35 Materials for the Study of Reef-building Corals (3) from Science of the South Sea (Kagaku Nanyo) Volume 5, Number 1, pp. 95-106 (September, 1942) Cover page of the original issue (in Japanese) 37 19. Extended and contracted polyps in the praetorta, as reported previously. daytime There are three types of polyp shape in fungiid As described in Report no. 1 in this issue, in some corals. Fungia actiniformis, which has very dense reef-building coral species, there is a tendency for zooxanthellae in the tentacles, fully extends its body zooxanthellae to assemble within the tentacles of in the daytime (Fig. 5; Discriptions in Fig. 5 indi- polyps that extend in the daytime. I assumed that cates Pocillopora damicornis instead of Fungia ac­ coral polyps harboring dense zooxanthellae assem- tiniformis). Fungia repunda does not have many blages would exhibit positive phototaxis like many zooxanthellae in its tentacles, and it ex tends its other zooxanthellate animals. In fact, many coral tentacles moderately in the daytime (Fig. 6). Fungia species having dense zooxanthellae within their echinata has very few zooxanthellae in its tentacles. tentacles extend their polyps in the daytime. How- This species has dense zooxanthellae in the oral disk, ever, there are some exceptions. Galaxea (Report no. and it completely contracts its tentacles in the 16) and Acropora have few zooxanthellae in their daytime (Figs. 7, 8). polyp bodies or tentacles, but they extend their Lobophyllia usually contracts its tentacles in the polyps in the daytime. Lobophyllia does not need to daytime. Assemblages of zooxanthellae exist in the extend polyps in the daytime, because dense zoo- tentacle tips, which are always facing outward even xanthellae assemblages are located in the tentacle if polyps are contracted. As shown in Fig. 9, zoo- tips that always face the outside of the colony. xanthellae assemblages are also located on the tips Similarly, coral species that have thin coenosarcs or of projected tissues of the coenosarc and oral disks epithelia often contract their polyps in the daytime, with red and green colors. These assemblages can be as sufficient sunlight may penetrate through them to observed by the naked eye as small dots. Zooxan- reach the zooxanthellae. Therefore, I hypothesized thellae are believed to align so as to utilize light in that the shape of coral polyps is determined by the the most effective way. When a polyp receives light need to maximize light reception by zooxanthellae. while extending its tentacles, other parts of the polyp Corals containing dense zooxanthellae in either retract to uncover the coenosarc and oral disk. I their polyps or tentacles extend their polyps in the observed a colony of Lobophyllia that fully extended daytime. The specific characteristics of these coral its tentacles in the daytime. This colony had dense species are either a large coenosarc compared with zooxanthellae in its tentacles. the size of the polyps or very large polyps with wide Acropora and Galaxea have tentacles or polyps oral disks. These corals are Acropora, Montipora, containing few zooxanthellae extending dur ing day- Goniopora, Porites, Pavona decussata, Fungia ac­ time. The phototaxic reaction caused by zooxanthellae tiniformis, Hydnophora, Euphyllia, Plerogyra, may not fully transmitted between the coenosarc and Physogyra, Acrhelia, Pocillopora, Seriatopora, and polyps, because polyps of these species are relatively Stylophora. distant from each other. Al ternatively, their trans- Porites are often observed to contract their polyps parent tentacles and polyps may function as a light in the daytime, though the polyps have dense zoo- diffuser. For example, Galaxea has dark-colored xanthellae. The contraction may be due to some type zoo xanthellae, which are similar to those of other of extrinsic physical stimuli or negative phototaxis coral species occurring in deep or shaded habitats. from strong sunlight. Strong sunlight is also a cause As described above, the difference between ex- of the decrease in the density of zooxanthellae in the tended and contracted polyps can be explained by cases of Pocillopora (Report no. 10) and Pavona positive phototaxis derived from symbiotic zooxanth- 38 Fig. 5 An extended polyp of Pocillopora dami­ Fig. 6 A part of a tentacle of Fungia repunda, cornis in the daytime. (a) The tentacle tips are which extends its tentacles in the daytime. The dark ened with dense zooxanthellae. (b) Assem- density of zooxanthellae in the tentacles of this blages of zooxanthellae can be observed on the species is intermediate between those of other coenosarc as dark bands, which are equivalent in species that fully extend tentacles in the daytime number to the tentacles (×65) and those that fully contract their tentacles. The round black dots are zooxanthellae. The long dots are nematocysts (×65) Fig. 7 Zooxanthellae are densely distributed on the oral disk of Fungia echinata, which does not extend its tentacles in the daytime. The black dots are zooxanthellae. A part of the oral disk is Fig. 8 Zooxanthellae are very scarce in the ten- darkened with zooxanthellae assemblages (lower tacles of Fungia echinata. The density of zoo xan- right) (×65) thellae is much lower in tentacles than in the coenosarc (see Fig. 7) (×65) ellae. There are a few similar examples in other zoo- xanthellate coelenterates. It is considered that many physiological processes are involved in the photo- taxic reactions of corals, although they are currently not well known. Based on his observations of Caulastraea furcata, AbeRef17 suggested that polyp extension at night is Fig. 9 A part of the oral disk of Lobophyllia caused by water absorption. He explained that the hemplichii, which does not extend its tentacles in osmotic pressure inside polyp tissues is decreased in the daytime. Zooxanthellae assemblages are lo- cated on the tips of projected tissues of (a) the oral the dark due to the low pH of the coelomic fluid, disks and (b) the coenosarc which dis solves unused carbon dioxide for assim- 39 ilation by zooxanthellae. Yonge was also interested digestion. The flatworms exhibit strong positive in this explanation; however, I find it rather ques­ phototaxis. When individual flatworms were placed tionable. approximately 50 cm away from a 60 W incandescent lamp in a Petri dish, they soon moved toward the Ref17 Abe N (1939) On the expansion and contraction of the light. They turned back in the opposite direction, at a polyp of a reef-coral, Caulastraea furcuta Dana. rate of 3 cm in 5-10 seconds, as soon as the Petri Palao Trop Biol Stn Stud 4: 651-670 dish was turned around. Eye-dependent phototaxis has been well studied in many other flatworm 20. Phototaxis in zooxanthellate animals species, but I was unable to find either eyes or Most reef-building corals harbor zooxanthellae eyespots in live specimens. The flatworm is and form colonies that face their polyps toward the commonly found on the colonies of reef-building light. Coral larvae also have zooxanthellae and settle corals in the genera Montipora, Lobophyllia, Stylo­ onto appropriate substrates according to their posi- phora, and Hydnophora. Based on my observations, tive photo taxic behavior (as I have reported pre- the flatworm appears to inhabit coral colonies with viouslyRef18). During the course of my studies on large co enosarcs and oral disks when the corals are zooxanthellate animals, all animals exhibited posi- in poor condition. A few other flatworm species are tive phototaxis. also found on coral colonies. These also have zooxanthellae and show weak positive phototaxis. i) Under laboratory conditions, reef-building corals iii) As previously reportedRef19, the upside-down jel- expelled zooxanthellae as their condition deter- lyfish Cassiopea harbors zooxanthellae and exhibits iorated. The expelled zooxanthellae initially settled positive phototaxis. Other zooxanthellate jellyfish evenly on the bottom surface of the container, but show similar phototactic behavior, as do soft corals after one or two days, they appeared to aggregate in and gorgonians harboring zooxanthellae species. spots. I found that the spots were individual slipper The positive phototactic behavior observed in Eu­ animalcules ingesting zooxanthellae. All of the dendrium and Pennaria by LoebRef20 may be as a paramecia with zooxanthellae in their guts displayed result of the existence of zooxanthellae. positive phototaxis. iv) YongeRef21 reported that giant clams (Tridacnidae) ii) Small individual flatworms*10 are often found on have a well-developed, beautifully colored mantle the surface of reef-building corals. The size of the con taining large numbers of zooxanthellae, and they flatworms range between 0.5 and 3 mm. They have generally occur on sea bottoms with sufficient light. very thin, egg-like, round bodies with tapering heads. Heart cockles (Corculum cardissa) also have zooxan- There are no visible internal organs. The flatworms thellae, but differ from giant clams in that they in- are brown and hence, are easily overlooked on the habit sunny reef flats. coral surface. How ever, they may become obvious in v) Aeolid nudibranchs occurring near coral colonies the field when they aggregate and fully cover the have zooxanthellae in their cerata. They also show surface of a coral colony*. The surface of such a positive phototaxis. coral colony turns a pale color. There are numerous vi) Cultured zooxanthellae tend to aggregate in the zooxanthellae inside the flatworm bodies. Zooxanth­ light. ellae near the body surface have a normal shape, but those in deeper parts of the body have relatively As described above, zooxanthellate animals show large diameters and appear to be in the process of positive phototaxis. The biological/physiological 40 pro cesses involved in phototaxis are unclear at this 21. Phototaxis as a factor in determining the stage, but I can state that they are somehow related to life form of reef-building corals the presence of zooxanthellae. I previously hypoth- As described in the last paper (Kawaguti 1937)Ref24, esized that all reef-building corals harboring zoo- phototaxis is the major factor determining the life xanthellae exhibit positive phototaxis.