Sea anemones are soft-bodied invertebrates belonging to the phylum Cnidaria, related to jellyfish and corals. These organisms are sessile polyps, anchored to a surface with a tube-like body topped by stinging tentacles. While their appearance suggests constant openness, the answer to whether they close at night is nuanced: many species retract their tentacles and column, but this behavior is not universal.
The Mechanics of Tentacle Retraction
Anemones lack a rigid internal skeleton, relying instead on a hydrostatic skeleton for support and movement. This system uses the incompressible seawater held within the central body cavity, the coelenteron. When expanded, this fluid is under pressure, maintaining the body’s inflated shape and extending the tentacles.
To retract, the anemone employs specialized muscles that work against the internal water pressure. The retractor muscles, longitudinal muscles running along the mesenteries, contract to shorten the body. This action forces water out of the coelenteron through the mouth, causing the body to deflate and the oral disc to pull downward.
A powerful sphincter muscle around the rim of the oral disc then contracts, cinching the top closed. This pulls the tentacles and oral disc completely inward, resulting in a small, compact ball of tissue. Re-expansion is a slower process, requiring the animal to relax the muscles and slowly pump water back into the column to re-inflate the structure.
Biological Drivers for Nocturnal Shelter
One primary reason for nocturnal retraction is energy conservation by reducing metabolic expenditure. Anemones are opportunistic predators that capture prey using their tentacles, a process less effective in the dark when plankton and small fish are less visible. By closing, the anemone conserves the energy it would spend maintaining its expanded posture and searching for food.
Retraction also functions as an effective predation avoidance strategy. While anemones have stinging cells, certain nocturnal predators, such as sea slugs and specialized snails, are adapted to feed on them. By pulling the vulnerable tentacles and oral disc into a compact, tougher ball, the anemone reduces its exposed surface area and protects its soft tissues from being grazed.
The closing behavior is often tied to the symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the anemone’s tissues and perform photosynthesis, providing the host with sugars and oxygen. Since zooxanthellae require sunlight, they become inactive once the light disappears. Closing at night reduces the exposed surface area and signals a temporary shutdown of the daylight-dependent food production process.
Species Differences and Environmental Variables
Not all anemone species exhibit nocturnal retraction, as their environment and primary food source dictate activity cycles. Species relying less on light-dependent zooxanthellae or adapted to deep water may remain open throughout the night, hunting for prey. For example, certain anemones capture significantly more prey, such as juvenile bivalves, during nighttime high tides, suggesting they remain active during dark feeding periods.
In the wild, tidal influence is a powerful driver that can override the light-dark cycle. Anemones living in the intertidal zone (the area exposed during low tide) close tightly regardless of the time of day to prevent desiccation. By retracting and sealing, they create a watertight barrier that retains internal moisture until the tide returns.
The behavior observed in a controlled setting, such as an aquarium, can differ from the natural environment. Constant, artificial light at night can disrupt the natural circadian rhythm and feeding activity. Additionally, the constant availability of food may lead some species to remain partially or fully open, overriding the natural energy conservation mechanism that typically triggers nocturnal closing.