Comb jellies, scientifically known as Ctenophora, are a phylum of marine invertebrates found worldwide. These gelatinous creatures, ranging from millimeters to 1.5 meters in size, have intrigued biologists for their unique characteristics and evolutionary position. This article delves into the diet and nutritional strategies of comb jellies, exploring their feeding habits, digestive processes, and ecological roles.
General Information
Ctenophores, commonly known as comb jellies, are marine invertebrates found in seas worldwide. Adult comb jellies range in size from a few millimeters to 1.5 m (5 ft), depending on the species. Their bodies are made up of a mass of jelly, with a two-cell-thick layer on the outside and another lining the internal cavity. Despite their soft, gelatinous bodies, fossils thought to represent ctenophores have been discovered in Lagerstätten (well-preserved fossil beds) dating back to the early Cambrian, around 525 million years ago.
Anatomy and Physiology Relevant to Diet
Like cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material. In ctenophores and cnidarians this middle layer is called the mesoglea. More complex animals have three main cell layers and no intermediate jelly-like layer.
The main axis of these almost radially symmetrical animals runs from the mouth to the opposite end (oral to aboral). However, ctenophores lack mirror symmetry, despite having rotational symmetry, because only two of the canals near the statocyst terminate in anal pores. The epidermis (outer skin) consists of sensory cells, mucus-secreting cells, and interstitial cells that can transform into other cell types. Specialized parts of the outer layer contain colloblasts (used for prey capture along the surface of tentacles) or cells bearing multiple large cilia for locomotion.
The internal cavity comprises a mouth (usually closable by muscles), a pharynx ("throat"), a wider area in the center that acts as a stomach, and a system of internal canals. These canals branch through the mesoglea to the most active parts of the animal. The inner surface of the cavity is lined with an epithelium, the gastrodermis. The mouth and pharynx have both cilia and muscles. The gastrodermis is different on the sides nearest to and furthest from the organ that it supplies in other parts of the canal system. The nearer side is composed of tall nutritive cells that store nutrients in vacuoles (internal compartments), germ cells that produce eggs or sperm, and photocytes that produce bioluminescence.
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Feeding Mechanisms
Almost all ctenophores are predators, with only one genus being partly parasitic. They are carnivorous and opportunistic, feeding on whatever passes them by. If food is plentiful, they can eat ten times their own weight per day. Ctenophores employ a variety of feeding strategies, including:
- Tentacles as "webs": Some species hang motionless in the water, using their tentacles as "webs" to capture prey.
- Ambush predators: Some are ambush predators, similar to Salticid jumping spiders.
- Sticky droplets: Some dangle a sticky droplet at the end of a fine thread, like bolas spiders.
Colloblasts
Many comb jellies have a single pair of tentacles (often each tentacle is branched, giving the illusion of many tentacles) that they use like fishing lines to catch prey. They are armed with sticky cells (colloblasts) and unlike jellyfish, the tentacles of comb jellies don’t sting. Comb jellies are distinguished from all other animals by possession of sticky cells called colloblasts and their eight rows of cilia.
Using sticky cells (colloblasts) that are lined on their tentacles, they can capture prey and then move the victim to the main body for digestion. Upon touch, a spiral filament automatically bursts out of colloblast cells that releases the sticky glue. Once an item is stuck, the comb jelly reels in its tentacle and brings the food into its mouth.
Predatory Behavior
While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplankton (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepods, amphipods, and even krill. Members of the genus Haeckelia prey on jellyfish and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblasts. The tentacle-less beroids depend on their large mouths. Instead of catching food with colloblasts, they swallow their prey (often other ctenophores!) whole and then clamp their mouths shut, giving them no escape route.
Diet Composition
The diet of comb jellies varies based on where in the ocean they live, but it is most often a diet of zooplankton, larvae, and sometimes small fish or other organisms. Their prey consists of zooplankton including crustaceans, other jellies, eggs, and fish larvae. This comb jelly is a voracious carnivore and a major predator of edible zooplankton consuming up to 10 times its weight per day. It prefers a broad-based diet of zooplankton including eggs and larval forms of various invertebrates and fishes, juvenile fish, copepods, sea jellies, and even other ctenophores.
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Digestive Process
When prey is swallowed, it is liquefied in the pharynx by enzymes and muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia and digested by the nutritive cells. The ciliary rosettes may help to transport nutrients to muscles in the mesoglea.
Recently, scientists discovered that comb jellies remove waste not through their mouths like similar invertebrate species, but instead release the indigestible byproducts of waste into the water through pores at the end of their bodies.
Nutritional Adaptations
Comb jellies have several adaptations that aid in their nutrition:
- Voracious appetite: They can consume up to ten times their weight in food per day when food is plentiful.
- Hermaphroditism and rapid reproduction: This allows them to quickly increase their populations when food is abundant.
- Size adaptation: They can shrink in size during food shortages to conserve energy and resume growth when food becomes available again.
- Tolerance to environmental factors: Their tolerance to wide ranges of salinity, temperature, and other environmental factors allows them to thrive in conditions unfavorable for other species.
Ecological Role
Comb jellies play a significant role in marine ecosystems:
- Predators of zooplankton: They can significantly impact zooplankton populations, affecting the food web.
- Prey for larger animals: They serve as a food source for sea turtles, fishes, marine mammals, and other comb jellies.
- Indicators of ocean health: Their abundance can indicate the quality of the ocean, with large populations sometimes suggesting diminishing ocean health.
Invasive Species
The comb jelly (Mnemiopsis leidyi), sometimes called the “sea walnut,” is a species of tentaculate ctenophores (stingless jellyfish). It is native to the western Atlantic Ocean but has become a notorious invasive species in the Black, Caspian, Mediterranean, and North Seas of Europe and Western Asia.
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The unintentional introduction of Mnemiopsis leidyi into the Caspian Sea had a catastrophic effect on the entire ecosystem. Several years later it was introduced into the Baltic Sea where it devastated the anchovy fisheries. More recent introductions into other parts of Europe have caused severe hardships in local fisheries. Large populations of voracious comb jellies significantly reduce the volume of fish eggs and larvae and also diminished other planktonic forms that these developing animals require for food.