The mesmerizing tentacles of sea anemones waving in the ocean current capture the imagination and invite curiosity into the mysterious lives of these prehistoric creatures. If you’ve ever wondered about the life cycle of sea anemones, you’re not alone!
Read on to uncover the complete life story – from polyp to full grown anemone – of these exotic coral reef dwellers.
If you’re short on time, here’s a quick answer to your question: The life cycle of a sea anemone begins when free-swimming larvae settle down on rocks or coral and transform into polyps. The polyp reproduces asexually by budding and splitting to form a colony of cloned polyps.
Eventually, polyps mature into adult sea anemones with tentacles used for capturing food and defense.
Sea Anemone Reproduction
Asexual Reproduction
Sea anemones can reproduce asexually through several methods. One common way is through longitudinal fission, where the anemone splits itself down the middle from oral disc to basal disc, creating two identical clones.
This usually happens when the anemone is stressed by things like lack of food, changes in water temperature, or damage. Another method is pedal laceration. In this process, small pieces of the pedal disc at the base break off and develop into new individuals. The original anemone is unharmed.
These fragments can attach to a substrate and regenerate all missing parts. A third asexual method is transverse fission. Here, the oral disc constricts and separates from the basal disc. Each half then regenerates the missing parts and develops into a whole new sea anemone.
Asexual reproduction allows sea anemones to quickly colonize an area. Each individual produced is genetically identical to the parent. This can be advantageous in stable environments but makes populations vulnerable if conditions change.
Some species like the beadlet anemone (Actinia equina) rely heavily on cloning for propagation. Researchers found genetically identical individuals up to 180 meters apart, suggesting extensive asexual spread.
Other species utilize asexual reproduction when needed but favor sexual reproduction when possible.
Sexual Reproduction
Most sea anemones reproduce sexually through spawning. Both males and females release sperm and eggs into the water, where fertilization takes place externally. Some brooding species keep eggs inside the gastrovascular cavity for a period before releasing swimming planula larvae.
Sexual reproduction promotes genetic diversity, allowing populations to adapt to changing conditions.
Sea anemones have complex sex lives. They can be male, female, or hermaphroditic. Some even change sex during their lifetime in a process called sequential hermaphroditism! The clownfish-hosting anemone Entacmaea quadricolor starts life as a male, then later becomes female.
Research shows the switch is influenced by the presence or absence of female anemones nearby. Pretty cool right!
Anemones only reproduce sexually when certain environmental cues are present. These signals vary by species but often include changes in water temperature, salinity, or food availability. Some species like the snakelocks anemone (Anemonia sulcata) spawn just once per year, while others can spawn multiple times annually when conditions are right.
Careful timing ensures larvae have the best chance for survival.
Larval Stage
Planula Larvae
After fertilization, sea anemone eggs develop into free-swimming larvae called planula larvae. These tiny larvae are oval-shaped and covered with cilia that help them move through the water. Planula larvae spend a few days to weeks floating in the plankton before settling on the seabed and undergoing metamorphosis.
While in the plankton, planula larvae are extremely vulnerable to predation. To improve their chances of survival, some species produce toxins or stinging cells called nematocysts while in the larval stage.
Other adaptations include transparent bodies, rhythmic swimming behaviors, and positive phototaxis (movement towards light) to reach optimal habitats.
Settlement and Metamorphosis
Once a planula larva finds a suitable surface, it settles and attaches using specialized attachment cells. Metamorphosis quickly follows, changing the free-swimming larva into a juvenile polyp anchored to the substrate. This transformation involves major structural and biochemical changes, such as:
- Loss of cilia and development of tentacles
- Formation of mesenteries (internal folds)
- Development of nematocysts and muscles
- Differentiation of digestive system
Metamorphosis is triggered by environmental cues like specific chemical signals or optimal light conditions. The process typically takes 24-72 hours, after which the tiny anemone begins feeding and growing into an adult.
Survival rates during settlement and metamorphosis are very low, estimated at only 1% in some populations. Finding an appropriate habitat is crucial, as the sessile juvenile will be stuck in that spot. Predation and other threats remain high during these vulnerable stages.
Polyp Stage
Budding and Splitting
Sea anemones reproduce asexually during the polyp stage through several methods like budding and splitting. In budding, a small bulb grows on the body wall of the adult anemone. This bulb eventually detaches and becomes a genetically identical clone of the parent anemone.
Budding allows sea anemones to rapidly populate an area. According to a 2009 study, some sea anemone species like Anthopleura elegantissima can bud up to 12 times per year.
Splitting is another asexual method where the pedal disc and tentacles of the adult anemone divide into two halves. Each half then regenerates the missing parts, resulting in two complete sea anemones that are clones of the original.
Splitting may occur when the parent anemone gets too large, allowing it to spread to new locations. A 2005 study on a species called Sococapsoides found it could split up to 16 times per year.
Growth into Adult Anemone
Once the small bulb or pedal disc has split from the parent anemone during budding or splitting, it begins growing into an adult sea anemone. Budded clones first form tentacle stubs and a mouth. Within a few weeks, they have fully developed into juvenile anemones.
They continue growing for up to a year before reaching sexual maturity. Anemones produced by splitting follow a similar progression, rapidly regenerating any missing parts before growing to adult size.
As the young anemone grows, it moves to a suitable hard surface like a rock or shell fragment. Here it attaches via the pedal disc to become permanently anchored. Different species grow at varying rates based on factors like water temperature, food availability, and genetics.
For example, according to a 1995 report, a cold water species called Metridium grows slowly, adding just 10-20mm to its size each year. But a tropical species like Condylactis gigantea can grow over 25cm per year, reaching over 1m in size as an adult!
Adult Sea Anemone
Anatomy
Sea anemones have a relatively simple body plan, with a column-shaped body and an oral disk surrounded by tentacles. The column contains the gastrovascular cavity, which functions as both a stomach and intestine, with a single opening that serves as both a mouth and anus.
Within the gastrovascular cavity is the pharynx, which pumps water and nutrients through the body. Suspended within the gastrovascular cavity are the gonads for reproduction. The tentacles contain specialized stinging cells called nematocysts that fire venom out to capture prey (the sea anemone’s tentacles provide it the means to feed).
At the base is an adhesive foot called the pedal disc that attaches the anemone to hard surfaces (Source).
Behavior and Habits
Sea anemones exhibit relatively simple behaviors mainly focused on food capture, defense, and reproduction. They typically anchor themselves to rocks or coral on the seafloor and extend out their brightly colored tentacles to attract fish and trap passing plankton for food.
Many sea anemones establish symbiotic relationships with certain fish species that seek refuge within their tentacles, with the fish gaining protection while the anemone gains scraps from the fish’s meal (NOAA). When disturbed, they quickly withdraw their tentacles back into their column.
Sea anemones also reproduce both sexually through gametes released into the water to be fertilized externally and asexually by splitting into clones through longitudinal fission.
Diet | Small fish, shrimp, plankton – captured using stinging tentacles |
Locomotion Method | Sessile – moves very little, anchored to rocks/surfaces by pedal disc |
Lifespan
The lifespan of sea anemones varies by species, ranging from one to over 50 years. Some sea anemones like Anthopleura elegantissima may live 80 to 100 years or more by continuously cloning themselves through longitudinal fission.
Their longevity enables sea anemones to form expansive undersea “clonal colonies” where all members are genetically identical. While the pedal disc usually detaches after splitting, sometimes the clones remain attached in colorful formations (McGill University).
By spreading clones around a region, sea anemones can better exploit microhabitats for resources and maximize their chance of survival as a genotype.
Conclusion
The exotic sea anemone exemplifies nature’s boundless diversity. Tracking the sea anemone life cycle from larvae to polyp to mature animal reveals strategies of survival perfected over eons. Sea anemones have lived in Earth’s oceans for over 500 million years, a testament to the success of their specialized adaptations.
Understanding the sea anemone life cycle provides a window into the interconnectedness of coral reef ecosystems.