How Do Sea Cucumbers Reproduce? (Explained)

Sea cucumbers are fascinating marine creatures thanks to their unique ways of adaptation and defense mechanisms. They also have a unique way of reproduction, so if you wonder how sea cucumbers reproduce, in this blog post, we’ll talk all about that. However, let’s being with a quick answer:

Sea cucumbers can reproduce sexually and asexually. When reproducing sexually, females release their eggs, and males release their sperm into the water column, where they meet and fertilize. When reproducing asexually, sea cucumbers divide their body into two or more parts and grow new individuals from each piece.

However, this certainly doesn’t tell the whole story. Below, I’ll explain more about how sea cucumbers reproduce sexually and asexually. Furthermore, I’ll explain when and why they reproduce. I’ll also answer the questions about whether we can define sea cucumbers’ gender or not and what their life cycle looks like. Read on!

Sexual Reproduction

Sexual reproduction is the most common way of reproduction for sea cucumbers and also other closely related echinoderms, such as sea stars, sea urchins, sand dollars, brittle stars, and crinoids. Below, I’ll explain the process and reproduction patterns.

Spawning

Sea cucumbers reproduce sexually by a process called “spawning”. During this event, several females release their eggs, and several males release their sperm into the water column. It’s usually a seasonal event, and animals release their gametes (reproductive cells) simultaneously, so the possibility that eggs will meet with the sperm and fertilize is higher.

It’s easy to see when sea cucumbers are spawning because they adopt a characteristic cobra-like appearance. They raise their front body from the bottom and release eggs or sperm that are white, spherical, and easily visible while scuba diving or snorkeling.

Gametes are released from their gonad, through the gonopore located in front of the body, right behind their mouth. Interestingly, one female can release about 1 million eggs during spawning, but it depends on the species and conditions. See the video below to see the sea cucumber spawning:

Brooding eggs

Interestingly, some species fertilize their eggs internally instead externally by realizing them into the water column. They fertilize eggs inside or on their body and form zygote – cells formed by fertilization. Next, they pick it up with their feeding tentacles that surround their mouth and keep it in brooding chambers.

The brooding chambers can be located in different places depending on the species. Most common are their tentacular crown, the body surface (on the stomach or dorsum), or inside their body. An example of brooding species is a red-chested sea cucumber.

Species that care for their babies, keep them inside their bodies, and give a birth. After the miniature sea cucumbers are ready, the animal gives birth through a small rupture in the body wall close to the anus. Below you can see the video of a sea cucumber giving birth to its babies.

Another example that cares for their young is a swimming sea cucumber Enypniastes eximia. This species lives in the deep sea, and as the name suggests – it swims most of its life. This sea cucumber looks very different from the sea cucumbers we know as it developed webbed swimming fin-like structures that enable it to swim up off the seafloor.

The swimming sea cucumber reproduces by fertilizing its eggs and keeping babies inside because of the low possibility of successful spawning. If it releases gametes in the ocean, they may never meet due to the strong current and lower concentration of sea cucumbers and their sperm/eggs in the deep water.

When do sea cucumbers spawn?

Most sea cucumbers reproduce in a discrete and predictable annual cycle, but many complex environmental factors control their reproduction frequency as they can cause spawning to be less successful. These factors include:

• water temperature,
• light,
• phytoplankton blooms,
• the phase of the moon,
• food availability,
• tidal conditions.

Many species of sea cucumber have an annual reproductive pattern with spawning peaks in spring and summer. In this case, the development of the gonads starts with the lower seawater temperatures and shorter days (during winter). Consequently, spawning occurs when the seawater temperatures rise, and the photoperiod approaches the maximum, meaning there’s more light during the day.

The water temperature may majorly affect reproduction, increasing food availability through benthic productivity. When the phytoplankton population peaks, it increases changes in larval development. The reason for that is that feeding larvae need phytoplankton for their development.

For example, H. mammata in Portugal shows an annual reproduction pattern with a long spawning period from April to November.

Asexual Reproduction

Asexual reproduction is another way of reproduction for sea cucumbers but also their close relatives, sea stars, sea urchins, or brittle stars. This remarkable skill is based on the mutable properties of their connective tissues. Sea cucumbers can reproduce asexually either by fission or fragmentation.

Fission

Fission occurs when a sea cucumber splits its body into two parts, and each half regenerates into a new and complete individual. It’s a very complex process that is accompanied by complex behaviors, such as softening the body, stretching, and twisting it.

Different species of sea cucumbers can use different combinations of methods in order to “clone themselves”. However, most species begin with forming a contraction. Next, they either deepen the constrictions, soften their body and move forward, or stretch and twist at the fission site.

The fission location depends on the species, but in most studied species, it occurs across the middle of the body. The scientists haven’t described the closure of the wound yet, but it probably results from the contraction of circular muscles in the body wall.

Furthermore, the duration of the process varies from a few minutes to 1–5 days and depends on the body wall and how quickly sea cucumber can soften it. For instance, the body wall in S. chloronotus can quickly soften, and the process takes only several minutes.

After fission, the animal begins regenerating lost organs. After developing internal organs, sea cucumber grows the body bigger. The growth duration of the body varies and depends on the species and conditions. Approximately sea cucumbers can grow their body back within a few months.

Fragmentation

Fragmentation occurs when an animal splits into multiple fragments. Instead of dividing themselves in half, some sea cucumber species can divide their body into more pieces. Next, they grow into new individuals, similar to the fission process.

Why do sea cucumbers reproduce asexually?

The unique ability to reproduce asexually can help sustain a species in challenging conditions. This is exactly one of the main reasons why sea cucumbers and other animals reproduce asexually. However, the reasons might be slightly different depending on the animal development stage.

Asexual reproduction factors in the adult stage

In the adult stage, the main factors that influence asexual reproduction are low environmental stability, high mortality, small individual body size, and low sexual reproductive activity. All these factors threaten the sea cucumber population, and asexual reproduction helps them to survive.

Bad conditions are not only a threat to adult species but also, if they spawn, there’s a low possibility that gametes will meet and fertilize. What’s more, larval development will be less likely to occur when the conditions are not sustainable due to low food availability, water temperatures, and more.

Asexual reproduction factors in the larval stage

In the larval stage, the factors are not completely known, but there are experimental data on other echinoderms that can help us to determine the reason for asexual reproduction. For instance, sea stars and sea urchin larvae clone at a higher rate when they’re under the most suitable habitat conditions.

Thanks to optimum conditions, the possibility of larvae’s successful development increases. Most likely, the metamorphosis and reaching the juvenile stage will be successful, eventually resulting in the growth of population size.

However, in other species, asexual reproduction occurs when the external mucus from predators (ex. fish) is present. In this case, fission is triggered to avoid predators. Moreover, the copied larva is about half the size of the original one, so they are a less obvious target and can escape more successfully.

Can you define sea cucumber genders?

Most sea cucumbers don’t show external morphological differences between the sexes. However, some species have genital papillae (modified tube feet) located in front of their bodies, near the feeding tentacles, that can assist in defining the gender.

The male papillae are often longer than those of females. During spawning, these papillae resemble raised cones with eggs and sperm emerging from their tips. In some species, the males use their long genital papillae to transform sperm into females, where the eggs are fertilized and young develop.

Interestingly, some sea cucumber species are protandric, meaning they can change their gender throughout their lifetime.

The life cycle of sea cucumbers

So, what happens next to released eggs and sperm after the spawning (sexual reproduction)? Once these free-floating gametes meet in the water column, they fertilize and form a zygote (fertilized egg). Next, the egg transforms into a larva that starts forming mouth, anus, and other parts of the digestive system.

The larva floats freely for about a few weeks, feeding on suspended food particles. At this stage, it can’t defend itself from predators (unless it reproduces asexually), so its mortality rate is very high. After a few weeks, it settles on the bottom of the ocean and begins a metamorphosis into a juvenile.

The juvenile sea cucumber looks like a miniature animal, reaching a length between 0.5 mm (0.02 in) and 1.5 mm (0.06 in). It forms the rest of the organs and grows very slowly for the next months or years. It mostly hides from predators under rocks, among seaweeds, or by burrowing itself in the sand until reaching the adult stage.

If you’re interested more in this topic, read my other article: “The Complete Life Cycle Of A Sea Cucumber”.

Sources

• Hélène Laguerre, Grégory Raymond, Patrick Plan, Nadia Améziane, Xavier Bailly, Patrick Le Chevalier, “First description of embryonic and larval development, juvenile growth of the black sea-cucumber Holothuria forskali (Echinodermata: Holothuroidea), a new species for aquaculture in the north-eastern Atlantic.” Aquaculture, Volume 521, 2020, 734961, ISSN 0044-8486, https://doi.org/10.1016/j.aquaculture.2020.734961.
• Igor Yu. Dolmatov, “Asexual Reproduction in Holothurians”, The Scientific World Journal, vol. 2014, Article ID 527234, 13 pages, 2014, https://doi.org/10.1155/2014/527234
• Venâncio, Eliana, Pedro M. Félix, Ana C. Brito, Francisco Azevedo e Silva, Tomás Simões, João Sousa, Susana Mendes, and Ana Pombo. 2022. “Reproductive Biology of the Sea Cucumber Holothuria mammata (Echinodermata: Holothuroidea)” Biology 11, no. 5: 622. https://doi.org/10.3390/biology11050622