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CHAPTER 3:
Mutualistic Relationship Between Sea Star Species Asterias jotarensis and Asterias kakyoinii
The coast of Morioh consists of a bay that stretches over a kilometer inland from the surrounding coastline and a series of large reefs that act as a natural barrier, isolating a network of smaller reefs and kelp forests inside the bay. This has created an ecosystem that has been ecologically isolated for at least 3 million years (Sato et al. 1997). This chapter focuses on the mutualistic relationship between two recently-discovered species of sea star endemic to Morioh Bay, A. jotarensis and A. kakyoinii (described in Kujo, 2001).
Species Overview
Despite being closely related species, A. jotarensis and A. kakyoinii are markedly different in appearance. A. jotarensis is somewhat larger in mass and has thicker arms, with a vibrant color pattern of various shades of blue to dark purple. Although able to move fairly quickly for its size, up to a meter per minute in short bursts, it tends to remain in a confined area, rarely straying more than about 5 meters from its “home” section of reef. It is an opportunistic feeder, capable of hunting small plankton or snails but also relying on algae and marine detritus suspended in the water or on the surface of the reef.
A. kakyoinii is smaller in mass, with arms that are longer but thinner than those of A. jotarensis. Its coloration consists of a variety of green hues overlaid with a pattern of narrow white stripes. Though it moves more slowly than A. jotarensis , it travels over a much broader territory, frequently venturing tens to hundreds of meters from the nearest reef. This is because it is a detritivore, relying almost solely on suspended organic matter in the water carried by tidal currents. To maintain this diet, it must spend most of its time in the open areas of the bay. However at tidal minima and maxima, it returns to a “home” reef as A. jotarensis does.
Reproduction
Though both species are capable of hermaphroditism, they primarily reproduce asexually through autotomy (separation of a limb). Many species of sea stars are capable of autotomy, but few use it as a primary means of reproduction. In fact, only two other species are known to rely primarily on autotomy, and both are in the sister genus Linckia (Edmondson, 1935), implying convergent evolution of autotomy as a mode of reproduction in the two genera.
Mutualism
A. jotarensis and A. kakyoinii exhibit a close mutualistic relationship, which is unusual for such closely related species with overlapping ecological niches. There are usually no more than two individuals of each species living in and around a given reef, but more notable is that each individual of one species is invariably accompanied by an individual of the other, both living on the same reef. The truly mutualistic behavior occurs when one individual begins the process of separating a limb. They will take shelter in a small crevice in the reef and remain for several days as the limb separates, after which both the individual and the limb remain in the crevice for up to a week after separation. Since the body is missing a limb, it is in a weakened state and more vulnerable to predation, and the limb is similarly handicapped as it regrows a body.
During this recovery period, the individual of the other species remains very close to the crevice, acting like a ‘guard’ for the recovering sea star, sometimes even physically covering the entrance to the crevice with their body and varying the color patterns on their back to ward off attackers. Once both the parent and offspring are recovered, they leave the crevice and remain near the reef. Very shortly afterward, sometimes in less than a day, the individual of the other species begins the autotomy of their own limb, often using the exact same crevice in the reef, with the first now acting as the guard. Once both individuals have reproduced, the new pair move together to a new reef elsewhere. Generally, it seems that A. kakyoinii is the first to reproduce, but A. jotarensis has also been observed doing so; more study is needed to determine the trigger for the beginning of their reproduction. This is the only known example of such a close and direct mutualism between sea stars, and the only example of such one-to-one pairing of individuals of different species in the animal kingdom.
References
Edmondson, Charles Howard. 1935. Autotomy and Regeneration in Hawaiian Starfishes. Bernice P. Bishop Museum Occasional Papers . 11 (8): 6–12.
Kujo, Jotaro. 2001. Description of two novel sea star species in Morioh Bay, Japan. Japanese Journal of Ecology. 51:13-19.
Sato, A. et al. 1997. U-Th dating of coral reefs in southeastern Japan. The Journal of the Geological Society of Japan. 106:177-184.