The sea anemone, also known as ortiguilla de mar or sea nettle, is widely known in the cuisine of southern Spain and Italy; however, they are also important for the functioning of coral reefs, and their toxins are being studied for their potential to combat diseases that affect both animals and humans.
These fascinating marine invertebrates are often found anchored to rocks, reefs, and even submerged objects. Although they may appear to be delicate flowers swaying in the ocean, these creatures are far from defenseless. With stinging tentacles, a symbiotic relationship with various species like the clownfish (popularized by the movie Finding Nemo), and a fundamental role in marine ecosystems, sea anemones are an incredible example of nature’s complexity.
However, sea anemones sold in the marine ornamental industry depend on the extraction of these organisms from the wild, which threatens their populations. In this sense, aquaculture is a possible solution to address overexploitation problems, and individuals can be used to supply the trade or to restore coral reefs (Scott, 2017).
On the other hand, sea anemones are seen as threats because they are part of the biofouling that attaches to marine aquaculture structures, ports, boats, etc. In the case of mussel farming, sea anemones often compete for space with the mussels, suggesting that this species can be an economically significant component of biofouling (Babarro et al, 2018).
This article aims to describe the main characteristics of sea anemones, advances in their aquaculture, and the cuisine associated with this species.
What is a sea anemone?
A sea anemone is a predatory animal from the phylum Cnidaria, the same group that includes jellyfish and corals. Sea anemones are part of the class Anthozoa, which is typically characterized by their colorful bodies covered in tentacles attached to a base called a pedal disc. Their tentacles are filled with cnidocytes, specialized stinging cells, which they use both to capture prey and for defense.
Taxonomy of Sea Anemones
- Domain: Eukaryotes
- Kingdom: Animalia
- Phylum: Cnidaria
- Class: Hexacorallia
- Order: Actiniaria, Hertwig, 1882
Sea anemones (actiniarians or actinias) are marine invertebrate animals that adhere to substrates, mainly rock, and sometimes to the shells of crustaceans or mollusks. Between 1,000 and 1,200 species of sea anemones have been reported worldwide, ranging in size from 1.25 cm to 2 m in diameter.
Classification and Types of Sea Anemones
Sea anemones are classified under the order Actiniaria, within the class Anthozoa. Their taxonomy is primarily based on the structure of their tentacles, body shape, and reproductive strategies. Some of the most well-known types include:
- Giant Carpet Anemone (Stichodactyla gigantea): A large, flat species that can host multiple clownfish species.
- Bubble-tip Anemone (Entacmaea quadricolor): Named for its distinctive bubble-shaped tentacle tips, commonly seen in home aquariums.
- Condylactis Anemone (Condylactis gigantea): A popular species in the Caribbean, known for its vibrant colors and size.
- Aggregating Anemone (Anthopleura elegantissima): Found in temperate waters and noted for its ability to form large colonies.
Tentacles and Cnidocytes: The Power of Defense
The tentacles of sea anemones are their most iconic feature, used to capture prey and for defense. These tentacles are armed with cnidocytes, specialized cells containing stinging organelles known as nematocysts. When triggered by touch, the nematocysts of the sea anemone explode outward, injecting venom into the target, whether prey or a potential predator. This venom paralyzes small fish or invertebrates, which are then drawn toward the anemone’s central mouth for digestion.
Habitat of Sea Anemones
Sea anemones can be found in a variety of marine habitats, from shallow coral reefs to the ocean floor. Most species prefer the warmer waters of tropical seas, but some are well adapted to colder environments, such as the plumose anemones (Metridium senile), which thrive in the colder waters of the North Atlantic and Pacific Oceans.
Although some species of sea anemones can swim freely, most adult sea nettles remain in one place. With over 1,000 species, sea anemones vary greatly in size, color, and shape. Some, like the Caribbean giant anemone (Condylactis gigantea), can reach impressive sizes, while others, like the aggregating anemone (Anthopleura elegantissima), are small but form enormous colonies.
Similar to many marine species, sea nettles are vulnerable to the effects of climate change. In this regard, Hobbs et al (2013) report that sea anemones are susceptible to increased severity and frequency of bleaching events. They indicate that population viability will be severely compromised if anemones, and their symbionts, cannot adapt to rising sea temperatures.
Meanwhile, Taira et al., (2024) documented the prevalence of bleaching and the recovery of host sea anemones, and quantified changes in clownfish abundance at two coral reefs in Singapore during and after the 2016 mass bleaching event. They concluded the following:
- Anemone bleaching is common: The study revealed that a significant number of host anemones, particularly Entacmaea quadricolor, suffered bleaching during the 2016 mass bleaching event in Singapore.
- Bleaching affects anemone size: Bleached E. quadricolor anemones exhibited significant size reductions, even after some regained pigmentation. This suggests that bleaching can have long-lasting effects on the host’s health and growth.
- Recovery rates vary: Recovery rates of bleached E. quadricolor differed significantly between the two study sites, indicating that local environmental factors may influence the resilience of these anemones to bleaching events.
- Clownfish populations are affected: The abundance of associated clownfish (Amphiprion frenatus) was closely tied to the bleaching status and size of their host anemones. This highlights the importance of healthy host anemones to support viable clownfish populations.
What do sea anemones eat?
Although sea anemones are primarily carnivorous and feed on small fish, plankton, and invertebrates, some species also rely on a symbiotic relationship with zooxanthellae, photosynthetic algae that live within their tissues. This relationship allows the anemone to harness energy from sunlight, providing them with additional nutrients beyond those captured through hunting.
For example, species such as the magnificent sea anemone (Heteractis magnifica) depend on both their tentacles to capture prey and their symbiotic algae for photosynthetic energy. This combination makes them suitable for a wide range of environments, from nutrient-rich coastal areas to more nutrient-poor reefs.
On the other hand, researchers Roopin & Chadwick (2009) report that sea anemones can absorb nutrients from the water, which serves as an important nutritional pathway in addition to capturing larger prey.
How do sea anemones reproduce?
In most species of sea nettles, sexes are separate, while other species are hermaphroditic. Anemones can reproduce either sexually or asexually.
In sexual reproduction, they release eggs and sperm into the water, where fertilization occurs. The fertilized eggs develop into freely swimming larvae called planulae, which eventually settle and grow into new anemones.
Asexually, sea anemones can reproduce through processes like fission, where an individual splits into two, or budding, where smaller anemones grow from the parent. This allows anemones to quickly colonize new areas, making them resilient in their environment.
In this regard, Moore et al (2020) evaluated the feasibility of asexual propagation of wild anemone breeders (Heteractis crispa and Entacmaea quadricolor) associated with cardinal fish (Pterapogon kauderni). They reported on the potential of the propagation method.
Symbiotic relationship with clownfish
One of the most famous associations in the marine world is the mutualistic relationship between sea anemones and clownfish. Clownfish, such as Amphiprion ocellaris, are immune to the stings of their host anemone thanks to a special mucus layer. In exchange for protection from predators, clownfish help keep the anemone clean by removing waste and may even attract prey towards the anemone’s tentacles.
This relationship is highly specific to each species. Different types of sea anemones host different species of clownfish. For example, Heteractis magnifica is known to host up to 12 species of clownfish. This relationship not only benefits the sea anemone but also plays a crucial role in the survival of clownfish in the wild.
In this context, Roux et al (2019) report two hypotheses regarding the symbiotic relationship between clownfish and sea anemones. The first suggests that the fish are protected from the sea anemone’s sting by their mucus, which prevents nematocyst discharge or protects the fish from the consequences of discharge, while the second hypothesis proposes that clownfish cover themselves with the anemone’s mucus, using it as a chemical camouflage.
Other animals associated with sea anemones include cardinalfish, gobies, various species of crabs, shrimp, and several marine snails. Schnytzer et al., (2022) report that crabs from the Polydectinae subfamily of xanthids (boxer crabs) have the notable habit of holding a sea anemone in each of their claws; for their part, Karplus (2024) describes the association between hermit crabs and sea anemones as a common facultative mutualism.
Care for Sea Anemones in Aquariums
Many sea anemones are popular in the aquarium trade, particularly those that host clownfish. The bubble-tip anemone is especially common due to its relatively easy care and compatibility with clownfish species. However, caring for sea anemones in captivity requires careful attention to water quality, light levels, and tankmates, as they can be aggressive towards other invertebrates.
Fraser et al. (2021) reported that the three most desired species of sea anemones by marine aquarists were Entacmaea quadricolor (bubble-tip anemone), Stichodactyla tapetum, and Heteractis magnifica; and that size preferences for the anemones were typically smaller (tentacle crown diameter of 100 to 200 mm).
Potential Medicinal Compounds Found in Sea Anemones
Frazão y Antunes (2012) report that various studies highlight the existence of around 250 compounds (peptides, proteins, enzymes, and protease inhibitors) and non-protein substances (purines, quaternary ammonium compounds, biogenic amines, and betaines) present in sea anemones, with 156 of these peptides being toxic, particularly actinoporins (PLA2s) of greater interest.
As mentioned earlier, the sea anemone uses toxins for prey capture, but also to deter potential predators and even in territorial disputes.
Ramkumar et al (2012) report on the use of venom from the sea anemone (Anthopleura asiatica) in mouse trials, with results showing necrosis in the brain, hemolysis in the heart, and hemolysis in the liver, among other symptoms.
However, the bacteria present in sea anemones may have antimicrobial potential. In this regard, León et al (2018) explored the antimicrobial activity of bacteria found in the microbiota of Anemonia sulcata and Actinia equina and identified antibacterial and antifungal activity from microorganisms affecting humans, fish, or other organisms.
Researchers at the Institute of Marine Sciences and Limnology (ICMyL) of the Autonomous University of Mexico studied the antitumor properties of compounds from the anemone Bunodeopsis globulifera (B. antilliensis), identifying 15 compounds with potential as drugs against cancer cells.
Furthermore, researchers from the Far Eastern Federal University (Russia) report that the sea anemone Heteractis magnifica contains neuroprotective peptides that slow down the inflammation process and neuronal degradation caused by Alzheimer’s disease.
Finally, Madio et al (2019) present a new organization of the components of sea anemone venom based on proteins and non-protein compounds, and secondly among enzymes and other proteins without enzymatic activity, their structure, and their molecular target.
Aquaculture of Sea Anemones
There is currently a growing interest in the aquaculture of sea anemones, not only to supply the marine aquarium industry but also for the extraction of compounds that have the potential to combat some diseases affecting humans. According to the results of the study by Fraser et al., (2021), marine aquarium hobbyists surveyed indicated that they would prefer to buy captive-bred anemones rather than wild-collected anemones (95% of hobbyists, 94% of businesses) and would pay more for the former (79% of hobbyists, 70% of businesses).
The first experiences in cultivating sea anemones occurred with the species known as “sea nettle” (Anemonia sulcata), developed by researchers at the University of Granada (Spain) through the spin-off iMare Natural S.L. since 2013.
Ahsin et al (2016) studied the survival and growth rates of three species of sea nettles: Stichodactyla gigantea, Entacmaea quadricolor, and Macrodactyla doreensis, cultivated in Indonesia. They found that asexual reproduction technology for anemone seed production was effective; however, S. gigantea and E. quadricolor had the highest growth and survival rates.
Watson y Younger (2022) explored the survival and growth of the anemone (Entacmaea quadricolor) with and without feeding. They recommend the fragmentation method to expand the cultivation of the sea anemone E. quadricolor. Meanwhile, Fraser (2022) reports that the ideal density for ex situ breeding of Radianthus crispa larvae is between 3000 and 4000 larvae·L-1. After settling, Fraser improved the growth of juvenile R. crispa by feeding them live Artemia nauplii or metanauplii every four or five days for the next three months.
Sea anemone aquaculture is still in its early stages; however, its greatest potential lies in being part of integrated multi-trophic aquaculture systems.
Consumption of Sea Nettles
Sea anemones are consumed in southern Spain and Italy. In Italy, the sea nettle is known as “capelli di Venere.”
In Spain, Anemonia sulcata is highly valued for its culinary merits and is found on the menus of many restaurants. Its consumption mainly occurs in the coastal areas of Cádiz province and the Ebro Delta.
Spaniards can buy sea anemones in supermarkets and restaurants; however, these sea nettles come from natural populations.
Aymerich (2021) reports that the most common way to consume sea nettles is breaded in flour and fried in olive oil; however, it is recommended to soak the nettles in cold seawater or to immerse and store them in vinegar and water for hours until they marinate a bit, in order to neutralize their stinging power.
Conclusion
Sea anemones are important species for Spanish and Italian cuisine, as a source of compounds for the pharmaceutical industry, for the functioning of coral reefs, and for the marine ornamental industry.
However, the dependence on natural populations of sea anemones has led to overexploitation and put this marine species at risk. In this context, the breeding of sea nettles emerges as an alternative.
Unfortunately, there has been little progress in the aquaculture of sea anemones; however, their greatest potential is observed as part of an integrated multi-trophic aquaculture system.
References
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