Pecten dijkstrai Duncan & Wilson, 2012
DUNCAN, P. F. & G. WILSON. 2012. A new species of Pecten (Mollusca: Bivalvia: Pectinidae) from northern Western Australia. Molluscan Research, 32 (1): 21-26, figs. 1-3. [p. 22, figs. 1, 2]
2012 Pecten dijkstrai Duncan & Wilson, 2012
P. F. Duncan & G. Wilson, 2012, figures 1, 2.
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«Pecten cf. excavatus Anton, 1839 [sic]; Taylor and Glover 2004: 262.
Pecten n. sp.; Duncan and Wilson 2007: 1306–1307. Description
Shell medium-sized, mean dorso-ventral height 42.8 mm (SD = 3.7, n=5), mean antero-posterior length 48.0 mm (SD = 4.8, n=5), and up to a maximum shell height of 55 mm based on specimen examined by Dijkstra and Beu (pers. comm.); typical of Pecten in form; sub-orbicular, strongly inequivalve, with equal, well-developed auricles, hinge line straight; weak byssal notch in anterior auricle of right valve. No ctenolium present. Right valve strongly overlaps left valve. Left valve strongly concave, with 14–16 prominent radial ribs (costae) of sub-angular profile. Weak secondary radial riblets present on some specimens. Commarginal lamellae well-defined in specimens from Gnaraloo, weaker in Exmouth Gulf specimens. Ground colour off-white, with predominantly pale pink to dark red-pink pigmentation graduated over entire valve. Distinct symmetrical white (un-pigmented) chevrons, opening ventrally, centred on primary costae; each chevron with black border on closed dorsal end. Black chevron patterns connect on many specimens, particularly close to anterior and posterior ends, forming distinctive, broadly concentric zigzag lines over valve surface. Umbonal region off-white or, on most specimens, pale pink, reflecting overall valve colour. Auricles lamellate; colour reflecting general valve colour, with dark pigmented lines in few specimens. Right valve strongly to very strongly convex, with 16-20 rather low radial ribs (costae). Ribs typically bifurcate, particularly prominent on specimens from Gnaraloo. Ground colour white or off-white, maculated extensively with small orange, pink or red blotches or striations, more prominent in Gnaraloo specimens, much paler in Exmouth Gulf specimens. Interior of both valves glossy white, with marginal pink coloration in strongly pigmented specimens. Adductor muscle scars prominent, with moderately callused margins. Juvenile stages up to around 20 mm shell height, weakly sculptured, and typical of genus (AMS C.146784, multiple specimens). Hinge prominent, with narrow, sharply defined dorsal, intermediate and resilial teeth and sockets, visible with difficulty inside right valve because of its strongly enrolled, convex shape. The dimensions of 5 specimens from off Gnaraloo and Exmouth Gulf are shown in Table 1. Type material
Holotype (pv) (DV 39.40 mm, AP 43.80 mm; hinge 25.10 mm, lateral width 16.40 mm) AMS C.462684, 30 paratypes (pv and v) (listed below). Type locality
Off Gnaraloo Station, Western Australia, 23º 52’ S, 113º 29’ E, 40–42 m depth in areas of coral sand around low-profile, sponge-encrusted limestone reef. Material examined
Western Australia, Off Gnaraloo Station, 130 km N of Carnarvon, 23º 52’S, 113º 29’E, dead, 40–42m, on areas of coral sand around low-profile, sponge-encrusted limestone reef (Holotype and 2 paratypes (pv), AMS C.462684, GW, PD); Exmouth, north of North-West Cape, approx. 21º 32’S, 114º 24’E, dead, 35–40m, on coral sand adjacent to low profile limestone reef (2 paratypes (pv), GW, PD); North West Shelf, 52 n.ml NNE of Port Hedland, 19°30.9’S–19°28.2’S, 118°49.2’E–118°55.4’E, dead, 36–37 m, sand, dead (2 paratypes, AMS C.149192; paratype, AMS C.149173); North West Shelf, 77 n.ml NNE of Port Hedland, 19°05’S–19°04.9’S, 118°58’E–118°58.2’E, dead, 82 m, sand & gravel (paratype, AMS C.149343); North West Shelf, 78 n.ml NNE of Port Hedland, 19°04.4’S–19°04.2’S, 119°04.4’E–119°00.7’E, dead, 82 m, sand (2 paratypes, AMS C.149327); North West Shelf, 80 n.ml NNE of Port Hedland, 19°03.6’S–19°03.4’S, 119°03.4’E–119°03.5’E, dead, 82 m, sand (20 paratypes [juv-adult], AMS C.146784). Additional material
(not examined by authors. Dijkstra and Beu; pers. comm. September 2011). Western Australia, West of Point Quobba, 50 km N of Carnarvon, approx. 24°27’S, 113°40’E, alive, 35–40 m (single specimen, HM); Exmouth, north of N Muiron Island, 21°39’S, 114°22’E, alive, 30 m, sand & gravel (single specimen, HM); NNE of Dampier Archipelago, 19°23’S, 117°21’E–117°23’E, alive, 101–102 m (single specimen, WAM S 30715); North West Shelf, 19°05’S, 118°50’E, alive, c. 80–100 m, muddy sand (single specimen, ZMA Moll. 409005); off Broome, 17°58’S, 122°14’E, alive, 12–18 m, coarse rubble (2 specimens, HM; single specimen, ZMA Moll. 409006). Distribution
We know of specimens of Pecten dijkstrai n. sp. from off Gnaraloo Station, 130 km N of Carnarvon, 23º 52’S, 113º 29’E to as far north as North West Shelf, 80 n.ml NNE of Port Hedland, 19°03.6’S–19°03.4’S, 119°03.4’E– 119°03.5’E. Dijkstra and Beu (pers. comm. September 2011) believe that the species also occurs slightly further south, (off Point Quobba, 50 km north of Carnarvon, WA (24º27’S, 113°40’E)) and slightly further north (Broome, WA (17º58’S, 122°14’E), and it seems likely that it occurs even more widely. Dijkstra (1991) reported Pecten excavatus from north-east of Sumba Island (9º52.8’S, 120º44.7’E) in the Savu Sea, Indonesia. His description of these specimens, reported as being similar to the original figured type specimens of P. excavatus, although less concave and with weaker radiation (radial costae or ribs), were considered to be within normal variation for this species. It was also a new record from Indonesia. However, we can find no other published records of Pecten spp. from Indonesia, Malaysia or the Philippines (Raines, in Poppe (2010) does not record excavatus from the Philippines). Pecten excavatus is recorded from Hong Kong (Bernard et al. 1993), but apparently not definitively between there and Indonesia, raising the unlikely possibility of a disjunct distribution for P. excavatus of more than 3500 km. Alternatively, given their description, particularly the weaker radial costae, it is possible that the Indonesian shells are Pecten dijkstrai n.sp. Importantly, statistical comparisons of multiple shell parameters between Japanese specimens (n=6) of P. excavatus and specimens of P. dijkstrai n. sp. (n=5) indicated several characteristics that can be used to differentiate the shells. There are significantly higher radial rib (costae) counts in P. dijkstrai n. sp. compared with P. excavatus (Mann-Whitney U test, P<0.01), (counts of 15.6±0.9 compared with 14.0±0 for external left valves, and 17.75±1.7 compared with 17.0±0 for external right valves respectively) (see also Table 1). In addition, the shells of these species can also be differentiated by the ratio of dorso-vental to antero-posterior dimensions, a measure of roundness, with P. excavatus being more round or, alternatively, P. dijkstrai n. sp. being longer (0.93±0.02 compared with 0.89±0.01 respectively) (Mann-Whitney U test, 2-tailed, corrected for ties, P<0.01)(see also Table 1). These data not only support their respective specific status, but also the possibility that P. excavatus does not occur in the southern hemisphere, there now being a probable alternative species to account for the Indonesian specimens. Further records of relevance to the potential distribution of Pecten dijkstrai n. sp. come from the Global Biodiversity Information Index (www.gbif.org) which records data from the Australian Government’s Bureau of Rural Sciences (National Commercial Fisheries Half-Degree Data Set 2000-2002). This database contains multiple unusual records of Pecten fumatus; two from the Torres Strait (9° 45’S, 14° 315’E; 10° 15’S, 143° 15’E), three from the Gulf of Carpentaria (15° 15’S, 137° 15’E; 13° 45’S, 136° 45’E; 13° 15’S, 136° 45’E) and one from north of Melville Island, near Darwin (11° 15’S, 131° 15’E). These identifications seem unlikely to be P. fumatus, with its well-established temperate distribution, but the recognition of it being a Pecten sp., given their characteristic appearance, appears more credible. We therefore speculate that these records may also refer to Pecten dijkstrai n. sp., and postulate that its distribution may extend as far east as the Coral Sea. Therefore, a possible revised distribution for the three Pecten species; excavatus, fumatus and dijkstrai n. sp. is proposed in Figure 3. Two species from the New Zealand region are also included for completeness.
If these proposed distributions are subsequently found to be correct, then it presents an interesting development, with potential implications for the phylogeography and evolution of Pecten within the Indo-Pacific region. It is perhaps noteworthy that another Australian scallop, Amusium balloti (Bernardi, 1861), shares a broadly similar Australasian and respectively Asian distributional relationship with its con-generic relative Amusium japonicum (Gmelin, 1791). Remarks
Compared to specimens from Gnaraloo, Exmouth Gulf specimens have a more strongly concave left valve and a more strongly convex right valve, resulting in a more significant overlap of the right valve margin (Fig. 2). Colouration and patterns are also generally less intense in the Exmouth than in the Gnaraloo specimens. Etymology
We have pleasure in naming this species in honour of Henk Dijkstra, in recognition of his significant contributions to pectinid taxonomy and allowing access to the information in his unpublished monograph of Australian Pectinidae (with Dr A. Beu) which included a provisional description of the species named herein. Discussion
Comparison between Pecten dijkstrai n. sp. and the only other recognised Australian Pecten species, the temperate P. fumatus, indicated more prominent and more angular radial costae, as well as a larger, more circular and thicker shell in P. fumatus. However, the right valve is more convex, and the left valve more concave in P. dijkstrai n. sp., and the shell colour and patterns are much more vivid than the subdued colours found in P. fumatus. The reported northern extent of the range of P. fumatus in Western Australia is Shark Bay (Raines and Poppe, 2006) (Fig. 3).
Similarly, comparisons between P. dijkstrai n. sp. and the two other Pecten spp. within the Australasian region, P. novaezelandiae (Reeve, 1852) and P. raoulensis (Powell, 1958), broadly indicate the same consistent differences as with P. fumatus. Specifically, prominent, narrower and more angular radial costae, larger and more robust shell of subdued brown-red colouration in the New Zealand species, and more convex right valve and concave left valve in P. dijkstrai n. sp. The reported distributions for P. novaezelandiae and P. raoulensis do not extend outside the adjacent waters of New Zealand and the Kermadec Islands respectively (Powell 1958; Raines and Poppe 2006; Dijkstra and Marshall 2008) (Fig. 3).
The most similar species to Pecten dijkstrai n. sp. appears to be the Asian species Pecten excavatus Anton, 1838 (= P. sinensis G. B. Sowerby II, 1842, = P. puncticulatus Dunker, 1877). The type locality of P. excavatus is “China”, and its distribution has typically been described as ranging from Japan and Korea, as far south as Hong Kong (Bernard et al. 1993). Other than Dijkstra’s (1991) record from nearby Indonesia, the only published reference we can find to its occurrence as far south as Australia is by Raines and Poppe (2006), who showed a distribution map for P. excavatus from southern Japan to Java, western Papua and north-western Australia. However, as indicated above, several shell characteristics enable differentiation of the two species, and therefore P. excavatus may not occur south of the equator, being replaced by P. dijkstrai n. sp. in Australasian and southern Indonesian waters. Specimens of Pecten dijkstrai n. sp. that we have seen are smaller than typical Japanese specimens of P. excavatus, and they are also longer and less robust than the Japanese shells. The radial ribs (costae) are weaker (lower) and less angular than in P. excavatus and the left valve of P. dijkstrai n. sp. may be more deeply concave in some specimens, although this feature appears to vary with locality, and is not diagnostic (see Fig. 2). The colour of P. dijkstrai n. sp. is also paler than in P. excavatus. Other characters appear similar in the two species. To date no tissue samples of P. dijkstrai n. sp. have been obtained and such samples would provide useful data on phylogenetic relationships with other species of the genus. The relatively isolated locality and habitat depth of this scallop limits bottom search time by scuba divers, and therefore specimen availability at this time. However, the possibility of tissue sample collection of Pecten specimens across the potentially wider range proposed above may clarify this issue in future. Differences in shell form between the Gnaraloo and Exmouth specimens of P. dijkstrai n. sp. may be related to local differences in substrate type or hydrology, as the relationship between shell form and such variables has been well described in the literature (Stanley 1970; Seed 1980; Dijkstra and Marshall 2008). However, there also seems to be a cline in shell shape and colouration from north to south, possibly influenced by temperature. Two of the original specimens of P. dijkstrai n. sp. exhibit drill holes, likely as a result of predation by octopods.» PETER F. DUNCAN & GARY WILSON, 2012
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«Description. Shell subcircular, thin, up to c. 55 mm in length, broader than high, subequilateral, strongly inequivalve. Left valve richly pigmented with brown streaks and creamy-white dots on cream or pinkish background; right valve paler to almost whitish.
Left valve strongly concave, with 12–13 low, almost flat-topped radial plicae, with 1-3 rudimentary plicae near anterior and posterior margins; interstices each broader than one plica. Delicate, closely spaced commarginal lamellae on entire disc, almost smooth on central part, more prominent near margins. Anterior auricle slightly larger than posterior one, anterior edge rather concave; both auricles covered with fine, close-set, unevenly developed, prominent commarginal lamellae; with or without 1-3 weak radial ridges near dorsal margin. Dorsal margin straight. Interior surface with paired angular internal rib carinae; adductor muscle scar well-defined; resilifer triangular; prominent resilial, intermediate and dorsal teeth developed on both sides of resilium. Right valve strongly inflated, with 12–14 broad, low radial plicae with rounded crests, some specimens with a narrow radial groove on plical crests near ventral margin, plus 1-3 rudimentary plicae near anterior and posterior margins; interstices each much narrower than one plica. Delicate, close-set commarginal lamellae present near ventral margin; central part of disc smooth. Auricles of some specimens with c. 5 weak radial riblets; most specimens with fine, closely spaced commarginal lamellae (less well-developed than on left valve). Byssal notch shallow, functional ctenolium absent in adult stage. Dorsal margin straight; teeth and sockets prominent, corresponding to those in left valve. Dimensions. Illustrated specimens: WA, Exmouth, Murion I., 8 km NE of North West Cape, 30–38 m (HM): rv, H 49.0, L 55.1, D (single valve) 23.3 mm; lv, H 41.5, L 54.1, D (single valve) 10.4 mm; WA, Exmouth, 7–8 km N of tip of North West Cape, 32–35 m (HM): rv, H 50.6, L 52.8, D (single valve) 24.6 mm; lv, H 41.6, L 51.4, D (single valve) 11.3 mm.
Habitat. Living in the sublittoral zone to the mid-continental shelf on soft sediment of sandy patches within flat reef areas.
Distribution. Continental shelf of Western Australia, from Carnarvon to northwest of Port Hedland. Possibly also southern Indonesia. Present specimens alive at 18–101 m (minimum depth range).
Remarks. Pecten dijkstrai is most nearly similar to Pecten excavatus Anton, 1838, known from southern Japan to Indonesia. Pecten dijkstrai is less solid and somewhat wider than P. excavatus. The left valve of P. dijkstrai is also more deeply concave and the radial plicae are weaker and less angular than in P. excavatus. Pecten dijkstrai is also paler in colour than P. excavatus. Other characters are identical.
Pecten dijkstrai differs strongly from P. fumatus in having a much more deeply concave left valve; it is almost flat in P. fumatus. The right valve of P. dijkstrai is also more strongly convex than that of P. fumatus. Pecten dijkstrai is also thinner (less solid) and smaller in size (up to c. 55 mm in length; P. fumatus up to c. 170 mm in length) and less nearly circular than P. fumatus. Pecten fumatus also has more strongly developed radial plicae than P. dijkstrai. For comparison with Pecten afribenedictus Kilburn & Dijkstra, 1995 from South Africa and Pecten erythraeensis G. B. Sowerby II, 1842 from the Red Sea see Kilburn & Dijkstra (1995).» DIJKSTRA, H. H. & A. G. BEU. 2018. Living scallops of Australia and adjacent waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae). Records of the Australian Museum, 70 (2): 113-330, figs. 1-102. [p. 225, 226, 227]
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Pecten dijkstrai Duncan & Wilson; H. H. Dijkstra & A. G. Beu, 2018, Living scallops of australia and adjacent waters, figures 57C, 57E, 57H, 58E-58I.
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