least one creeps through silt and mud. Some have long tendrils that catch and bind prey (though as mentioned, they do not have stingers). All are unified by the shared bell/balloon structure, the mouth at the apex, the eight rows of beating cilia combs, and by their predatory carnivore habits. Having often started reproducing at a pre-adult size, comb jellies will continue to produce eggs and sperm as full grown adults. In times of bountiful food supply they will produce eggs and sperm for weeks. Interestingly, however, in times of famine the ctenophora will shrink in size not simply as though thinner, but in all aspects of their body form. With no unalterable features such as skeletons they are capable of simply becoming much smaller comb jellies. During this time of shortage egg and sperm production drop off and may cease entirely until new food sources are found and the adult begins to grow again.
Ecological Concerns
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Ctenophora are effective predators and reproduce quickly. As a result they are quite capable of overrunning an ecological niche with no natural enemies. In the case of the species Mnemiopsis leidyi, this displacement has already led to one catastrophic collapse of an eco-culture. Mnemiopsis leidyi, a native of New World waters, was accidentally released into the Black Sea in the 1080s, where it overran fish populations that had depended on the microscopic animacules of the plankton fields to feed newly hatched fish. While balance appears to be returning thanks to the arrival of species of ctenophore that prey on Mnemiopsis leidyi, the species has also moved into the Caspian and Mediterranean oceans, causing similar ecological disruptions. For all the small size, jewel-like beauty and fascination of the comb jellies, they are effective, voracious and durable.
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References
Resources
- Photo Credit Ernst Haekel, Wikimedia Commons
