Amphistegina d'Orbigny 1826

Amphistegina d’Orbigny 1826

Amphistegina lessonii d’Orbigny 1826 (Parker, Jones & Brady 1865) (Fig. 23:8–10)

1826 Amphistegina lessonii d’Orbigny, in Parker, Jones & Brady, 1865, p. 304, pl. 3, fig. 92. 1978 Amphistegina lessonii d’Orbigny; Hallock & Hansen, p. 103, fig. 5c, d. 1979 Amphistegina lessonii d’Orbigny; Hallock & Larsen, p. 34, fig. 1: 5, 5A. 1991 Amphistegina lessonii d’Orbigny; Van Marle, p. 80, pl. 21, figs 7, 8.

1984 Amphistegina lessonii d’Orbigny; Hallock, p. 251, fig. 1: 8.

1993 Amphistegina lessonii d’Orbigny; Hottinger et al., p. 132, pl. 184, figs 1–11; pl. 185, figs 1–7. 1994 Amphistegina lessonii d’Orbigny; Loeblich & Tappan, p. 156, pl. 340, figs 1–9. 1999 Amphistegina lessonii d’Orbigny; Harney et al., p. 64, fig. 1.

1999 Amphistegina lessonii d’Orbigny; Hohenegger et al., p. 141, fig. 19.

2004 Amphistegina lessonii d’Orbigny; Saraswati et al., p. 340, pl. 1, fig. 5. 2004 Amphistegina lessonii d’Orbigny; Resig, p. 225, pl. 1, fig. 7, 9, 11.

2007 Amphistegina lessonii d’Orbigny; Horton et al., p. 343, pl. 1, fig. 11.

2009 Amphistegina lessonii d’Orbigny; Parker, p. 498, fig. 355a–d.

2010 Amphistegina lessonii d’Orbigny; Narayan & Pandolfi, p. 2076, pl. 1, fig. 27. 2012 Amphistegina lessonii d’Orbigny; Debenay, p. 215, pl. 20.

Description. See Hohenegger et al. (1999, p. 141, fig. 19), Hottinger et al. (1993, p. 132, pl. 184, figs 1–11; pl. 185, figs 1–7) and Van Marle (1991, p. 80, pl. 21, figs 7, 8).

Remarks. Amphistegina lessonii d’Orbigny 1826 has a lenticular, slightly unequally biconvex or planoconvex test and an irregular star-shaped umbo with faintly perforate ornamentation. The periphery is smooth with a pinched margin; sutures are flush and the aperture is an interiomarginal low slit-like arch with a pustulose lip and pustulose ornament that radiates from the aperture (Fig. 23:8–10).

Amphistegina lessonii differs from Amphistegina lobifera Larsen 1976 by its more compressed, sharply lenticular-shaped test. Amphistegina lobifera has a more inflated biconvex test with a much larger and deeper lobate umbo. As noted by Parker (2009), the relatively slight variation between these two species may be a gradation in morphology between two morphological extremes within the same speces, making these two taxa potentially conspecific. It is noticeable in synonymised examples of A. lessonii that the small umbo ranges in size from barely visible to approximately 1/10th test diameter. Therefore, the point where the lobulate test shape and umbo size becomes large enough to identify A. lobifera as opposed to A. lessonii is still very difficult to establish. For the CG populations, the two taxa are separated according to the relative size of lobulate ornament and whether the test is planoconvex to weakly biconvex or inflated and strongly biconvex.

Significant work by Hallock et al. (Hallock & Hansen 1978; Hallock 1979;1984; Hallock et al. 1986) documented the influence of environment on test thickness in A. lessonii. Specimens showed a reduction in thickness of secondary laminae within chamber walls with depth (Hallock & Hansen 1978) and individuals exposed to saturated light and subjected to gentle water motion produced greatest test thickness to diameter ratios (Hallock et al. 1986). Furthermore, specimens subjected to water motion possessed test lamellar thickness up to 50% thicker than specimens grown in conditions with no water motion (Hallock et al. 1986).

Amphistegina lessonii was originally reported from Mauritius by d’Orbigny (1826) which is the type location. Specimens collected by Brady (1884) from the Bermudas and Admiralty Islands came from depths of 795 m (leg 33) and 27–83 m (leg 218A), respectively. Studies by Hallock & Hansen (1978) and Hallock & Larsen (1979) focused on specimens collected from the Gulf of Aqaba by Larsen (1976) who additionally examined the paratypes of A. lessonii. Van Marle (1991) reported specimens from eastern Indonesia within Middle Miocene-Quaternary and recent island sediments. Recent island sediments commonly have A. lessonii at depths ranging from 60–150 m with scattered occurrence as deep as 495 m (Van Marle 1991). Hottinger et al. (1993) found this species in the Gulf of Aqaba, Red Sea and Loeblich & Tappan (1994) from a depth of 31 m at the eastern Van Diemen Rise, 146 m at the Western Timor Sea and 24 m north of Melville Island, southeast Timor Sea. Specimens collected from the CG are similar to the biconvex form of A. lessonii described by Hohenegger et al. (1999) from Sesoko Island, Okinawa, Japan. These specimens were common at depths down to 50 m and were most abundant on the northern side of the island (Hohenegger et al. 1999). Saraswati et al. (2004) also found A. lessonii further south, on Akajima Reef Flat, Okinawa. Resig (2004), Hallock (1979;1984), Hallock & Larsen (1979), Hallock et al. (1986) and Harney et al. (1999) studied specimens from Hawaiian beaches, particularly Oahu, and Palau within the Western Caroline Islands, finding it a typical reef slope species with depths ranging from 5 – 20 m. Horton et al. (2007) collected specimens from shelf subtidal sediments and found A. lessonii to be a dominant middle-shelf zone species with maximum abundance at a depth of 38.5 m. Parker (2009) collected specimens from Ningaloo Reef, Western Australia and Narayan & Pandolfi (2010) recovered specimens from Moreton Bay, GBR. Debenay (2012) reported this species from the southwestern lagoon, back-reef and areas under influence of the open sea in New Caledonia.

Distribution within study area. Amphistegina lessonii was collected from all CG reefs. Abundance was lower on reef flats (one to seven specimens per site) compared to lagoons (one to nineteen specimens per site), but the greatest abundance was from the channel sample with 68 specimens (20% of total counts). This distribution and abundance supports previous data that shows A. lessonii is most common below tidal influence in lower energy regimes at depths ca. 35 m.

Amphistegina lobifera Larsen 1976 (Fig. 23:11, 12)

1976 Amphistegina lobifera Larsen, p. 4, pl. 3, figs 1–5; pl. 7, fig. 3; pl. 8, fig. 3. 1978 Amphistegina lobifera Larsen; Hallock & Hansen, p. 103, fig. 5a, b.

1979 Amphistegina lobifera Larsen; Hallock & Larsen, p. 34, fig. 1: 3, 3A. 1984 Amphistegina lobifera Larsen; Hallock, p. 251, fig. 1: 8.

1993 Amphistegina lobifera Larsen; Hottinger et al., p. 133, pl. 186, figs 1–11; pl. 187, figs –7; pl. 188, figs 1–6. 1999 Amphistegina lobifera Larsen; Harney et al., p. 63, fig. 1.

1999 Amphistegina lobifera Larsen; Hohenegger et al., p. 140, fig. 17.

2002 Amphistegina lobifera Larsen; Yordanova & Hohenegger, p. 191, pl. 30, figs 14–16. 2004 Amphistegina lobifera Larsen; Resig, p. 226, pl. 1, figs 1–6, 10.

2009 Amphistegina lobifera Larsen; Parker, p. 498, fig. 355e–o.

2012 Amphistegina lobifera Larsen; Debenay, p. 216, pl. 20.

Description. See Hottinger et al. (1993, p. 133, pl. 186, figs 1–11; pl. 187, figs 1–7; pl. 188, figs 1–6) and Parker (2009, p. 498, fig. 355e–o).

Remarks. Amphistegina lobifera has a distinctly perforate test that ranges from lenticular to subglobular in outline with a smooth periphery and peripheral margin that varies from sub-acute to rounded. The umbo is large, rounded and imperforate giving the test an inflated appearance. The aperture is similar to A. lessonii with an interiomarginal slit-like aperture with a lip and pustulose radiating ornament (Fig. 23:11–12).

As discussed above, the difference between A. lessonii and A. lobifera is the lobulate state of the umbo. As seen with CG collected specimens, Parker (2009) noted a significant variation in lobate development of A. lobifera ranging from highly developed lobulate sutures to only a few lobulate sutures on the umbilical side of the test, features that greatly resemble A. lessonii. Additionally, A. lobifera possesses a much more inflated and broad umbo than the distinctly more lenticular and planoconvex A. lessonii. Work by Hallock (1979; 1984) and Hallock et al. (1986) found A. lobifera to typically occur in shallow, more turbulent waters than A. lessonii and to grow more quickly within moving waters. Like A. lessonii, Hallock & Hansen (1978) found that the secondary laminae within A. lobifera test walls reduce with depth as there is a decreased amount of symbiont activity.

Amphistegina lobifera was originally reported by Larsen (1976) from the Gulf of Aqaba and these specimens were also utilised in investigations by Hallock & Hansen (1978) and Hallock & Larsen (1979). Amphistegina lobifera was later described in great detail by Hottinger et al. (1993) from the same region in the Red Sea. Specimens have been investigated extensively from around the Pacific (Hawaii and Western Caroline Islands— Hallock 1979, 1984, Hallock & Larsen 1979, Hallock et al. 1986, Harney et al. 1999 & Resig 2004; Ryukyu Islands—Hohenegger et al. 1999, Yordanova & Hohenegger 2002; Ningaloo Reef—Parker 2009; New Caledonia to 30 m—Debenay 2012). Hohenegger et al. (1999) found A. lobifera in shallow water environments with high abundance down to ca. 20 m deep with significantly reduced abundance down to 50 m. Yordanova & Hohenegger (2002) found a similar distribution of living specimens as Hohenegger et al. (1999), but dead test distribution around the island was influenced by the bathymetry of the immediate area (Yordanova & Hohenegger 2002). Parker (2009) found A. lobifera as the most abundant Amphistegina species in Ningaloo Reef, but was restricted to water depths <10 m and always occurred in close proximity to reef crest and back reef environments.

Distribution within study area. Amphistegina lobifera is the most abundant Amphistegina species in the CG. Similar to other reported occurrences of A. lobifera (Hohenegger et al. 1999; Yordanova & Hohenegger 2002; Parker 2009), the highest abundance was found in reef flat and lagoon samples compared to deeper water samples (such as the channel sample) greater than 10 m. Amphistegina lobifera was very abundant on Sykes Reef with all three sites having the highest abundance observed in the CG.

Amphistegina radiata (Fichtel & Moll 1798) (Fig. 23: 13, 14)

1798 Nautilus radiata Fichtel & Moll, p. 58, pl. 8, figs a–d.

1884 Amphistegina quoyii ? d’Orbigny; Brady, Fig. 3 a–c.

1976 Amphistegina radiata (Fichtel & Moll); Larsen, p. 7, pl. 5, figs 1–4; pl 6, figs 1, 2; pl. 7, fig. 5; pl. 8, fig. 5. 1984 Amphistegina radiata (Fichtel & Moll); Rögl & Hansen, p. 43, pl. 10, figs 4, 5, tfs 15, 16. 1992a Amphistegina radiata (Fichtel & Moll); Hatta & Ujiié, p. 196, pl. 42, fig. 5. 1994 Amphistegina radiata (Fichtel & Moll); Loeblich & Tappan, p. 157, pl. 341, figs 8–10. 1999 Amphistegina radiata (Fichtel & Moll); Hohenegger et al. p. 143, fig. 20.

2001 Amphistegina radiata (Fichtel & Moll); Lobegeier, p. 301, pl. 17, figs 3–4. 2002 Amphistegina radiata Fichtel & Moll; Yordanova & Hohenegger, p. 193, pl. 31, figs 13–17. 2009 Amphistegina radiata Fichtel & Moll; Parker, p. 499, fig. 355a–d.

2012 Amphistegina radiata Fichtel & Moll; Debenay, p. 216, pl. 20.

Description. See Debenay (2012, p. 216, pl. 20); Hohenegger et al. (1999, p. 145, fig. 20); Larsen (1976, p. 7, pl. 5, figs 1–4; pl 6, figs 1, 2; pl. 7, fig. 5; pl. 8, fig. 5) and Yordanova & Hohenegger (2002, p. 193, pl. 31, figs 13–17).

Remarks. Specimens from the CG were assigned to A. radiata based on their almost flat, lenticular test shape (especially when compared to A. lessonii and A. lobifera) and numerous, distinct, radiating sutures that cover the spiral side of the test (Fig. 23:13, 14). These sutures can be either flush (Fig. 23:13) or slightly raised with papillate ornament (Fig. 23:14) in CG specimens. This species distinctly differs from A. lessonii and A. lobifera through their more compressed test, short, peripheral aperture and greatly reduced field of surrounding papillae (Fig. 23:14), and more numerous chambers with accompanying radiating sutures.

Amphistegina radiata ’s original type locality is unknown as the type material was collected from a gastropod shell from the Arabian Sea that potentially came from a merchant ship (Larsen 1976; Parker 2009). Larsen (1976) was unsuccessful in finding A. radiata from the Gulf of Aqaba but published extensive illustrations and descriptions from Indonesian specimens that were later designated lectotypes and paralectotypes by Rögl & Hansen (1984). Barker (1960) commented that some of Brady’s (1884) specimens of Amphistegina quoyii d’Orbigny 1926, were likely A. radiata and were considered synonyms. Rögl & Hansen (1984), Loeblich & Tappan (1987) and Parker (2009) additionally considered the two species synonymous and the morphological differences intraspecific variation. Brady’s (1884) specimens of A. radiata (under A. quoyii) were retrieved from a depth of 22 m off Fiji. Loeblich & Tappan (1994) collected this species from the Van Diemen Rise, eastern Timor Sea (depth 57.3 m) and north of the Sahul Rise (depth 31.08 m) whilst Hatta & Ujiié (1992a), Hohenneger et al. (1999) and Yordanova & Hohenegger (2002) all collected A. radiata from the Ryukyu Islands, Japan. Lobegeier (2001) collected specimens from Green Island Reef, GBR, Parker (2009) found them only sparingly from Ningaloo Reef, Western Australia and Debenay (2012) from the southwestern lagoon and south of the Grande Terre, New Caledonia between depths of 10–80 m.

Distribution within study area. Amphistegina radiata was found sparingly with no more than 6 specimens per sample through the CG except for the channel site where 29 specimens were collected. Apart from the channel site, this species was most commonly found at Sykes Reef. Amphistegina radiata is the least abundant Amphistegina taxon collected from the CG.