By Catherine Alves, Marine Ecologist
Description
Ctenophores (the “C” is silent – pronounced “Teen – oh – fore”) are a type of gelatinous marine plankton, also known as comb jellies. They can be found in marine habitats from shallow coastal waters to the deep sea. The name ctenophore comes from the Greek, meaning “comb bearer” because they have plates of giant fused cilia (known as “combs” or “ctenes”) along their body.
Many people get ctenophores confused with their jellyfish relatives, but they are in different phyla – jellyfish are in Cnidaria while comb jellies are in Ctenophora. Both have existed for 500 million years. Ctenophores are invertebrates, so they don’t have a backbone and have pretty simple body plans. However, they still play an important role in coastal and marine ecosystems and have an interesting biology.
Anatomy and Movement
Like their jellyfish relatives, ctenophores have gelatinous body plans, but unlike jellyfish, the comb jellies you would find in Rhode Island can’t sting you! They do not possess the stinging cells that jellyfish have on their tentacles. The most distinctive feature on ctenophores is their eight rows of cilia, that they move like little oars to propel them through the water. Ctenophores have two major layers of cells: the epidermis (on the outside) and the gastrodermis (on the inside). Although they live in the water, they do not possess gills or lungs. Instead, they are able to “breathe” through the cells; i.e. oxygen and nutrients easily pass through the cell layers. Their bodies are made primarily of water, with several structural proteins, muscle cells and nerve cells making up the internal skeleton. They do contain one mouth-like opening, through which food and waste pass.
Feeding
All ctenophores are carnivores, which means they are animals that eat other animals. Their prey consists of zooplankton including copepods, krill, amphipods, and clam and snail larvae. Because of this generalist feeding behavior, they are able to optimize what they eat based on prey availability. Some ctenophores have tentacles with colloblasts lining them. These colloblasts release a glue-like substance to stick to the prey item, which the comb jelly then reels in to their mouth (like a fishing line, sticking to food, and reeling it in). Other ctenophores have two flattened lobes that reach below their mouths. Special cilia lining the lobes move, creating a current (like a vacuum), through which planktonic food gets pulled between the lobes and into the mouth. Additional ctenophores without tentacles simply use their mouths to open wide, swallow their prey whole, and then close their mouths shut. These ctenophores have small cilia in their mouths that act like teeth to pull the food apart and direct it to the gut (the gastrodermis).
Defense Mechanisms
Ctenophores are preyed upon (eaten) by some jellyfish species, sea turtles, and some fish. They have adapted the ability to produce light, likely to avoid getting eaten, and in some cases, to attract prey. The ability of animals to produce light is known as bioluminescence and is made possible by special proteins that produce light from a chemical reaction. Often this is a response to an external stimulus, like touch or water agitation. Ctenophores contain genes that code for the green-fluorescent protein (GFP), which glows bright green, and enables them to be bioluminescent. However, the bioluminescence can only be seen in the dark/at night. You might be able to see the rows of cilia illuminating/reflecting light during the day. That is due to visible light scattering and reflecting off the moving cilia within the ctenophore’s clear body. Other phyla that can also produce GFP include Cnidaria (jellyfish, corals), Arthropoda (copepods, crabs, bugs), and Chordata (tunicates, vertebrates).
Reproduction
Ctenophores spend all of their life cycle as plankton, meaning they are “holoplanktonic.” Plankton are small plants and animals that live in the water and cannot swim against a current (so mostly move with the water). Most ctenophores are hermaphroditic, making them able to release egg and sperm into the water, as frequently as every day. The egg and sperm cells eventually find other sex cells in the water. Once the egg gets fertilized by the sperm, the embryo develops into a small larva that resembles a small adult. Throughout its life, it just grows larger.
Ecology
The most common ctenophore species found in Rhode Island coastal waters is the sea walnut (Mnemiopsis leidyi). It is native to the east coast of North and South America. The ctenophore is most abundant during the warm spring through early fall, primarily in temperate waters. During the summer months, they can dominate the planktonic communities, becoming the food source for a variety of fishes and turtles, and compete with other fish for zooplankton food. The winter is too cold for them to reproduce, so they often move offshore to deeper waters. They can tolerate temperatures between 0 – 32 degrees Celsius (32 – 89.6 degrees Fahrenheit), and a wide range of salinities; from nearly freshwater to hypersaline lagoons. Therefore, ctenophores can even be found in estuaries, like the Narrow River Estuary, which has brackish (half fresh, half salt) water.
Interestingly, the ctenophore was introduced to the Black Sea in the early 1980s in ship ballast water. Due to their generalist diet on a variety of zooplankton species, they outcompeted the native fishes, leading to the rapid decline of commercially important fish species. This marine invasion resulted in the collapse of fisheries in six countries! Scientists are especially interested in studying what made this invasion successful to learn more about marine plankton and predict what could happen if ctenophores (and other plankton) expand their ecological ranges. This is especially timely research given the proposed range expansions of many marine species due to climate change.
Resources
https://ocean.si.edu/ocean-life/invertebrates/jellyfish-and-comb-jellies
http://faculty.washington.edu/cemills/Ctenophores.html
https://ucmp.berkeley.edu/cnidaria/ctenophora.html
Haddock, S. H. D., Mastroianni, N. and L. M. Christianson. 2010. A photoactivatable green-fluorescent protein from the phylum Ctenophora. Proceedings of the Royal Society B. 277:1155-1160.
