''Heterocentrotus'' is a genus of slate pencil urchins, part of the familia

Echinometridae The Echinometridae are a family of sea urchins in the class Echinoidea. Characteristics All Echinometridae have imperforate tubercles and compound ambulacral plates.Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...

basin, especially in Reunion or

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

. This genus appeared in the Miocene and spread throughout the warm Indo-Pacific.

Description

''Heterocentrotus'' are brightly colored tropical sea urchins with very thick spines that have given them the nickname "pencil urchins". The genus consists of sea urchins with rounded (but slightly elliptical)

test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...

, with the peristome (mouth) located in the center of the oral surface (lower) and the periproct at opposite, at the apex of the aboral (upper) face. The apical disc is dicyclic, with a reduced periproct. The peristome is also reduced and elliptical, with limited mouth notches. The

ambulacrum Ambulacrum is an architectural word that denotes an atrium, courtyard, or parvise in front of a basilica or church that is surrounded by arcades or colonnades, or trees, and which often contains a fountain A fountain, from the Latin "fon ...

are polygeminate, with between 9 and 14 pairs of pores per plate, arranged in more or less regular arcs, sometimes almost in bands. All ambulacral plates bear a single, large primary tubercle, occupying almost the entire surface of the plate. The interambulacral plates bear a single, massive nipple, flanked by secondary tubercles on the aboral surface. The radioles are very thick and dense, usually three-faceted (or two or four, or round, mainly in H. mamillatus). They are never modified into scales or plates (unlike '' Colobocentrotus''), but the secondary, short radioles can have an almost rounded shape

List of species

This genus contains two species: These two species are very close visually, and often difficult to distinguish, since they generally share the same distribution area. The main criteria for in situ identification are the radioles, which are generally longer (especially on the aboral side) and more clearly triangular in ''H. trigonarius'', which never has bifacial radioles on its underside. ''H. trigonarius'' is also usually darker (brown, dark orange...), but the great variability of colorations in this genus makes this criterion delicate. ''H. mamillatus'' often has the secondary radioles white or the base of the primary radioles surrounded by a white ring, which never seems to be seen in ''H. trigonarius'', which is almost always entirely monochromatic (with perhaps the test being slightly darker than the radioles primary). Mamillatus radioles also often bear lighter rings, which does not seem to be seen in ''H. trigonarius'' either. Skeletal-wise, ''H. mamillatus'' has less petalloid aboral ambulacral zones than its sister species, and fewer pore pairs per ambulacral plate (9-12 versus 15-16). File:Radioles d'Heterocentrotus mamillatus sculptées.JPG, Radiols (spikes) of ''

Heterocentrotus mamillatus ''Heterocentrotus mamillatus'', commonly known as the slate pencil urchin, red slate pencil urchin, or red pencil urchin, is a species of tropical sea urchin from the Indo-Pacific region. Description This species is a large sea urchin, wi ...

'' sculpted Image:Heterocentrotus mammilitatus.jpg, ''

'' Image:Radioles d'Heterocentrotus mamillatus.JPG, Radiols (spikes) of a "slate pencil sea urchin" (''

''). They are a classical souvenir.

References

{{echinoidea-stub