THE SCLERACTINIAN CORAL FAUNA OF RAKITU ISLAND, NORTH-EASTERN NEW ZEALAND. by F.J. Brook

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1 TANE 28,1982 THE SCLERACTINIAN CORAL FAUNA OF RAKITU ISLAND, NORTH-EASTERN NEW ZEALAND by F.J. Brook Department of Geology, University of Auckland, Private Bag, Auckland SUMMARY Four species of ahermatypic coral occur in the waters around Rakitu Island. Flabellum rubrum (Quoy & Gaimard) and Culicia rubeola (Quoy & Gaimard) live attached to rocky substrates and extend from the immediate subtidal to depths greater than 30 m. Sphenotrochus ralphae Squires and Kionotrochus suteri Dennant have a largely nonoverlapping range on sediment substrates to the west and north-west of Rakitu Island. An account is given of growth of the attached trophozoid stage of K. suteri. INTRODUCTION A brief study has been made of the Recent scleractinian coral fauna of Rakitu (Arid) Island following a visit by the Offshore Islands Research Group from 30 December 1980 to 8 January Rakitu Island lies off the east coast of Great Barrier Island 100 km north-east of Auckland, New Zealand, at latitude 36 08'S and longitude 'E (Fig. 1). It is a small (3.5 km') steep-sided island which drops away close inshore to water depths of between 20 and 50 m. Corals from sediment substrates on the western side of Rakitu Island were obtained from dredge samples collected for studies of macrofaunal associations by Hayward al (1982). The hand-hauled dredge samples an area of up to m at seafloor. The station numbers shown in Fig. 1 1 are the same as those in Hayward et al (1982). In addition three SCUBA dives were made to assess hard-substrate coral faunas on the north-west and eastern sides of Rakitu Island (Fig. 1). SEDIMENT SUBSTRATE FAUNAS Sediments fine northwards on the north-western and western sides of Rakitu Island (Fig. 2). The underwater flanks of the island are draped with medium to coarse sands which locally can be of almost wholly bioclastic origin. On the steep north-western side of the island this belt extends down to as much as 50 m depth whereas on the western side it 163

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3 Fig. 2. Sea bottom sediments west of Rakitu Island showing distribution of ralphae Squires and Kionotrochus suteri Dennant. Sphenotrochus 165

4 occurs only to m depth. Further offshore in the narrow strait between Rakitu Island and Great Barrier Island current-swept gravelly sediments occur (20-28 m depth). These fine northwards with increasing water depth through medium to coarse sands (28-34 m depth) into fin sands (deeper than 34 m). Beyond 60 m depth fine sands are generally muddy. A small cove o: the north-western side of the island is floored with fine to coarse, locally; muddy sands. Two species of coral occur commonly in the area studied. These are the turbonoliines Sphenotrochus (S.) ralphae Squires and Kionotrochus, (K.) suteri Dennant. The latter was found only as dead specimens, but both living and dead S. ralphae were obtained. However dead coralla of both species are mostly fresh and unworn and are probably not far removed from their place of growth. At Rakitu Island the two species have a largely non-overlapping distribution with respect to depth and substrate type, and occurs at m depth on a wide range of substrate types from gravelly sands to fine sand (Table 1). Dead specimens are most abundant on medium to coarse often gravelly sand while living specimens were obtained at 28 m and 31 m depth on gravel and coarse sand (Stations 13, 30) and at 58 m depth on fine sand (Station 34), Table 1. Occurrence of Sphenotrochus ralphae Squires at Rakitu Island Station Depth Sediment Number of live Number of dead (m) specimens specimens 8 45 gravelly crs. sand fine sand sandy gravel sandy gravel crs. sand crs. sand crs. sand gravelly med. sand « fine sand 1 Kionotrochus suteri occurs below 50 m depth on fine, often slightly muddy sand. At stations where this species is common (11,19, 21), both the free-living form and the small attached form (Squires 1964) are represented (Table 2). Trophozoids were only found attached to dead bivalve shells or shell fragments. However this is understandable as these latter comprise the bulk of the coarse sediment fraction at stations with K. suteri. The majority of trophozoids are attached to sculptured surfaces, with less common attachment on unsculptured valves or on the insides of valves. An attachment surface having relief may thus be better suited for settlement of planulae. 166

5 Table 2. Occurrence of dead Kionotrochus suteri Dennant at Rakitu Island Station Depth (m) Sediment Number of specimens (trophozoids) fine sand 4 muddy fine sand muddy fine sand muddy fine sand medium sand Number of specimens (freeliving) In addition to these two corals, rare, worn fragments of Culicia rubeola (Quoy & Gaimard) and Flabellum rubrum (Quoy & Gaimard) occur sporadically in bioclastic sediments close inshore to Rakitu Island. A fifth species is represented by fragmental material at stations 24 and 35. Better preserved specimens are known from the Mokohinau and Poor Knights Islands where the species lives attached to shallow subtidal rocky substrates. It is apparently a rhizangiid coral previously unrecorded in the New Zealand fauna and will not be treated further here. An account of the total softbottom macrobenthos around Rakitu Island is given in Hayward et at (1982). Kionotrochus suteri is restricted to their Cuspidaria - Amphiura - Notocallista association. Sphenotrochus ralphae occurs mainly in a Selenaria squamosa association but also in a Selenaria - Zeacolpus association and rarely in the Cuspidaria-Amphiura - Notocallista association. The absence of living K. suteri in the samples studied is interesting. Certainly some coralla are corroded and filled with sediment and thus may be part of a relict fauna. However many appear freshly dead and a few trophozoids are attached to fresh bivalve shells. The bivalve Saccella bellula (A. Adams) has a similar occurrence in the Cuspidaria - Amphiura - Notocallista association. Freshly dead valves are extremely common at stations 11, 19, 20, 21 yet only one Living specimen was recovered. Thus if K. suteri is living at Rakitu Island as seems probable, it must have a patchy, localised distribution. This could easily occur if it had only limited larval dispersal, and patchiness would certainly be enhanced if each tropozoid budded off more than one free-living corallum as suggested by Squires (1964). HARD SUBSTRATE FAUNAS Two species of coral occur commonly in subtidal rocky habitats at Rakitu Island. These are the cup coral Flabellum rubrum (Quoy & Gaimard) and the encrusting coral Culicia rubeola (Quoy & Gaimard). On the exposed northern coast at locality A (Fig. 1) both are common in shaded rock crevices and under overhangs below c.6 m depth. At the more sheltered locality B (Fig. 1) both species occur commonly from 1 m 167

6 depth down to the shellsand seafloor at 25 m depth. Here F. rubrum occurs only in crevices above c. 15 m depth, but below this occurs also oi open, flat rock surfaces. Culicia rubeola occupies similar habitat encrusting vertical to horizontal rock surfaces below 15 m, and crevice in shallower water. However it also occurs in more open habitats t depths as shallow as 1 m. Some interesting occurrences of F. rubrum were noted at this locality. One specimen was found at 20 m depth lyin prone on a sediment substrate. It had a heavily calcified base with i flattened attachment area and had evidently been broken off a hare substrate. When found it had tentacles extended and was apparently feeding. This accidental habit probably closely parallels the life habit o: asexually budded flabellids. Three other living specimens were seen al 20 m depth attached to a horizontal rock surface on which a covering oi shellsand extended almost to the hps of the calices. The species thus appears fairly tolerant of sediment movement although smothering would undoubtedly cause death. Locality C is a narrow seacave in a small cove on the east side o: Rakitu Island. The cave is 50 m long and shelves from a water depth of m at the entrance to a beach 10 m from the end. It opens towards the south-west and is thus protected from direct onshore wave action However, a side fissure opening to the north-east ensures a constant circulation of well-oxygenated water through the cave. Within the cave a zonation is evident with barnacles and corals as the dominant fauna. The highest band is formed by a dense encrustation of the barnacle Balanus trigonus Darwin that extends from intertidal to the immediate subtidal. It is underlain by a band c.30 cm wide dominated by Culicia rubeola. Both species disappear 10 m inside the cave leaving bare rock. Below the Culicia band, and extending almost to the floor of the cave, is a zone with scattered Flabellum rubrum as the dominant species. Flabellum rubrum continues to be common 20 m into the cave after which it disappears. Associated taxa in both the Culicia and Flabellum "zones" include serpulid worms and small encrusting sponges. TAXONOMIC NOTES Family RHIZANGIIDAE d'orbigny 1851 Genus Culicia Dana 1846 Culicia rubeola (Quoy & Gaimard 1833) Specimens obtained from different locations at Rakitu Island show variety in the maximum diameter attained by coralla. In specimens from 8-10 m depth at locality A the maximum diameter is c.6.0 mm while the average diameter is c.4.0 mm. However at locality B at 3 m depth, coralla only attain a diameter of c.4.0 mm while the average diameter is c.3.0 mm. Coralla at the former locality are generally more erect than those at locality B. At both localities colonies formed isolated 168

7 masses having internally consistent corallum diameters. However in the seacave at locality C the species forms an almost continuous cover in a 30 cm wide zone immediately below low tide level. Here fragments from different parts of the zone show widely differing corallum diameters up to a maximum of c.6.0 mm. From this I believe that the species may produce larger coralla in habitats sheltered from wave action than it does in more exposed habitats. However, there are insufficient data to verify this. Family CARYOPHYLLIIDAE Gray 1847 Subfamily TURBINOLIINAE Milne-Edwards & Haime 1857 Genus Sphenotrochus Milne-Edwards & Haime 1848 Sphenotrochus (S.) ralphae Squires 1964 All previously figured specimens of S. ralphae show a corallum that tapers gradually from a greatest diameter near the calyx to a sharp to bluntly rounded base, or one with a basal spine (Squires 1964, Squires and Keyes 1967). However several specimens from Rakitu Island expand rapidly from the base at c.60, and then become sub-parallel (Fig. 4a). None of the specimens from Rakitu Island possesses a basal spine. The polyp of this species was not seen expanded but within the calice it appears pale-orange. The individual from station 30 (Fig. 1) alters the known depth range of living specimens to m (previously m; Squires and Keyes 1967). Genus Kionotrochus Dennant 1906 Kionotrochus (Kionotrochus) suteri Dennant 1906 A plot of corallum width by corallum height for Rakitu Island specimens shows that corallum height increases more rapidly than does width (Fig. 3). Small free-living coralla are of discoidal form and have a plano-convex base. With further growth the corallum acquires a bowlshaped or low conical form. Only in larger specimens does the corallum become turbinate, having a height/width ratio of 1.0 or greater (Fig 3.) As the corallum increases in size, downwards growth of costae forms a pointed base (Squires 1964). However some coralla up to 4.0 mm in height retain a plano-convex base while others less than 3 mm in height have a pointed base. The lack of transitional forms between the fixed trophozoids (Squires 1966) and free-living forms in Fig. 3 results from the sorting of sediment from dredge samples through a sieve with a mesh diameter of 2.0 mm. Trophozoids were retained in the sieve as the shell fragments to which they are attached are of greater diameter than the mesh size. Trophozoid specimens collected from Rakitu Island give additional insight into growth of the attached stage. The earliest formed corallum is up to 2.0 mm in diameter, and has a wall less than 0.5 mm high that is 169

8 Height of corallum (mm) Fig. 3. Scattergram showing the all metric relationship between height and width of coralla of Kionotrochus suteri Dennant from Rakitu Island. Heights of trophozoids were not measured but diameters are plotted on the y-axis. usually constricted upwards. Beads are arranged on the outside of the wall to form low, closely spaced ridges, or a malleated surface. The basal plate generally extends just beyond the base of the corallum wall. The six protosepta are exert and are non - or weakly spinose. Polycylic bases commonly develop after formation of the corallum wall and before second cycle septa (Fig. 4b). Squires (1964) claimed that this species had pah developed before second cycle septa. However, one specimen from Rakitu Island clearly shows development of second cycle septa, yet lacks pah (Fig. 4c). Horizontal lappets similar to those described by Squires (1963) for Flabellum rubrum can occur in the juvenile corallum. No "juvenile" trophozoids were seen to have third cycle septa although many had pah and second cycle septa. Further growth occurs by cylindrical upwards extension of the corallum, often with increased diameter, to a height of c.1.0 mm. This stage has highly exert spinose septa of three cycles with pah and a weakly developed columella. In young specimens costae extend down to the top of the juvenile corallum. The latter may be partially resorbed and secondarily costate. Older specimens have weakly costate bands of thickened stereome atop the juvenile state. Again septa are highly exert. One trophozoid specimen collected clearly shows the onset of transverse fission with a small discoidal corallum sitting on the juvenile corallum (Fig. 4d). Following fission the fixed corallum has only a very 170

9 Fig. 4. (a) Sphenotrochus ralphae: Station 17, x 6; Kionotrochus suteri: fb) polycyclic juvenile trophozoid, Station 11, x 6; (c) juvenile trophozoid, Station 20, x 6; (d) trophozoid showing partial transverse fission Station 11, x 6; (e) trophozoid showing intratentacular budding, Station 19, x 6. low edge zone and the interior of the calice is concave (fig. 10,11 Squires 1964). Thus fission apparently does not occur in a plane but along a concave surface. A further trophozoid specimen shows intratentacular budding with trabecular linkage (Fig. 4e). This is the first record of that form of reproduction in K. suteri Family FLABELIIDAE Bouine 1905 Flabellum rubrum (Quoy & Gaimard 1853) Highly variable corallum morphology is well documented in this species. Squires (1963) concluded that specimens from deep water on stable substrates are generally pedicellate - cuneiform while those on 171

10 unstable substrates or in high energy areas are more cylindrical and have a basal attachment thickened by additional epithecal material. Most specimens seen during the present study had ovate calices and all had thickened basal attachments. However rare specimens from the seacave (locality C, Fig. 1) are cuneiform, albeit with a thickened base. A cylindrical or ovate corallum is obviously better adapted to a turbulent environment than is a flattened cuneiform corallum. This is reflected in the absence of cuneiform coralla from more exposed habitats around Rakitu Island. However above records show that cuneiform F. rubrum can occur in water depths as shallow as 1 m in a suitably sheltered microhabit e.g. crevice. The occurrence of F. rubrum in the seacave is of interest also with regard to polyp colour. Approximately 20% of specimens within the cave have white polyps, with the remainder having pale orange polyps. Immediately outside the cave specimens have bright red or scarlet polyps as do all other specimens seen around Rakitu Island. Powell (1947) described shallow subtidal F. rubrum from the Hauraki Gulf as having scarlet polyps while intertidal specimens have salmon or dull Vermillion polyps. Squires (1963, p.22) described polyps from 55 m depth in Cook Strait as being "variously a dull orange, a salmon, and a light yellow; the lesser colour intensities being associated with the larger specimens". There are also records of a form having a polyp banded in white and scarlet (Suter 1906) but this requires confirmation. From the above it appears that there may be a correlation between polyp colour and light intensity, with red or scarlet polyps occurring in illuminated habitats and salmon, pale orange, yellow, or white polyps in less illuminated habitats. Light intensity varies with water depth, turbidity, and shade and thus less brightly pigmented polyps may be expected to occur in deeper or more turbid waters, or in shaded habitats e.g. in caves, under stones etc. ACKNOWLEDGEMENTS I would like to thank Jack Grant-Mackie for critically reading the manuscript and suggesting improvements. Thanks also to Roy Harris for draughting the figures, Ross Weber for photographs and Denise Coldham for typing the manuscript. REFERENCES Hayward, B.W.; Brook, F.J.; Grace, R.V. & Bull, V.H., 1982: Soft bottom macrofauna and sediments off Rakitu Island, north-east New Zealand. Tane Powell, A.W.B., 1947: "Native Animals of New Zealand". Auckland Museum Handbook Zoology. Auckland, 96 p. Squires, D.F. 1963: Flabellum rubrum (Quoy and Gaimard). New Zealand Department of Scientific and Industrial Research Bulletin 154 : (Memoir of the New Zealand Oceanographic Institute 20). 172

11 Squires, D.F. 1964: New stony corals (Scleractinia) from north-eastern New Zealand. Records of the Auckland Institute and Museum 6:1-9. Squires, D.F. & Keyes, I.W. 1967: The marine fauna of New Zealand : Scleractinian corals. New Zealand Department of Scientific and Industrial Research Bulletin 185 : 1-46 (Memoir of the New Zealand Oceanographic Institute 43). Suter, H. 1906: On Flabellum rugulosum Tennison-Woods. Transactions of the New Zealand Institute 38:

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