Invertebrate Files: Graeffe's Sea Cucumber
Graeffe's Sea Cucumber (Pearsonothuria graeffei)
This post is all about the Graeffe’s Sea Cucumber (Pearsonothuria graeffei), also known as the Blackspotted Sea Cucumber. I frequently came across this species while diving in the Seychelles, Africa and later in Koh Tao, Thailand. To be completely honest, when I saw it for the first time, I was pretty weirded out. I vividly remember reviewing my GoPro footage and thinking it looked like something straight out of an alien movie. Yet the more I saw it, the more interested I became. Each encounter raised more questions for me, and and with every closer look, I found myself increasingly fascinated by the Graeffe’s Sea Cucumber.
Habitat
The Graeffe’s Sea Cucumber is widely distributed throughout the tropical waters of the Indo-Pacific region, where it usually inhabits coral reefs or the sandy seabeds nearby. It is rarely found at depths greater than 25 metres.
Graeffe's Sea Cucumber (Pearsonothuria graeffei)
Appearance
Its roughly cylindrical body is white and pale brown in colour, covered with numerous small black spots. The pale-brown patches feature thorn-like protrusions, which are clearly visible in the photo below. This distinctive colouring and textured surface allows the Graeffe’s Sea Cucumber to blend into its natural environment, providing effective camouflage.
On the underside of its body, it has multiple rows of tube feet (like little legs), each with a sucker at the end. These tube feet function like suction cups, primarily allowing it to anchor securely to its surroundings, while also helping its movement across the substrate. At one end of the body is the mouth of the Graeffe’s Sea cucumber, surrounded by a ring of up to 24 leaf-like tentacles with black stalks and white tips, which are used to capture prey (see the picture below). At the other end of the body is the anus, also known as a ‘cloaca’. This opening serves several purposes, including respiration, reproduction, defence and waste excretion.
Photo by: Mai Lazarus
Behaviours
Now let’s talk about this sea cucumber’s butt (don’t worry, I’ll switch to the more scientific term, cloaca, from here on out to keep things professional and, hopefully, less uncomfortable! 😄). When extremely threatened, this species can eviscerate itself, ejecting its internal organs through its cloaca. These organs are called Cuvierian tubules, which are sticky, white threads (see the picture below). This extraordinary defence mechanism not only distracts predators but also entangles and immobilises them, thanks to the tubules' adhesive and toxic properties. What’s even more impressive is that the sea cucumber can regenerate its Cuvierian tubules in a matter of weeks after they’ve been ejected, making it a highly effective and resilient survival strategy.
Photo by: Paddy Ryan
As I mentioned previously, the Graeffe’s Sea Cucumber also breathes through its cloaca. It possesses specialised respiratory structures called respiratory trees (similar to our lungs), located at the posterior end of its body. Seawater is drawn in through the cloaca and the trees absorb the oxygen from the water. Now for something even more bizarre! While there are currently no studies specifically on the Graeffe’s Sea Cucumber, it’s known that Pearlfish often live in the cloaca of various sea cucumber species. As the sea cucumber opens its cloaca to take in water for respiration, a Pearlfish can swim in to take shelter. To get rid of the intruder, the Graeffe’s Sea Cucumber can use its unique defence mechanism: evisceration. It ejects its internal organs, which also forces the Pearlfish out of its cloaca.
As a juvenile, this species relies on a different primary defence mechanism. Its bright yellow, white and black colouring mimics that of the toxic Varicose Wart Slug (Phyllidia varicosa). When threatened, this sea slug secretes a toxic mucus that is capable of killing a variety of marine predators. By imitating the wart slug's colouration, pattern and body shape, the juvenile Graeffe’s Sea Cucumber deters potential predators, offering crucial protection during this vulnerable early stage of its life cycle.
Juvenile Graeffe’s Sea Cucumber. Photo by: Matt Biegner
Varicose Wart Slug (Phyllidia varicosa). Photo by: Dzivula Gube
Diet
This species is described as a scavenger, feeding on tiny particles of organic material, such as plankton and detritus. It captures organic matter suspended in the water column and also sieves through sandy seafloors, filtering the substrate for nutrients.
Reproduction
The Graeffe’s Sea Cucumber is gonochoric, meaning individuals are either male or female. It commonly reproduces through synchronised broadcast spawning, a form of sexual reproduction in which both males and females release their gametes (sperm and egg cells) into the water column simultaneously. External fertilization occurs, and the embryos develop into auricularia larvae (the first larval stage), followed by doliolaria larvae (a later developmental stage). The free-swimming larvae eventually settle on a suitable substrate and undergo metamorphosis into juvenile Graeffe’s Sea Cucumbers.
Although there is no significant evidence supporting this in the Graeffe’s Sea Cucumber, it has been demonstrated that some Holothurian species (sea cucumbers) can reproduce asexually through processes known as transverse fission and fragmentation. During fission, the body undergoes constriction, twisting or stretching, resulting in the formation of separate fragments. Usually, this process produces two sections: an anterior and a posterior fragment. However, some Holothurians are capable of dividing into multiple fragments simultaneously. As an asexual reproductive mechanism, each fragment is genetically identical to the parent sea cucumber. After fission is complete, the body begins to grow and regenerate its internal organs, restoring its full structure.
IUCN Red List Status
Least Concern (LC)
Taxonomy
Domain: Eukaryota
Kingdom: Animalia (Animals)
Phylum: Echinodermata (Enchinoderms)
Class: Holothuroidea (Sea Cucumbers)
Order: Holothuriida
Family: Holothuriidae
Genus: Pearsonothuria
Species: P. graeffei
References
JungleDragon (n.d.) Graeffe's Sea Cucumber. Available at: https://www.jungledragon.com/specie/10315/graeffes_sea_cucumber.html
Dolmatov, I. Y. (2014) ‘Asexual Reproduction in Holothurians’, The Scientific World Journal. doi: 10.1155/2014/527235
SeaLifeBase (n.d.) Pearsonothuria graeffei. Available at: https://www.sealifebase.se/summary/Pearsonothuria-graeffei.html
Flammang, P., Ribesse, J. and Jangoux, M. (2002) ‘Biomechanics of Adhesion in Sea Cucumber Cuvierian Tubules (Echinodermata, Holothuroidea)’, Integrative and Comparative Biology, 42(6), pp. 1107-1115. doi: 10.1093/icb/42.6.1107
Rothschild, A. (2017) Sea Cucumber Butts. Available at: https://www.pbs.org/wgbh/nova/nature/sea-cucumber-butts.html
Netchy, K. (2017) Sea cucumber, Pearsonothuria graeffei. Available at: https://www.floridamuseum.ufl.edu/iz/discover-invertebrates/pearsonothuria-graeffei/