Red slate-pencil urchins (Heterocentrotus mamillatus) often have permanent housemates (called obligate simbionts) a small shrimp (Levicaris mammillata) or the aptly-named red slate pencil urchin shrimp. While the relationship hasn’t been thoroughly researched, it seems that the shrimp keeps things nice and tidy, while the red-slate pencil urchin protects the shrimp from predators.
[Photo and caption by Karli Chudeau]
Fujino, T., & Takeda, M. (1977). Levicaris mammillata (Edmondson), a Gnathophyllid shrimp associated with slate-pencil sea urchin, Heterocentrotus mammillatus (Linnaeus), from the Ogasawara and Ryukyu Islands. Bulletin of the National Science Museum. Series A, Zoology, 3(3), 131-140.