Description
This species is a large sea urchin, with some specimens reaching over 8 cm in diameter, with spikes up to 10 cm. Most specimens are bright red, but brown and purple colorations are also seen. The spines may have a different color from the body. Spines have a white ring at their stem and have alternating light and dark rings. The spines are rounded to triangular in cross-section and taper towards the tip. Surprisingly during the night the red spines turn into a chalky pink. Specimens from Hawai‘i tend to have bright red spines, while specimens from other parts of the Pacific may have yellowish or brown spines. Other variations of ''H. mammillatus'', such as those living on theRange and habitat
This species can be found throughout the tropical waters of the Indo-Pacific region (from the east coast of Africa to the Pacific archipelagos), but is especially abundant inDiet and behavior
''H. mammillatus'' predominantly feeds on encrusting coralline algae, but has been noted to consume small amounts of other algae like ''Pterocladia'' and ''Ulva''. Sea urchins are primarily marine grazers and tend to eat the algae in closest proximity to them. Thus, several species live a rather sedentary lifestyle. However, ''Heterocentrotus mammillatus'' appears to be somewhat active in comparison to other urchins like ''Echinothrix calamaris, Echinometra mathaei'', and ''Echinometra oblonga''. ''H. mammillatus'' was observed to travel up to 600 cm away from its starting site and moved an average of 96 cm per day (Ogden et al., 1989).Predators
''Heterocentrotus mammillatus'' shares many of the same predators as the other tropical sea urchins living in its habitat. For example, fish, such as those belonging to the families Balistidae, Labridae, and Lethrinidae, are known to prey on sea urchins. ''H. mammillatus'' is also collected and eaten by humans, though not as often as ''Tripneustes gratilla'' is.Spines
''Heterocentrotus mammillatus'' has thick, rounded spines that allow it to bore into hard substrates and defend itself against predators, wave drag, and pressure. Similar to other sea urchins, its spines are made of magnesium calcite arranged into a porous network, allowing for the passage of fluids and organic matter within the spines. The spines have a dense base and become more porous up towards the tip. However, ''H. mammillatus'' is unique in how its spines contain other layers of dense material organized throughout, sandwiching the more delicate porous layers. The layered structure allows for graceful failure, preserving the bottom layers of the spine which are farthest from the point of pressure. This graceful degradation occurs because energy is forced to travel through several layers and dissipates. The layered constitution of ''H. mammillatus''’ spines also allows for crack deflection.Locomotion
Bilateral symmetry creates an anterior “front” end and posterior “back” end in living organisms. The anterior end of bilateral organisms often undergoes cephalization and becomes a “head”, hosting a cluster of sensory organs, the mouth, or both. This head often becomes a locomotor anterior in which the organism orients itself, moving forward so that the head of the body is first to approach the upcoming environment. However, despite being bilaterally symmetrical, ''Heterocentrotus mammillatus'' does not possess a fixed locomotor anterior like many other bilaterally symmetrical organisms do, which allows it to proceed with equal ease in whatever direction it chooses. Instead, ''H. mammillatus'' seems to display ''temporary'' locomotor anteriors when moving, orienting itself in the direction that it first moves. Therefore, ''H. mammillatus'' often insists on continuing in its initial direction of movement, even if it is interrupted by being picked up or placed down in a different orientation.Reproduction
Like other sea urchins, ''Heterocentrotus mammillatus'' periodically develops gametes to be released into the water for fertilization and undergoes a rest period in order to resume the cycle again. However, the timing and duration of these cycles varies from urchin to urchin. ''H. mammillatus'' from a reef near where the Gulf of Aqaba meets the Red Sea displayed an annual reproductive cycle. Its resting period is from October to January, accumulating nutrients in its gonads in preparation for gametogenesis. It begins gametogenesis in January and finishes in May, during which its gonads are nearly completely filled with gametes. Spawning usually occurs between May and June, but sometimes can take until October for all the urchins to finish releasing their eggs and sperm. The start and end times of these periods are not always rigid and can vary by a couple months. This data on ''H. mammillatus''’ reproduction indicates a possible lunar or semilunar cycle in spawning and gametogenesis, meaning that ''H. mammillatus'' may take temporal cues from the moon. Given that spawning did not always happen during the same phase of moon, other factors may contribute to the timing of ''H. mammillatus''’ reproductive stages (such as the tidal cycle, for example).Organisms associated with ''H. mammillatus''
The small shrimp, ''Levicaris mammillata'', has been found clinging to the spines of ''Heterocentrotus mammillatus'' in Hawaii, the Ogasawara Islands, and theReferences
* Hoover, John P. (1998). ''Hawaiʻi's Sea Creatures: A Guide to Hawaiʻi's Marine Invertebrates''. Mutual Publishing. .External links
* {{Taxonbar, from=Q277658 Heterocentrotus Animals described in 1758 Taxa named by Carl Linnaeus