The Complete Life Cycle Of A Sea Cucumber
Sea cucumbers are one of the most fascinating marine creatures thanks to their unique ways of adaptation and defense mechanisms. They also have a unique way of reproduction, and if you wonder what the life cycle of the sea cucumber is and how they reproduce, in this blog post we’ll talk all about that. However, let’s begin with a quick answer:
The life cycle of sea cucumbers has five stages: spawning, egg, larvae, juvenile, and adult. They can reproduce sexually and asexually through the fission or fragmentation of the body, and they can live for about 5-10 years.
However, this certainly doesn’t tell the whole story. Below, I’ll explain more about each stage of the sea cucumber’s life cycle and their reproduction method. Furthermore, I’ll explain more about how they reproduce asexually and about the brooding species that lay eggs inside their bodies. Read on!
Sexual reproduction of a sea cucumber
Sexual reproduction is the most common form of reproduction, with animals having two separate sexes. However, some sea cucumber species are protandric, which means they can change their sexes at some point in their life. We can divide sea cucumber sexual reproduction into five stages: spawning, egg, larvae, juvenile, and adult.
Sea cucumbers reproduce by a process called spawning – the first stage of sea cucumbers’ life cycle. During this event, several females release their eggs, and several males release their sperm into the water column, all at the same time. It’s a common way of reproduction for all echinoderms (a group of animals that sea cucumbers belong to), such as sea stars, sand dollars, and sea urchins.
During spawning, sea cucumbers adopt a cobra-like appearance, with the front end of the body raised from the bottom. They release eggs or sperm from their gonad located in front of the body, right behind their mouth. See the video below to see the sea cucumber spawning:
Most sea cucumbers come together during reproduction, and billions of gametes (reproductive cells) are spewed into the water. This increases the likelihood that eggs will be successfully fertilized and won’t be eaten immediately by predators at the bottom of the ocean.
Furthermore, most sea cucumbers reproduce in a discrete and predictable annual cycle, but many complex environmental factors control their reproduction frequency, such as water temperature, light, phase of the moon, food availability, and tidal conditions.
Interestingly, one female during spawning can release about 1 million eggs, but it depends on the species and conditions. Eggs are white, spherical, and visible to the naked eye.
Once free-floating eggs and sperm interact, they successfully fertilize and form a zygote (fertilized egg). Successive cell divisions divide the zygote into smaller and more numerous cells forming two, four, eight, and more cell embryos.
The following stages of egg development are blastula and gastrula, which begin to form 35 hours after fertilization and take on the appearance of a ball.
The next stage of the sea cucumber’s life cycle is the larval stage. The first typical larval stage is auricularia which is only around 1 mm (39 mils) in length. This larva is characterized by a functional, visible mouth, other parts of the digestive system, and hair-like organelles used for feeding and locomotion.
At this stage, these larvae eat suspended food particles and float freely for about a few weeks before transforming into the next larval stage, called doliolaria. Doliolaria is a non-feeding larva that has a barrel-shaped body and three to five separate rings of cilia.
After a few days, the larva transforms into the last larval stage called pentactula. This larva starts developing tentacles surrounding sea cucumbers’ mouths. It also changes from planktonic larvae to benthic larvae, which means it settles on the bottom of the ocean. Once on the bottom, the larva begins a metamorphosis into a juvenile.
Interestingly, there are some species that fertilize their eggs internally, and larval development may be slightly different. We’ll talk more about brooding species below.
After the metamorphosis, the animal’s body begins looking like a miniature sea cucumber, reaching a length between 0.5 mm (0.02 in) and 1.5 mm (0.06 in). It forms the rest of the organs that later help the animal to function.
The juvenile sea cucumbers grow very slowly for the next months or years. They mostly hide from predators under rocks, among seaweeds, or by burrowing themselves in the sand. Moreover, they extend their tentacles to feed on suspended food particles and algae. After a few years, sea cucumbers reach sexual maturity and begin spawning.
Adult sea cucumbers normally grow to about 10 to 30 centimeters (4-12 in) in length. However, their size depends on the species, and some can grow even to 2-3 meters (6.5 – 10 ft) like the tiger’s tail or the giant red sea cucumber.
Most sea cucumbers live for 5 to 10 years. They adapted to many different habitats, living in almost all depths and water temperatures. Their main habitat includes coral reefs, sandy bottoms, rocky intertidal shores, seagrass meadows, mud, and even kelp forests. You can read more about it in my other post: “Where Do Sea Cucumbers Live?”.
Sea cucumbers survive thanks to their unique defense system. They can swim away when threatened, release toxins and their internal organs to scare away predators, or liquefy their bodies and escape through narrow spaces.
During adult life, sea cucumbers move slowly around the sea floor, collecting organic matter and carrying out the decomposition process. Because of their diet, they’re classified as decomposers and play an essential role in our ecosystem.
Asexual reproduction of a sea cucumber
Sea cucumbers and their close relatives, such as sea stars, sea urchins, or brittle stars, have the unique availability to reproduce asexually. This remarkable skill is based on the mutable properties of their connective tissues. Sea cucumbers can reproduce asexually either by fission or fragmentation.
Fission occurs when the animal 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.
The fission location depends on the species, but in most studied species, it occurs across the middle of the body. 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.
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 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.
Asexual reproduction factors
There are a few factors that influence asexual reproduction in sea cucumber species. In the adult stage, the main factors are:
- Low environmental stability,
- High mortality,
- Small individual body size,
- Low sexual reproductive activity.
Larval development will be less likely to occur when the conditions are not sustainable. This is why asexual reproduction can be a more effective way.
In the larval stage, the factors are unknown, 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.
These optimum conditions increase the possibility of larvae’s successful development, metamorphosis, and reaching the juvenile stage, so it eventually results 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, the 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.
Some species, such as red-chested sea cucumber, fertilize their eggs internally instead of externally in the water column. After they fertilize eggs and form a zygote, they pick it up with their feeding tentacles and keep it in brooding chambers such as on their tentacular crown or the body surface (on the stomach or dorsum.
Some species keep their babies inside their bodies. 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.
The same behavior we can observe in some sea star species that I wrote more about in this article.
- 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
- Giménez, J., Penchaszadeh, P.E. Brooding in Psolus patagonicus (Echinodermata: Holothuroidea) from Argentina, SW Atlantic Ocean. Helgol Mar Res 64, 21–26 (2010). https://doi.org/10.1007/s10152-009-0161-z
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