Sealife guideThe marine echinoderms

The phylum of echinoderms, literally « hedgehog skin », includes 5 classes and totals around 6,000 species. The most well-known of them is undoubtedly the starfish, characterized by its star-shaped form with 5 arms, or the sea urchin with its spines.

Taxonomy

Echinoderms are divided into 5 classes:

the asteroids. This class includes the most well-known echinoderms, the starfish.
the crinoids. This class includes sea lilies and feather stars.
the echinoids. Along with the asteroids, this class includes the most famous echinoderms, the sea urchins.
the holothurians. This class includes sea cucumbers or holothurians.
the ophiuroids. This class includes brittle stars, which closely resemble starfish with their flattened body and long, thin arms. Also included in this class are gorgonocephalans.

Description

Echinoderms, as the etymology of their name suggests, are animals whose outer skin surface is covered with spines, like a hedgehog. Echinoderms are primarily characterized by:

their shape, which displays a 5-fold radial symmetry
the presence of an internal skeleton made of calcareous plates or spicules
the presence of an aquifer system, called the ambulacral system, which allows them to move and feed. The aquifer system is essentially a hydraulic system that enables the movement of suckers.

Range

Echinoderms can be found throughout all the oceans and seas across the globe, at all latitudes and depths, even at very great depths.

Habitat

A red starfish under a rocky overhang

Echinoderms are sedentary and benthic animals, meaning they live resting on the seafloor. Echinoderms can be found on all types of substrates. All echinoderms are mobile, with the exception of crinoids, which are fixed to a substrate.

Diet

The diet of echinoderms is very varied depending on the species: decomposing organic matter, mollusks, …

The presence of spines in echinoderms keeps many potential predators at bay due to their size. Ultimately, echinoderms are preyed upon by very few predators.

Reproduction

Echinoderms primarily exhibit:

sexual reproduction through the release of gametes into seawater followed by external fertilization. However, to ensure the species' survival, this requires a significant quantity !
for certain classes like starfish and brittle stars, asexual reproduction through the splitting of part of the body and regeneration of the individual from that fragment.

Tips for observing

Starfish and sea urchins are often easily observable from the beach with a mask and snorkel, but be careful with your feet !

A dried starfish on a beach littered with shell fragments !

Otherwise, take a walk on the beach and carefully observe the sand and you'll surely discover some hidden treasures of nature, like a sea urchin test or a starfish dried by the sun !

Some marine echinoderms to discover

Pineapple sea cucumber
(Thelenota ananas)

Necklace sea star
(Fromia monilis)

Red cushion sea star
(Oreaster reticulatus)

Smooth starfish
(Hacelia attenuata)

Purple sea star
(Pisaster ochraceus)

Vermillion sea star
(Mediaster aequalis)

Black sea urchin
(Arbacia lixula)

Red sea urchin
(Mesocentrotus franciscanus)

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