Diadema setosum

As a member of the class Echinoidea, the anatomy of Diadema setosum is that of a typical sea urchin. All of the animal's internal organs are enclosed within the near-spherical, black test that is essentially the body and skeleton of the organism. However, the body is not perfectly spherical – Diadema tests are slightly dorso-ventrally compressed. Protruding outwards from the central body are the long spines iconic of a sea urchin's appearance. Like the other members of the family Diadematidae, the spines of D. setosum are extremely long and narrow in proportion to its body. The spines, often black but sometimes brown-banded, are hollow and contain a mild venom. D. setosum can be distinguished from other species in the genus Diadema by the presence of five white spots on the animal's test, strategically located between the urchin's ambulacral grooves. In addition, a clear distinguishing characteristic of the species is the presence of a bright, orange ring around the urchin's periproctal cone (also known as the "anal cone"), which is often mistaken for an "eye". A few other minor characteristics in D. setosum include bluish spots on the organism's genital plates and similar blue spots (iridophores) arranged in linear fashion along its test. An apical ring is absent in the species, along with calcareous platelets on its apical cone. Sexually mature Diadema setosum specimens average from in weight. Image:Searchin' for an urchin (Diadema setosum).jpg|General look Image:Diadema_setosum_qtl1.jpg|Detail of the anal papilla. Image:Diadema setosum apex.jpg|Close-up on the classical characteristics : orange ring, five white spots, blue iridophores. Image:Diadema setosum 1.jpg|Specimen with white spines. ==Distribution and habitat==

Diadema setosum is a widely distributed species of sea urchin. Its range stretches throughout the Indo-Pacific basin including Malaysia, longitudinally from the Red Sea and then eastward to the Australian coast. Latitudinally, the species can be found as far north as Japan and its range extends as far south as the southern tip of the African east coast. Diadema setosum is commonly associated with coral reefs, but is also found on sand flats and in seagrass beds. Along with the other members of the family, D. setosum is a prolific grazer. They are known to feed on a variety of algal species common on tropical coral reefs. The ecological importance of the taxon as a whole has been stressed because of its herbivorous habits. ==Biology==

The species has been known to spawn both seasonally and year-round depending on the location of the spawning population. It has been suggested that Diadema setosum populations are temperature-dependent in their spawning seasonalities. Temperatures higher than have been cited as a possible spawning cue. Equatorial populations are those recorded to spawn at no particular times throughout the entire year. This is true for the Philippine populations of D. setosum. For a population in the Persian Gulf, spawning occurs during the months of April to May. Other cues, such as the phases of the moon have been observed to affect the spawning of D. setosum populations. The species has been found to trigger spawning events in concordance with the appearance of a full moon. They are broadcast spawners. Evolutionarily, Diadema setosum is considered one of the oldest of the known extant species in the genus Diadema. Genetic analysis of the Diadema have placed D. setosum at a basal branch on a cladogram, having it as the sister group to all the other remaining members of the genus. Morphological analysis confirms this conclusion, adding weight to the concept of D. setosum being the most basal of the Diadema and possibly the oldest extant species in the genus. In terms of behavior, D. setosum has been observed to be able to avoid danger by rapidly inverting its body and "running" on the tips of its longest spines. This behavior is triggered by sudden impacts and the snapping of one or more of its spines. During the process, upon encountering small obstacles, D. setosum will adopt a rolling motion. It has been observed to move in just 7 seconds using this method of locomotion. It is also worth noting that this particular variety has some of the very best vision observed among sea urchins and will regularly redirect spines toward passing fish as a defense mechanism. Although unclear, this vision seems to be achieved by the sensing of light that enters each spine and it transmitted down the spines shaft and sensed at the base, although further scientific research is required. Goose barnacles of the genus Microlepas are known to parasitize D. setosum. ==Gallery==

File:Diadema setosum in Zanzibar.jpg|From Zanzibar File:Diadema setosum face aborale.jpg|From la Réunion File:Diadema setosum (Kenya).JPG|A specimen encountered in Kenya. The 5-fold symmetry is obvious. File:Diadem urchins.jpg|D. setosum along with Diadema savignyi : test shape, spines length and orange ring help telling them apart. File:Diademseeigel.jpg|A classical specimen with long, black spines. The characteristic orange ring is visible in the photo. ==Bibliography==