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BULLETIN OF MARINE SCIENCE. 33(4): 905-918.1983 PAPER

TAXONOMY, ECOLOGY AND PHYSIOLOGY OF THE GEOGRAPHICALLY RESTRICTED SCLERACTINIAN SPECIES CHAGIUS MA TTHAI

C R. C Sheppard, Z. D. Dinesen and E. A. Drew

ABSTRACT The scleractinian coral Ctenella chagius is unusual in having a very restricted geographical range (, Indian Ocean) and for being the only extant Indo-Pacific member of the family . Several aspects of its biology were examined including its depth distribution, population structures, competitive ability, and rates of photosynthesis and cal- cification. In all these aspects the species falls well within ranges exhibited by the majority of coral species. No abnormalities were found which could account for its limited range. Although it is amongst the 25 commonest coral species in Chagos (out of 200) its taxonomic position suggests that it is a relict population.

Ctenella chagius Matthai is a unique coral for two reasons. Not only is this monospecific genus restricted to a single archipelago in the middle of the Indian Ocean, but it is also the sole Indo-Pacific member of the family Meandrinidae which is otherwise represented only in the Atlantic (Dinesen, 1977; Sheppard, 1981). C. chagius has received little attention since Matthai's (1928) description although several remarks have been made about its limited distribution (Rosen, 1971a; b; Dinesen, 1976; 1977; Bellamy, 1979; Sheppard, 1981). An opportunity for detailed study of the ecology and physiology of C. chagius occurred during the 1978/79 U.K. Joint Services Chagos Research Expedition which visited several in the Chagos Archipelago (Fig. 1). Collection of specimens, underwater observations, and experiments in situ have allowed: (1) redescription of the species and reassessment of its systematic position; (2) an autecological study of its distribution within the Chagos Archipelago and of its photosynthetic and calcification physiology; and (3) consideration of possible factors causing its restricted geographical distribution despite its local abundance.

METHODS

The systematic study was based on detailed examination by light and scanning electron microscopy ofa suite of 36 specimens, including type material. The specimens, from seaward and lagoonal habitats at Egmont and Peros Banhos atolls, were collected from low water mark to over 45 m, but mostly between 5 and 25 m. Systematic terminology follows that of Moore et al. (1956) and classification that of Wells (1956). Measurements of thecae, valleys and septal numbers were made both at the margins and elsewhere on coralla. Extreme ranges of all measurements were noted, and average values obtained from 15-20 measurements per specimen. The depth distribution and percentage presence of C. chagius were calculated from transects carried out on the seaward reef fronts of Peros Banhos, Salomon and Blenheim atolls and on reef slopes and isolated lagoon knolls at Peros Banhos. A total of25 transects were examined at the seaward sites at depth intervals of 3 or 6 m to a maximum of 45 m depth, and 30 transects were similarly examined at the lagoon sites to the sandy floor (always shallower than 45 m). Size-frequency histograms were constructed from the diameters of 50 adjacent colonies measured at 10m depth (that of greatest abundance of C. chagius) both on the exposed seaward reef front and on a protected lagoon knoll at Peros Banhos. Direct underwater observations were made of the expanded and contracted states of the polyps, dimensions of their tissues, contraction speeds of tentacles, and of interaction with other, closely adjacent species. Attempts to observe this species in open circuit aquaria failed consistently due to death of the colonies very soon after transfer. The aquaria supported many other species ofscleractinian , but no amount of care or speed in transfer overcame the difficulty with C. chagius. Careful

905 906 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.4, 1983

speakersQ ) Colvocoresses

OBlenheim Bena.,::es('-1 L,.,) t>Salomon

Peros Banhos QVictory

••

~Egmont

Chagosi:

Figure I, The location of the Chagos Archipelago (inset) and the atolls and submerged banks on which Ctenella chagius has been observed,

relocation of colonies a few meters along the reef did not cause death, but more severe disturbance such as transport to the aquarium was always fatal. An in situ experiment to determine rates of photosynthesis and calcification was carried out at 6, 13 and 25 m depth on a knoll in Peros Banhos lagoon. The methods described by Drew (1973) were used. Small intact colonies were collected from 10 m and incubated in sealed 500-ml jars for 6.5 h. Changes in dissolved oxygen were determined using the Winkler technique with modifications de- scribed by Drew and Robertson (1974). The radioisotopes "C (as 10 {.lCiNaHl·C03) and 45Ca(as 25

{.lCi "CaCI2) were injected into the jars at the start of incubations, and their incorporation into living tissue and skeleton respectively was subsequently determined. Small blocks of known surface area (approximately I-cm cubes) were cut from the oven-dried (80°C) experimental samples with a hacksaw SHEPPARD ET AL.: BIOLOGY OF CTEN/:,LLA CliAGJUS 907 and the radioisotopes fractionated and measured using standard techniques. Three replicate blocks were cut from each of the three replicate incubations carried out at each depth and also in the dark. Summation of net oxygen production in illuminated jars and oxygen utilization in the dark was considered a measure of gross photosynthesis, as was total carbon fixation in the light, determined as the difference between I'C incorporation in the light and dark. Total incorporation of ,sCa was considered a reasonable measure of actual calcification rates as incorporation in the dark was less than 5% that in the light. Oxygen data, obtained from whole, rounded colonies, was converted to an areal basis by direct measurement of their horizontal projected areas and to carbon metabolized using the factor I Itl O2 = 0.46 Itg C, which assumes glycerol to be the initial stable product of photosynthesis.

RESULTS Skeletal Structure and Affinities A detailed systematic description of Ctenella chagius is given in the Appendix. No evidence was found to suggest that this coral has closer affinities with the Eusmiliinae than with the Meandrinidae, and it closely resembles the Atlantic genus Meandrina. Ctenella chagius must therefore remain in the Meandrinidae. Thus it is the sole Recent Indo-Pacific representative of this family; its closest extant relative being the Atlantic Meandrina.

Polyp Appearance and Behavior Polyps of Ctenella chagius were cream to light brown in color, with transparent tentacles. Dark brown patches on colonies, tentatively attributed to higher den- sities of , were common, especially on shaded parts, and in those areas tentacles were brown, but with colorless tips. The corallum was usually penetrated by siphonaceous green algae and commonly had patches of strong green coloration indicative of this, which sometimes showed through the living colony but which were most evident after bleaching. The oral openings were oval, about 1.5 mm long and spaced 1.5 to 2.0 mm apart, or one every 2 to 3 septa along the valleys. Observations of this coral during darkness were not made, so the degree of night-time emergence remains unknown. However, tentacles were always extended to 2 to 3 mm above the calices during day-time. They were smooth cylinders with rounded tips, arranged in two rows. A row of longer tentacles showed slight but fairly rapid vibratory movement which did not appear to be the result only of water movement over the coral. Retraction of the tentacles upon mechanical irritation was rapid. After disturbance, a delay of less than I sec was followed by a virtually instantaneous «0.5 sec) withdrawal of tentacles to below the level of the walls over the whole colony regardless of size, although on larger colonies a very rapid ripple effect could be seen. In one dumbbell-shaped colony, stimulation on one end resulted in immediate withdrawal at both ends, transmission through the "waist" of the dumbbell occurring across a constriction only 2 cm wide and crossed at right angles by walls and valleys. Hence, neurotransmission occurred with great rapidity over adjoining walls as well as along polyps. The speed of the response and the ease with which it was elicited appears to be more rapid than in most other corals (Horridge, 1957). Extension of tentacles was also rapid. A delay of 5 to 30 sec occurred after cessation of stimulation, and then re-extension to the normal day-time length occurred over 1 to 2 sec, followed almost immediately by onset of the vibratory movement described above. C. chagius was observed interacting with 12 other species growing very close to it. Data in Table I show that it killed and was killed by an equal number of other species. The small number of observed interactions limited the validity of 908 BULLETIN OF MARINE SCIENCE. VOL. 33. NO.4. 1983

Table 1. Scleractinian corals observed in competitive interactions with C. chagius on 3 or more occasions, and outcome of interaction

Subordinate to C. chaglUs Dominant over C chagms Pocillopora verucosa Ellis and Solander Stylophora pistillata Esper Seriatopora hystrix Dana palifera (Lamarck) varians Verrill Acropora reticulata (Brook) Gardineroseris planulata (Dana) Montipora spp. Porites lutea (Milne-Edwards and Haime) astreata (Lamarck) Leptastrea transversa K1unzinger glabrescens (Chamisso and Eysenhardt) conclusions, but the species clearly showed some aggressive capability in terms of extracoelenteric digt:stion. It has been tentatively categorized as similar in this respect to several other common species in the Chagos Archipelago (Sheppard, 1979). Atlantic members of the Meandrinidae also include species known to be aggressive (Lang, 1973).

Local Distribution C. chagius was one of the 25 most common species amongst the 200 species of scleractinians present in the Chagos Archipelago; it has been observed at all the localities named in Figure 1 and appeared equally common at all of them. At Peros Banhos, it was equally abundant on the seaward reef fronts and on lagoon reef slopes and knolls; it was, however, absent from reeftlats and grew only where it was never emergent. Its absence from the of Salomon and Blenheim atolls, which contained more fine sediment than Peros Banhos, indicated an intolerance for slightly turbid water. The distribution of C. chagius with depth on both the seaward reef front and in the lagoon at Peros Banhos is shown in Figure 2. On the reef front it was abundant between 6 and 18 m depth, scarce both at 3 m and between 18 and 30 m and virtually absent below that depth. In the lagoon the distribution was very similar except for particularly high abundance at 3 m and scarcity below 24 m. Although observed deeper than 40 m on the Great Chagos Bank, this species was thus usually absent below 30 m and did not inhabit the particularly turbulent zone above 6 m at the seaward sites. The size-frequency distributions (Fig. 3) show a unimodal distribution both on

100 b

80 20 a b

N 60 15 ('l\ of total) N 40 10

20

3 6 9121824303643 3 6912182430 m. deep m. deep seaward lagoon seaward lagoon

Figure 2. (Left) The depth distribution of Ctenella chagius on seaward reef slopes (a) and lagoon reef slopes and lagoon knolls (b). y axis is % occurrence in about 40 collections (2 X 2-m2 quadrats) at each depth. Figure 3. (Right) Size-frequency distributions of Ctenella chagius on a seaward reef slope and a lagoon reef slope in southwest Peres Banhos . SHEPPARD ET AL: BIOLOGY OF CTENELLA CHAG/L'S 909

Table 2. Photosynthesis and respiration rates in Ctenella chagius and certain other massive corals

Oxygen/Unit Volume Basis (~I 0, ml-' h-')

Depth Temp lrradiancc Net Gross (m) ("C) (mW cm-1 PAR) Respiration Photosynthesis Photosynthesis Ctenella (Chagos)* 6 29 20.0 5.0 ± 0.8t 13.7 ± 2.5 18.7 13 29 10.0 5.0 ± 0.8t 10.8 ± 2.0 15.8 25 29 3.6 5.0 ± 0.8t 9.9 ± 0.7 14.9 Goniastrea (Eilat):J: 5 21 5.6 16.3 Favia (Low Is1es)§ 4 29.5 6.3 15.3 Carbon/Unit Area Basis

Carbon Metabolism Depth (m) From 02 Data From 14C Data

Daylight hours (/lg C cm-1 h-') 6 18.2 29.1 ± 15.5 13 15.2 35.6 ± 11.2 25 13.8 11.8 ± 2.0 Daily accretion (/lg C cm-2 day-I)\! 6 52.8 183.6 13 16.0 261.6 25 0.0 -24.0 Dark -165.6 • Duration 6.5 h (lOIS to 1645 h local time; solar noon 1330 h). t Incubated at 6 m depth only. ~ Data from Drew (1973). § Data from Yonge and Nicholls (1932). ~ Assuming 12 h photosynthesis at hourly rale and 24 h dark respiration.

the seaward reef front and on a protected lagoon knoll at Peros Banhos. This suggests either an irregular spawning cycle with only one main spawning event reflected in the present population; or regular spawning followed by variable recruitment success; or rapid growth of the young colonies into an adult, slower- growing population. The first two alternatives, both of which relate to irregular recruitment into the population, are probably not the cause of the single mode in this case as a similar pattern was observed for several other coral species on these reefs. The consistency of this pattern amongst non-crowded species suggests more rapid growth of the smaller, younger colonies, and arguments supporting this are given in Sheppard (1980).

Photosynthesis and Calcification Rates Oxygen production per unit volume of C. chagius colony at three depths is set out in Table 2A together with comparable values from other studies using corals of similar morphology. Despite difficulties associated with use of a volume basis for comparison, due to inclusion of indeterminate amounts of inert skeletal ma- terial, all three corals showed similar metabolic rates. C. chagius produced an excess of oxygen over respiratory demand at all three depths during this experi- ment.

Data for carbon metabolism per unit area, obtained by both the O2 and 14C methods, are compared in Table 2B. The two methods showed some difference 910 BULLETIN OF MARINE SCIENCE. VOL. 33. NO.4. 1983

Table 3. Calcification rates measured using 45Ca method

Total Calcif1c.Hion Depth Dark as °,0 T olal (m) (~g Ca em-2 h-') (~g Co mg N-' h-') (~g Co mg N 'h ')

Ctenella (Chagos) 6 52.2 ± I\.6 23.7* 3.0 13 58.8 ± 5.9 26.7* 2.7 25 32.4 ± 7.0 14.7* 4.9 Dark \.6 ± \.0 0.7* Massive corals in (Goreau and Goreau, 1959) Calpaphyllia nalans 12.7 19.8 Diplaria labyrinthifarmis 17.9 Diplaria slrigasa 5.9 Mantaslrea annularis 9.3 3.2

• Values per unit tissue nl1rogen calculaled assuming N = II % organic m.Hler. which was laken 10 be the organic residue from HCI decalcification X 2; mean organic residue for all samples = 10.1 ± 4.1 mg cm-2• in absolute magnitude as is often found in such comparisons, but were of the same order and indicated considerable reduction of photosynthetic carbon fixa- tion, on an hourly basis, at the deepest station. !fit is assumed that photosynthesis continued at these hourly rates for 12 h whilst dark respiration continued for 24 h per day, the daily carbon accretion rates also set out in Table 2 are obtained. Both the O2 and 14Cmethods showed positive daily accretion at 6 and 13 m depth but not at 25 m. Thus, C. chagius cannot be expected to survive in an autotrophic mode at or below that depth, and indeed, as was shown in Figure 2, it was not usually found any deeper. Incorporation of 45Ca into the skeleton of C. chagius was very much faster in the light than in the dark. The measured rates of skeletogenesis (Table 3) were relatively high at all three depths in the light and were considerably higher than those obtained by Goreau and Goreau (1959) for similar massive corals in Ja-

maica. At the two shallow stations, a mean deposition rate of 1713 /lg CaC03 cm-2 day-I was indicated for a 12 h light-12 h dark daily cycle. With a corallum density of about 1.25 g cm-3, a I-em cube could therefore be laid down in about 730 days and colony diameter would consequently increase by 1 cm per year, reducing to 60% of that rate at 25 m depth. Such growth rates are within the range of 0.25 to 2.0 cm diameter year-I quoted for massive corals by Stoddart (1969).

DISCUSSION The taxonomic studies (see Appendix) show that Ctenella chagius must still be considered a member of the Meandrinidae, a family otherwise confined to the Caribbean. This monospecific genus is widespread only within the confines of the Chagos Archipelago, in the relative isolation of the central Indian Ocean. There, C. chagius thrives in well illuminated water but is limited by turbidity, reduced irradiance below 25 m depth, and exposure to strong wave action and emergence. It shares these characteristics with at least 100 other coral species in Chagos or 50% of the total (Sheppard, 1981). It also has a population structure similar to several other common species. It is certainly not at the bottom of the aggression hierarchy but appears comparatively less aggressive than Caribbean meandrinids (see Lang, 1973). Photosynthetic, respiratory and calcification rates in this coral are comparable to many species with widespread distribution in the Indo-Pacific, and its growth rate appears to be quite normal for massive corals. SHEPPARD ET AL.: 1JI0LOGY OF CTENELLA C1IAG/l'S 911

The hemispherical shape and the high proportion of oral disc area to total surface area suggest that, according to the autotrophic-heterotrophic scale of Porter (I976), C. chagius should exhibit significant zooplankton-capturing activity. How- ever, although it presumably obtains a zooplankton supplement, it can satisfy its energy requirements by photosynthesis to about 25 m, and indeed seldom grows much deeper. According to Jaubert (1977), Porites convexa exhibited similar metabolic characteristics, but that coral also showed a marked change in colony shape from its shallow to deepest extremes, a feature common to many corals (Roos, 1967; Barnes and Taylor, 1973). No similar flatteningofC. chagiuscolonies occurred at its deeper limit and this failure to adapt to increased depth may contribute to its scarcity below 25-30 m. Calculations based on size frequency histograms and measured calcification rates indicate that the oldest size class at 10 m depth represents corals about 90 years old, assuming constant growth rates. However, the unimodal curves suggest that growth may in fact be considerably faster up to a third or half maximum size (Sheppard, 1980). Since small colonies were used in the calcification experiments, the oldest size class may therefore have taken up to twice as long to attain their large dimensions. According to density measurements, the mass of limestone deposited by the oldest colonies is about 230 kg and it is at least in part due to their substantial size that C. chagius is visually one of the more conspicuous corals on the shallow reef slope. Since none of the measured parameters suggest that C. chagius has any phys- iological weakness or lack of competitive ability, other than its inability to survive away from the reef in aquaria, there must be other reasons for its geographical isolation. This coral does not even occur on the nearby southern islands of the , which have been extensively studied by Wells and Davies (1966). The species living alongside C. chagius in the Chagos Archipelago do not differ mark- edly from those in many geographical areas from which it is absent. As Chagos has a particularly high diversity of scleractinians (Sheppard, 1981), and hence a potentially high level of inter-specific competition, it is not reasonable to suppose that this particular coral is absent elsewhere due simply to stronger competition or to a lack of suitable habitats. There seems to be no reason why it should not thrive on other reefs were it to settle on them, but perhaps its larvae cannot normally reach them. Unfortunately, the failure of adult colonies to live in aquaria prevented direct observations on larval production and longevity. It may be significant, however, that C. chagius is relatively abundant only within a 300- km-diameter area of scattered reefs, wherein no reef is more than 50 km from another. These reefs may act as "stepping stones" within the Archipelago, allowing extensive local colonization. No such aids to dispersal exist in the 400-km stretch of deep ocean to the nearest islands, the Maldives, and at present the species appears to be unable to traverse that distance. Little information is available on the evolutionary and zoogeographic history of Ctenella, although research into this is currently in progress (B. R. Rosen and J. Darrell, personal communication). The first appearance of its family was in the Cretaceous, and that of closely related extant Caribbean species was in the Eocene, both antedating the separation of the tropical Atlantic and Indo-Pacific provinces (Wells, 1956). The present wide geographical separation of the family, the re- stricted distribution of this monospecific genus, and the absence of close relatives elsewhere in the Indo-Pacific until the subordinallevel is reached, all suggest that the abundant and conspicuous Chagos population is a relict one, effectively cut off from other reefs in the province. The progressive retreat of C. chagius from other reefs which is implicit in this suggestion is even more difficult to reconcile 912 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.4, 1983 with this coral's relatively robust physiology than is its inability to spread outwards from the Chagos Archipelago. This uniquely restricted coral requires further study if this dilemma is to be resolved.

ACKNOWLEDGMENTS

Field work was carried out on the Joint Services Chagos Research Expedition, 1978/79; the numerous sponsors are acknowledged individually in the Expedition report. In particular we would like to thank R. Rayner, J. Robinson and A. Sheppard for assistance in the field. Systematic work was carried out at the Australian Institute of Marine Science, and for advice on , we are very grateful to Drs. M. Pichon, F. Bayer and J. Veron. Dr. C. Wallace assisted with SEM work. Radioisotope analyses were done at the Gatty Marine Laboratory, St. Andrews, Scotland, using equipment provided by the U.K. Natural Environment Research Council.

LITERATURE CITED

Barnes, D. J. and D. L. Taylor. 1973. In situ studies of calcification and photosynthetic carbon fixation in the coral ]I;fontastrea annularis. HelgoHinder wiss. Meeresunters 24: 284-291. Bellamy, D. J. 1979. Half of paradise. Cassells, London. 192 pp. Dinesen, Z. D. 1976. A study of the taxonomy and distribution of hermatypic corals of the Chagos Archipelago, Indian Ocean. M.Sc. Thesis, University of Durham, 76 pp. --. 1977. The cora] fauna of the Chagos Archipelago. Pages 155-161 in D. L. Taylor, ed. Proc. Third Int. Symp. 1. Bio]ogy. Univ. of Miami, Miami, . Drew, E. A. 1973. The biology and physiology of alga-invertebrate symbioses. III. In situ measure- ments of photosynthesis and calcification in some hermatypic corals. J. Exp. Mar. BioI. Ecol. 13: 165-179. --- and W. A. A. Robertson. 1974. A simple field version of the Winkler determination of dissolved oxygen. New. Phytol. 73: 793-796. Goreau, T. F. and N. Goreau. 1959. The physiology of skeleton formation in corals. II. Calcium deposition by hermatypic corals under various conditions in the reef. BioI. Bull. Mar. BioI. Lab. Woods Hole 17: 239-·250. Horridge, G. A. 1957. The co-ordination of the protective retraction of coral polyps. Phil. Trans. Roy. Soc. Lond. B 240: 495-529. Jaubert, J. 1977. Light, metabolism and growth forms of the hermatypic scleractinian coral Synaraea convexa Verrill in the lagoon of Moorea (French ). Pages 483-488 in D. L. Taylor, ed. Proc. Third Int. Cora] Reef Symp. I. Biology. Univ. of Miami, Miami, F]orida. Lang, J. C. 1973. Cora] Reef Project-papers in memory of Dr. Thomas F. Goreau. II. Interspecific aggression by scleractinian corals. 2. Why the race is not only to the swift. Bull. Mar. Sci. 23: 260-279. Matthai, G. 1928. A monograph of the recent meandroid Astraeidae. BM(NH) Cat. Madreporarian Corals (London) 7: ]-288, pis. 1-72. Moore, R. c., D. Hill and 1. W. Wells. 1956. Glossary of morphological terms applied to corals. Pages F245-F25I in R. C. Moore, ed. Treatise on invertebrate paleontology, Part F, Coelenterata. Geol. Soc. Am. and Univ. Kansas Press. Pichon, M. M. 1964. Contribution a ['etude de la repartition des madreporaires sur Ie recifde Tu]ear, Madagascar, Recl. Trav. Stn. Mar. Endoume. Fasc. Hors Ser. Suppl. 2: 79-204. Porter, J. W. 1976. Autotrophy, heterotrophy and resource partitioning in Caribbean reef building corals. Amer. Nat. 110: 731-742. Roos, P. J. 1967. Growth and occurrence of the reef coral Porites astreoides Lamarck in relation to submarine radiance distribution. Drukkerij, Elinkwijk, Utrecht. 72 pp. Rosen, B. R. 1971a. The distribution of reef coral genera in the Indian Ocean. Pages 243-299 in D. R. Stoddart and C. M. YOJ)ge, eds. Variation in Indian Ocean cora] reefs. Symp. Zool. Soc. Land. 28. ---. 1971b. Annotated check list and bibliography of corals ofthe Chagos Archipe]ago (including recent collections from ) with remarks on their distribution. Atoll Res. Bull. 149: 67-88. Sheppard, C. R. C. 1979. Interspecific aggression between reef corals with reference to their distri- bution. Mar. Ecol. Prog. Ser. I: 237-247. ---. 1980. Coral cover, zonation and diversity on reef slopes of Chagos atolls, and population structures of the major species. Mar. Ecol. Prog. Ser. 2: 193-205. ---. 1981. Reef and soft-substrate coral fauna of Chagos, Indian Ocean. J. Nat. Hist. 15: 607- 621. SHEPPARDETAL.:BIOLOGYOF CTENELU CHAGlUS 913

Stoddart, D. R. 1969. Ecology and morphology of recent coral reefs. BioI. Rev. 44: 433-498. Vaughan, T. W. and J. W. Wells. 1943. Revision of the suborders, families and genera of the . Geol. Soc. Am. Spec. Pap. 44, 363 pp. Wells, J. W. 1956. Scleractinia. Pages F328-F444 in R. C. Moore, ed. Treatise on invertebrate paleontology, Part F, Coelenterata. Geol. Soc. Am. and Univ. Kansas Press. --- and P. S. Davies. 1966. Preliminary list of stony corals from Addu Atoll. Atoll Res. Bull. 116: 43-55. Yonge, C. M. and A. G. Nicholls. 1932. Studies on the physiology of coral. VI. The relationship between respiration in corals and the production of oxygen by their zooxanthellae. Gt. Barrier Reef Exped. Sci. Rep. 1: 213-251.

DATEACCEPTED: September 9, 1982.

ADDRESS: Australian Institute oj Marine Science, P.M.B. No.3, Townsville M.S.O .. 4810. .

ApPENDIX 1 Systematic Description Genus Ctenella Matthai, 1928

Ctenella Matthai, 1928, p. 171; Vaughan and Wells, 1943, p. 220; Wells, 1956, p. F4l5. Vaughan and Wells (1943) placed Ctenella in the Eusmiliinae but Wells (1956) later moved the genus to the Mcandrinidae. A second species, C. laxa. has been reported from the Saya de Malha Bank (Matthai, 1928) and Madagascar (Pichon, 1964), but this probably belongs to the eusmiliinid genus Gyrosmilia (Dinesen, 1977).

Ctenella chagius Matthai, 1928 Figures 4-6

Ctenella chagius Matthai, 1928, p. 172, pI. 54, fig. 2. Material Examined.-From British Museum (Natural History): 28.3.1.61 (holotype); 28.3.1.60 (fig- ured Matthai, 1928, pI. 54, fig. 2), 1928.4.18.235,28.4.18.591 (paratypes); 1982.2.17.29-1982.2.17.39. From Tyne and Wear County Council Museum, Sunderland, U.K.: F2252-2270. One specimen each from private collections of E. A. Drew and M. M. Pichon. Measurements of thecae and valleys, and septal counts, are shown in Table 4. Description.-Coralla are usually massive, although small specimens are submassive or encrusting. The shape is rounded and approximately hemispherical, but larger examples often become elongated and slightly greater than hemispherical in height. The largest specimen examined (collection Drew) is 22 X 20 X 18 cm high, the smallest (1982.2.17.35) is 1.0 X 0.9 X 0.8 cm high. Most coralla are <20 cm in greatest dimension. Colonies over I m across have been observed, though the surfaces of the larger colonies commonly contain extensive dead patches. Colony formation is by polystomodaeal, intramural, intratentacular budding (Wells, 1956), and coralla usually form a very regular, meandroid, "brain" pattern, with long valleys. Two, or rarely three, orders of septa are distinguishable. Primary septa are generally slightly thicker and more exsert than secondary septa, and extend further towards the columella. Although primary septa may alternate with secondaries, on most coralla the primary septa outnumber secondaries. Adjacent septal series meet on summits of thecae, and are rarely separated by ambulacra. Individual

-1 Figure 4. Ctenella chagius: a, b, Typical hemispherical colony shape and regularly meandroid "brain" pattern (F2252, XO.7, X3.0); c, Elongate colony shape with parallel valleys (F2254, XO.3); d, Unusually narrow valleys and thecae, where valleys are parallel (F2255, X3.0).

Figure 5. Ctenella chagius: a, b, Profile of septa descending to meet columella (F2265, X35, SEM); b, Detail of above specimen, showing denticles on septal margin (X70, SEM); c, Broken septa, some with hollow interior (F2270, X17, SEM); d, Section showing broken, solid septa and endothecal dissepiments (F2260, X12, SEM). 914 BULLETIN OF MARINE SCIENCE. VOL 33. NO.4. 1983 SHEPPARD ET AL.: BIOLOGY OF CTENELLA CIiAG/US 915 916 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.4, 1983 SHEPPARD ET AL.: BIOLOGY OF CTF:NELLA C/IACIUS 917

Table 4. Measurements (mm) of thecae and valleys, and septal counts, for Clenella chagius

Corallum Margin Elsewhere on Camllurn

Maximum Range Usual Range Maximum Range Usual Range

Septa cm-I 8-18 12-16 8-20 12-16 Width across theca 1-7 2-3 0.5-7.0 2-3 Width across theca plus septa* 2-9 4-6 2-9 4-6 Valley widtht 3-9 4-6 2-9 5-7 Valley depth:!: 2-5 3-5 3-6 3-5 " Distance between columellae of adjacent series. t Distance between summits of two adjacent thecae. t Depth from top of columella to summa of adjacent theca. septa are occasionally united over thecae, then primary septa may unite with either primary or secondary septa over thecae. However, septa usually alternate across thecae; and since septa are often slightly exsert, the inner ends of septa from adjacent series may overlap. Septal thickness rarely exceeds 0.5 mm, is frequently <0.25 mm, and never approaches the 1.0 mm described by Matthai (1928). Septa are usually solid, but some hollow septa were observed in some coralla. Septa descend almost vertically towards the columella, the margin curving smoothly from horizontal to vertical. The apparently angular profile of some septa seems due to breakage of tops of septa. However, where exsert septa descend to meet the tops of thecae, they may have an angular or rounded profile. The septal margin has minute denticles, numbering approximately 12-15 mm-I (counted by light microscope). The septal surface is ornamented with minute spinules, which are low and conical with blunt or pointed tips. Faint striations, indicating the diverging trabecular structure, may be evident. The columella is thin, lamellar, and usually continuous. Usually 50% or more of the septa (i.e., most primaries) meet the columella, which is rarely composed of more than one lamellar layer. The upper part of the theca is usually rounded, though on thinner thecae the tops may be acute. On most specimens a narrow ridge occurs along the summit of the theca over part or all of the corallum. All coralla seem to have well developed endothecal dissepimcnts, and the surface of the thecae is sometimes brokcn, revealing "blisters." Valley width is usually similar to the width across thecae plus septa (sce Table 4), enhancing the very regular meandroid appearance of coralla. Variation. - The most obvious variation among coralla is that of valley pattern. Whereas most spec- imens are meandroid, about one-third of them have small areas of straight valleys, especially at their margins. Some larger examples have extensive areas of straight valleys up to 23 cm in length. Here valleys and thecae may be narrower, and this may be due to the elongate colony shape. Apart from the presence of some hollow septa in 40% of the suite, including the holotype, specimens are very consistent in septal structure and arrangement. Among coralla there are some slight differences in average thecal width, and width and depth of valleys; and the extent to which thecal summits are rounded or acute, and ridged or unridged. Accordingly, there is slight variation in the profile of the septal margin, whether more or less "broad-shouldered." Several coralla show a tendency, principally at the margin, for adjacent series to be separated by a narrow region of ambulacrum. Thecae here tend to be unridged and slightly wider, and septa are less exsert. On one unusual example (1982.2.17.36), septal series are separated by narrow ambulacra distinct over the entire corallum. This specimen is also remarkable for its columella, which consists throughout ofa double, often twisted, layer of lamellae. A few other specimens (e.g., paratype 28.4.18.591) have areas where the columella is a double layer. Although the columella is typically continuous, in several specimens it is interrupted in places or not developed. Here septa may unite across valleys. Despite this variability, Clenella chagius is a very distinctive species showing remarkably little

~ Figure 6. Clenella chagius: a, Section showing lamellar columella united to septa, note also spinu1es on septal surface (F2265, x35, SEM); b, Unusual columella of double lamellar layer (1982.2.17.36, X35, SEM); c, Typically continuous columella and closely united septal series (holotype, 28.3.1.61, X3); d, Septal series atypically separated by narrow ambulacra (1982.2.17.36, X3). 918 BULLETIN OF MARINE SCIENCE. VOL. 33. NO.4. 1983 intraspecific variation. Much of the variation noted may be encountered within a single specimen. Such variation as exists could not be related to any obvious differences in habitat. Affinities. - The rather smooth septal margin and lightweight coralla (compared to many members of the Faviina), and the sometimes hollow septa, certainly suggest affinities with the Eusmiliinae. How- ever, although Meandrina has a much denser skeleton, Ctenella chagius bears very close resemblance to this Atlantic coral, both in growth form and in septal structure, notably the minutely dentate septal margIn. To determine beyond doubt the systematic position of Ctenella will involve a major revision of both the Meandrinidae and the Eusmiliinae. During this study, coralla of Meandrina and various eusmiliinid genera were examined, and no evidence was found to suggest that C. chagius has closer affinities with the Eusmiliinae than the Meandrinidae. Ctenella must therefore remain in the Mean- drinidae.