Echinoderms: Sea Cucumbers
Holothuria (Sea-Cucumbers)
Despite their very different appearance to Sea-Stars and Urchins, these are also echinoderms. Their pentameral symmetry is not so obvious but it exists in transverse section- the elongated body exhibits a bilateral symmetry in the longitudinal axis but a cross section of the body would show pentameral/pentaradial symmetry.
The skeletal structure is not rigid but is composed of spicules throughout the body, which is flexible. As with other echinoderms, the degree of flexibility/rigidity of the soft tissue can be adjusted by changes to the Mutable Collagenous Tissue, making the body more or less rigid as required.
Two species are very common in Mallorca:
Holothuria tubulosa is one of the most common sea cucumbers in the Mediterranean Sea, generally associated with organically enriched coastal sediments and seagrass beds. It grows to a length of between 20 centimetres (7.9 in) and 45 centimetres (18 in), and a diameter of 6 centimetres (2.4 in). It is roughly cylindrical with a flattened base on which there are three longitudinal rows of tube feet. It has a tough, leathery skin. The general colour is a shade of brown and the surface is covered with numerous dark-coloured, conical, thornlike projections known as papillae. It often appears greyish, as it secretes a protective film of mucus to which bits of seaweed and sediment may adhere. It has a fringe of short, flattened tentacles around the mouth at the anterior end and an anus at the posterior end.
Holothuria Sanctori is found in fewer numbers but is, nonetheless, a common species. It is similar in size, form and behaviour to the tubulosa but there are some differences, the most obvious being the coloration, although this is highly variable: it usually has pale yellow-white coloured plates around the base of its papillae; the photos show the variability of this feature. It generally prefers rock substrates but is otherwise similar to tubulosa.
In both species, their passivity belies some interesting defence mechanisms:
Physical Defences:
Cuvierian Tubules: When threatened, H. tubulosa can forcefully expel a mass of long, sticky white tubules (Cuvierian tubules) from its anus. These tubules rapidly expand in the water, forming a tough, elastic net that entangles and immobilizes predators such as fish and crabs, allowing the sea cucumber to escape. The lost tubules can regenerate within a few weeks.
Evisceration (partial): As part of the Cuvierian tubule release, some internal organs connected to the cloaca (posterior opening) may also be expelled. While this may sound extreme, the sea cucumber is not permanently harmed and can regenerate its missing organs over time.
Cryptic Behaviour: This species often exhibits nocturnal behaviour, hiding during the day under rocks or in crevices to avoid predators.
Chemical Defences
Toxins (Holothurin): The Cuvierian tubules and body wall of H. tubulosa contain potent toxins, primarily triterpene glycosides (saponins), known as holothurin. These compounds are noxious to many marine organisms and can cause severe pain or even death in higher quantities, acting as both an antipredatory and antimicrobial agent.
Innate Immunity: The sea cucumber also possesses a complex, innate immune system within its coelomic fluid, which helps it fight off pathogens and environmental stresses through cellular and humoral (chemical) components, including antimicrobial peptides and proteins.
Note:
1. I am not a Marine Biologist- I have an amateur fascination in this subject and any corrections to my Blog are welcome.
2. All images by Sophia Chapman
