Sea anemones are predatory marine invertebrates belonging to the phylum Cnidaria. Unlike their free-swimming relatives, anemones spend their adult lives anchored to a hard surface, existing as a single, stationary polyp. Anemones spread definitively, primarily through efficient and prolific self-cloning. This ability to multiply rapidly allows them to maximize their presence in favorable environments.
Asexual Reproduction The Primary Methods of Spreading
Asexual reproduction bypasses the need for a partner. The most visually dramatic form of this cloning is fission, where a single polyp physically divides itself into two genetically identical individuals. This division can occur longitudinally, where the anemone splits down its central axis, or less commonly, transversely. This results in two complete, yet smaller, organisms.
This splitting process is often observed in popular species like the aggregating anemone, Anthopleura elegantissima, which forms dense clonal colonies in the wild. The newly created clones are identical to the parent, allowing a successful genetic line to quickly colonize an area. This method is incredibly efficient because it requires very little energy compared to creating and raising sexual offspring.
Another highly effective asexual strategy is pedal laceration. As the anemone moves or contracts its body, small fragments of tissue can break off from its pedal disc, the muscular foot it uses for attachment. These tiny pieces of basal tissue are capable of regenerating into fully formed, miniature anemones. This low-energy fragmentation allows a single anemone to passively scatter numerous offspring across a substrate, a method commonly employed by species like the pest anemone, Aiptasia.
Environmental Factors Triggering Division
Anemones are sensitive to their surroundings, and their decision to divide asexually is often a direct response to environmental cues. Both optimal and stressful conditions can trigger a splitting event. For species that host symbiotic algae, high light levels, stable temperature, and abundant feeding can lead to rapid growth and subsequent splitting. This maximizes resource utilization and expands territory when conditions are ideal for survival.
Conversely, sudden environmental shifts or poor water quality can also induce asexual reproduction as a survival mechanism. A drastic change in temperature or salinity may prompt an anemone to divide. This increases the chance that at least one clone survives the stress. The rate of pedal laceration, in particular, has been observed to increase significantly when anemones are attached to unstable substrates or are under some form of stress.
Sexual Reproduction and Genetic Diversity
While asexual cloning accounts for most rapid spreading, sea anemones also engage in sexual reproduction to ensure genetic diversity. This involves the release of gametes—eggs and sperm—into the water column. Gonads develop on the internal mesenteries, and the resulting gametes are typically expelled through the mouth.
Fertilization occurs externally in the water, forming a zygote that develops into a tiny, free-swimming organism called a planula larva. These larvae drift in the water before settling onto a hard surface. There they metamorphose into a new, genetically unique polyp.
Spawning events are often seasonal, cued by factors such as rising water temperatures. This sexual method is generally slower and far less predictable than asexual fission, meaning it is rarely the source of a sudden population boom in a localized area.
Controlling Anemone Proliferation in Captivity
Attempting to scrape or tear an anemone from a rock can be highly counterproductive. The remaining fragments of tissue will often regenerate into multiple new individuals via laceration. This response mimics the natural process of fragmentation and can lead to a population explosion of clone anemones.
A more effective approach involves targeted chemical or biological intervention. Many hobbyists use targeted injections of calcium hydroxide solution or concentrated lemon juice directly into the anemone’s oral disc. This method kills the anemone completely without leaving viable fragments to regenerate.
Introducing natural predators, such as the Berghia nudibranch or certain species of peppermint shrimp, can also provide continuous, long-term control over problematic species like Aiptasia. Additionally, reducing excess feeding in the aquarium can slow the growth rate of anemones, thereby reducing the likelihood of a self-splitting event.