Sea anemones, often mistaken for plants due to their colorful appearance, are actually predatory marine invertebrates belonging to the order Actiniaria. Named after the terrestrial anemone flower, these creatures are closely related to corals, jellyfish, and other cnidarians. Found worldwide in diverse marine habitats, sea anemones exhibit fascinating feeding strategies and dietary preferences.
Anatomy and Feeding Mechanisms
A typical sea anemone consists of a single polyp attached to a hard surface by its basal disc, though some species inhabit soft sediments or float freely. The polyp features a columnar trunk topped by an oral disc, surrounded by a ring of tentacles and a central mouth. These tentacles, which can number from a dozen to hundreds, are armed with cnidocytes, specialized stinging cells used for both defense and prey capture.
Cnidocytes contain nematocysts, capsule-like organelles that discharge a harpoon-like filament upon the slightest touch. This filament injects venom, composed of actinotoxins and neurotoxins, into the prey, paralyzing it. The tentacles then transport the immobilized prey to the mouth, where it is ingested into the gastrovascular cavity for digestion.
Dietary Preferences and Feeding Habits
Sea anemones are carnivores, preying on a variety of organisms ranging from tiny plankton to small fish, crabs, and even jellyfish. Their diet largely depends on their size and the availability of prey in their surrounding environment. Smaller anemones consume microscopic organisms, while larger ones can ingest sizable creatures.
The giant plumose anemone, Metridium farcimen, has a surprisingly diverse diet. Research has revealed that they consume arthropods, especially crab larvae, barnacle larvae or molts, copepods, and insects. Notably, a significant portion of their diet, about 10% in one study, consisted of ants, specifically the pale-legged field ant (Lasius pallitarsis). Spiders and other insects that inadvertently fall into the water may also become part of their diet.
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Anthopleura nigrescens, a common sea anemone found in Costa Rica's rocky intertidal habitats, preys mainly on cypris larvae, juvenile bivalves, and copepods. The predominance of cypris larvae in their diet may be linked to the reproductive cycles of local barnacle populations and larval settlement patterns. Juvenile bivalves are likely captured during their settlement process, while copepods are caught while swimming in the water column.
The Role of Symbiosis in Nutrition
Many sea anemones form mutualistic relationships with single-celled algae, such as zooxanthellae and zoochlorellae, that reside within their gastrodermal cells. These algae provide the anemone with oxygen and food in the form of glycerol, glucose, and alanine, products of photosynthesis. In return, the anemone offers the algae protection from micro-feeders and ensures reliable exposure to sunlight.
The color of some anemones, like the aggregating anemone (Anthopleura elegantissima), is largely determined by the types and proportions of symbiotic algae present. Some species, such as the hidden anemone (Lebrunia coralligens), have specialized pseudotentacles rich in zooxanthellae to maximize photosynthetic benefits.
Feeding Strategies
Sea anemones are typically sessile, attaching to a hard surface and remaining in the same spot for extended periods. They rely on their stinging tentacles to ensnare prey that comes within reach. The current helps to rid the anemone of waste products and mucus, helping to repel bacteria. A moderate to brisk flow is generally appreciated, but it does vary between anemones.
In addition to capturing prey, sea anemones can absorb dissolved organic compounds from the water, supplementing their nutritional intake. This is particularly important for species that live in nutrient-poor environments.
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Special Relationships
Sea anemones are known to form symbiotic relationships with other animals, their most well-known alliance is with clownfish. Because of a protective mucous coating, clownfish are immune to an anemone’s stinging cells. Several species of fish and invertebrates live in symbiotic or mutualistic relationships with sea anemones, most famously the clownfish.
Reproduction and Feeding
Anemones can reproduce sexually or asexually, depending on species and it's environment. Splitting in the aquarium may not always be related to reproduction, per se, and may be a survival tool that anemones use to ensure a continuation of genes.
Aquarium Diet
With adequate lighting, anemones do not need such gross feeding as once a day or once a week even. If an anemone is healthy it will grow. This can be a good indicator if it is receiving enough food. Some grow by utilising tank lighting only. If an anemone shrinks, then it is not receiving enough food (and or light).
Threats to Anemones
Besides their collection from the wild for use in reef aquaria, sea anemones are also threatened by alterations to their environment.
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