Grazing effect of the sea urchin Echinometra mathaei (Type A) on coral recruits
Satoshi Nojima and Tomoaki Hirakawa
Amakusa Marine Biological Laboratory,
Graduate School of Kyushu University, Japan
Echinometra mathaei (Type A) is one of the most common type of sea urchins in the reef slope and reef flat in Okinawa. The sea urchin normally feeds on micro algae on the reef floor during night and its density reaches up to 70 m-2 in Okinawan reefs. Because of this, it can be easily seen how the grazing effect of the sea urchin is also harmful to newly settled coral recruits. A cage experiment is the most common method to test the grazing effect. However, there are disadvantages to this method. One example is the mesh problem - a certain mesh size inevitably excludes not only the sea urchin but also other grazers of the same size or larger. In such case, it is impossible to relate on effect unequivocally to the sea urchin.
In order to eliminate this problem, the following two treatments were carried out before starting the experiment. (In Okinawan reefs, experimental plates made of rhyolite attract plenty coral larvae as same as that of bed rock of reef. For settling experiments, two experimental plates are usually set on concrete blocks with a gap of 1 cm made by using nuts and bolts, and most recruits settle on the underside of the experimental plates.
1) Two series of experimental plates (10 x 10 cm, i.e. 100 cm2) were prepared in such a way that the gap between the lower plate and concrete block was 1 cm and the gap between upper and lower plates was adjustable from 1 - 4 cm.
2) One series was submerged at a reef flat with sea urchin under natural conditions. Another series was set in adjacent where adults of Echinometra mathaei (Type A) had been removed. The setting of the plates was carried out just before mass spawning of Acropora and the plates were collected after three months. A number of juvenile colonies were counted in each family or genus.
From these experiments, the following results were obtained:
a) In the area where sea urchins had been removed, the upper surface of the upper plates was covered with a dense turf of algae. No turf of algae covered the upper surface of the upper plates in the area under natural conditions.
b) No coral colony was seen on the upper side of the upper plates in either areas.
c) In both normal and sea urchin-removed areas, many coral colonies were counted on the underside of the upper plates when the gap was between 1 and 2 cm. Many juvenile corals also attached themselves to the underside of the lower plates in both areas.
d) In the natural area, any coral colonies also appeared on the underside of the upper plates when the gap was between 3 and 4 cm. In the sea urchin-removed area, no colony was observed on the underside of the upper plates, in spite of the fact that many colonies had attached themselves to the underside of the lower plates when the gap was 1 cm.
Results a) and b) show that Echinometra mathaei (Type A) frequently grazes on microalgae on the surface of the reef floor, and algae covers function to inhibit the settlement of coral larvae on the sunny surface. Result c) indicates that smaller grazers and juvenile sea urchins do not have a harmful effect on coral recruits. Though Result d), it is apparent that only adult sea urchins have harmful influence on juvenile colonies.
