1 Coral Polyp Expulsion

scientific correspondence

1 in areas that suffered chronic levels of sedi Coral polyp expulsion mentation, during the summer and autumn months when the water temperatures are n coral reef environments that suffer regu dant in sandy shallow tropical waters, show highest (26 °C in Eilat and 29 °C in the I 1 lar physical disturbances, the fragmenta only minimal fragmentation • Here we Mediterranean). We did not see polyp expul tion of established colonies is believed to be report a mode of asexual reproduction in sion in specimens from waters deeper than important in the recruitment (recoloniza massive and encrusting corals, which 4 m in the Red Sea and 7 m in the Mediter tion) of the local area by branching and enables them to sustain local populations in ranean, indicating that this phenomenon free-living corals1--4. High rates of wave physically disturbed environments. might be unique to shallow water, where action and sedimentation, for example, We have observed an unusual form of physical disturbances are more pronounced. would favour fragmentation over the settle budding in massive and encrusting corals Histological sections revealed that dur ment and metamorphosis of sexually pro from shallow waters, particularly in Favia ing the peak reproductive season, polyps duced larvae1• Nonetheless, massive and favus from the northern Red Sea and in that were about to be expelled from the encrusting coral species, which are abun- Oculina patagonica from the Mediterranean 'mother' colony contained no gonads, coast of Israel. Individual polyps, including whereas surrounding polyps did. It is thus their calices, lift on elongated calcareous possible that the expelled polyp concen stalks (Fig. la) before detaching and settling trates its stem-cell pool and energetic elsewhere. The area of polyp expulsion resources for use in reattachment and (Fig. 2) regenerates within two weeks. The growth rather than for reproduction. Polyp new propagule settles, its tissues spread over expulsion may increase the survival rate of the stalk and onto the substrate (Fig. lb), the parental genotype in a disturbed envi and the polyp forms a new colony through ronment by ensuring that genotypically budding. In the laboratory, newly settled identical offspring are present at different propagules of 0. patagonica and F. favus stages in their life cycle5• developed and grew into new colonies with This phenomenon differs from the polyp in two months. 'bail-out' process, found in several branching We also observed polyp expulsion in situ, pocilloporid coral species6.7. In polyp bail out, disintegrating corals release whole polyps from their calices, some with planu lae, and the parent colony dies. In contrast, polyp expulsion occurs in physiologically healthy corals, and whole polyps, including their calices, are expelled. Unlike fragmenta tion, this process seems to be regulated by the colony and therefore may be considered a form of asexual reproduction. As the main mode of reproduction in stony corals is sexual2, the question arises as to how successful recruitment1 occurs in chronically disturbed environments, where gametes and planulae may be swept away, dislodged or smothered by sediment. Our findings show that massive and encrusting stony corals which are abundant in such environments may overcome the difficulties of planular recruitment by reproducing asexually by polyp expulsion. As expelled polyps are larger than sexually derived propagules, they begin their sessile lives at the juvenile stage, avoiding the rigours of larval development, settlement and meta morphosis. This may be a form of size refuge, increasing the corals' chance of sur vival in disturbed environments. E. Kramarsky-Winter, M. Fine, Y. Loya Department ofZoolo gy, Figure 1 Polyp expulsion in the encrusting coral Tel Aviv University, Tel Aviv 69978, Israel Oculina patagonica. a, An expelled polyp at the e-mail:[email protected]. ii time of release. The tissue covering the calcareous stalk is visible ( x 6.5). b, A newly settled expelled I. Highsmith, R. Ma r. Ecol. Prog. Ser. 7, 207- 226 (1 982). 2.. Harrison, P. L & Wallace, C. C. in Ecosystems oft he World 25: Coral polyp, with tissues beginning to spread over the Reefs (ed. Dubinski, Z.) 133-206 (Elsevier. Amstersdam, 1990). stalk and eventually over the substrate ( x 6.0). The Figure 2 The massive cora l Favia favus. a, An 3. Scheer, G. Naturwiss. Ver. Darmstadt Ber. 1958/59, 50---60 (1959). prevalence of expulsion was recorded along 10-m elevated polyp before expulsion; and b, the area of 4. Rosen, B. R. & Taylor, J. D. Science 166, 119- 121 ( 1969). 5. O rive, M. E. Am. Nat. 145, 90--!08 (1995). line transects8. Expulsion was observed in 34 of the detachment immediately after expulsion ( x 0.6). 6. Sammarco, P. W. Mar. Ecol. Prog. Ser. 10, 57-65 ( 1982). 178 colonies of shallow-water 0. patagonica recor• Expulsion was seen in 10 of the 25 F favus colonies 7. Richmond, R. H. Mar. Ecol Prog. Ser. 22, 181- 185 (1 985). ded in 12 transects. All photographs by A Shoob. recorded in nine transects. 8. Loya, Y. Mar. Biol 13, 100-123 ( 1972).

NATURE I VOL 387 I 8 MAY 1997 137