Diving with Longspine Sea Urchins

Lanta Marine Life | Diadematidae

Longspine Sea Urchins are a family of around 32 species of spiny, spherical, or globe shaped animals with hard shells which live on the seabed. Spotted everywhere, on all of our Koh Lanta dive trips.

As with other echinoderms, most urchins have a five part symmetry, with five equally sized parts radiating out from their central axes. The mouth is at the base, and the anus at the top.

The long spines are usually hollow, and most species have two series of spines, primary (long) and secondary (short), distributed over the surface of the body, with the shortest at the poles (top and bottom) and the longest at the equator (middle).

Like sea stars and sea cucumbers, sea urchins have small tube feet which can be used to 'walk' across the seabed, often aided by movement of their spines. An inverted Longspine Sea Urchin can right itself by using its tube feet and manipulating its spines to gradually roll its body upright.

Like other sea urchins, longspine sea urchins are sensitive to touch, chemicals and can detect light. They have eye spots which can detect light, with the spines being manipulated to allow light to fall onto the eye spots. When detecting shadows, they can flee from a perceived threat, or move toward perceived shelter.

Sea urchins feed mainly on algae, but also eat other slow moving of stationary invertebrates they may encounter. Like other families of urchins, they are eaten by sea stars and triggerfish.

Dense populations of Longspine Sea Urchins can quickly devour important marine algae communities in a coral reef ecosystem, destroying the ecosystem balance in the particular areas where these aggregations occur.

2 species found on this page:

Double-Spined Urchin (Echinothrix calamaris)
Black Longspine Sea Urchin (Diadema setosum)

Double-Spined Urchin

(Echinothrix calamaris)

Double-Spined Urchin @ Koh Bida

The Double-Spined Urchin, or Banded Sea Urchin, has a slightly oval test (shell) which can grow up to 5 cm in diameter. As with all other members of the family Diadematidae, the hollow spines are two different lengths, and very obvious in this species.

The short spines are yellow to dark brown and can deliver a very painful sting. The longer spines can be light coloured, dark coloured, or banded light and dark, and grow to 15 cm in length. Juvenile cardinal fish (family Apogonidae) may hide in the spines for protection.

As with other sea urchins and echinoderms, the Double-Spined Urchin displays five-part symmetry which is clearly visible with the shorter spines.

The mouth is on the lower body surface, and the large anal opening on the top is a generally light colour, with many black and white speckles and spots. The anus is surrounded by a ring of light sensitive receptors which provide rudimentary vision.

The Double-Spined Urchin is nocturnal, feeding at night and usually seeking shelter during the day.

Black Longspine Sea Urchin

(Diadema setosum)

Diadema setosum @ Koh Bida

The Black Longspine Sea Urchin is common to all dive sites in Thailand. The central test (body shell) is almost spherical in shape (slightly squashed vertically), normally black, sometimes dark grey, and grows to 8 cm in diameter.

As with other Echinoderms, this species displays five-fold symmetry which can be seen in the five equally-spaced white dots on the body.

The Black Longspine Sea Urchin has long, hollow, black or white, occasionally banded spines. The spines are slightly venomous and can easily inflict a painful sting, though non fatal.

There is an orange to white ring around anal sac opening, and the body is covered with light receptors (eye spots) providing rudimentary vision. The entire body, is in effect, a very basic ‘eye’.

This species is found on sandy, rubble or rocky bottoms and can use it’s longer spines to move quickly along the seafloor.