# World of Mutualism between Corals and Crabs

A variety of living organisms, including crabs, shrimps, univalve shells and small fish, use the space between coral branches as their habitats. Based on research on the relationship between Pocillopora damicornis and Trapezia cymodoce, we introduce the mutualism (the relationship of harmonious coexistence that brings mutual benefits) between them in this column.

Trapezia cymodoce feeds on mucus made by corals. This is the advantage of living with corals. Corals, on the other hand, are protected by Trapezia cymodoce from acanthasters, their natural enemy. Scenes of Trapezia cymodoce beating off acanthasters that approached to eat corals by cutting off their ambulacral feet and grabbing and cutting their needles are observed. All of more than 10 types of Trapezia cymodoce confirmed to live in coral reefs in Okinawa behave like this.

On the other hand, what remains less well understood is cases where congeneric multiple species of Trapezia cymodoce live in the same coral colony, which is contrary to the principle that “species with the similar mode of life do not live in the same habitat.” One tentative theory under study is: “The coexistence relationship between corals and Trapezia cymodoce is related to the existence of acanthasters. When acanthasters are around, many species of Trapezia cymodoce gather to protect corals. If acanthasters are not around, Trapezia cymodoce do not have to exert efforts to beat off acanthasters, so species of Trapezia cymodoce start fighting among them and only strong species survive. Then, individuals in the surviving species start fighting and a pair of a large-size male and female occupy the coral colony.”

If the above phenomenon is unraveled, clues to the protection of corals from acanthasters may be obtained. Besides this example, there are a lot of unknowns about the mechanism of ecosystems. Destroying biodiversity that brings immeasurable benefits to us without elucidating these unknowns would be a great loss to all living organisms on the earth, including humans.

Multiple species of Trapezia cymodoce confirmed to live in the same coral colony

Photo: Professor Makoto Tsuchiya, Faculty of Science, University of the Ryukyus

## Section 2 Biodiversity and Global Warming

According to the IPCC Fourth Assessment Report, the risk of species extinction is projected to increase with increase in global average temperature. Droughts and wildfire associated with climate change are also threatening food production and ecosystems, while the

## 1 Impact of global warming on biodiversity

The IPCC Fourth Assessment Report states that annual average Arctic sea ice extent has shrunk by 2.7 $ [2.1 \text{ to } 3.3] $ % per decade, with larger decreases in summer of 7.4 $ [5.0 \text{ to } 9.8] $ % per decade (Numbers in square brackets indicate a 90% uncertainty interval around a best estimate). The U.S. Fish and Wildlife degradation on biodiversity such as decreasing forest area is accelerating global warming. Therefore, it is necessary to implement measures to conserve biodiversity and measures against global warming by linking them.

Service (FWS) estimates that sea ice changes as projected, two-thirds of the global population of polar bears will be lost by around the mid-21st century. The IPCC Fourth Assessment Report states that increases in sea surface temperature of about 1 to 3°C are projected to result in more frequent coral bleaching events and