Chitons (Mollusca, Polyplacophora) from Rapa,
the southernmost island of Polynesia
Enrico SCHWABE
Zoologische Staatssammlung München,
Münchhausenstrasse 21, D-81247 Munich (Germany)
enrico.schwabe@zsm.mwn.de
Pierre LOZOUET
Muséum national d’Histoire naturelle, Département Systématique et Évolution,
case postale 51, 55 rue Buffon, F-75231 Paris cedex 05 (France)
lozouet@mnhn.fr
Schwabe E. & Lozouet P. 2006. — Chitons (Mollusca, Polyplacophora) from Rapa, the
southernmost island of Polynesia. Zoosystema 28 (3) : 617-633.
KEY WORDS
Mollusca,
Polyplacophora,
French Polynesia,
Rapa,
brooding species,
new species.
ABSTRACT
he RAPA 2002 workshop permitted the first comprehensive collection of
polyplacophorans from the island of Rapa (Austral Islands, South Pacific) to be
made. he collection includes three genera and three species, two of the species
being new to science. he isolation of Rapa may explain the uniqueness of this
small polyplacophoran fauna. he two new species are: Leptochiton hiriensis n. sp.
and Chiton (Rhyssoplax) rapaitiensis n. sp. he latter species, which is the most
common (83% of the specimens), is a brooder. Leptochiton hiriensis n. sp. differs
from congeneric species of the same size in the sculpture of the valves, which
show concentric growth marks in the central area and concentrically arranged
nodules in the head valve, postmucronal region of the tail valve and the lateral
areas. Chiton (Rhyssoplax) rapaitiensis n. sp. is compared to the related species
C. (R.) linsleyi (Burghardt, 1973), C. (R.) rhynchotus (de Rochebrune, 1884), and
C. (R.) maldivensis (E. A. Smith in Gardiner, 1903). In addition to its smaller
size, the new species differs in tegmental sculpture and in the ornamentation
of the dorsal girdle scale. Onithochiton lyellii (Sowerby, 1832), a species also
known from the Gambier Islands and Pitcairn Islands, is restricted herein to the
Polynesian region. In addition to the descriptions of the new species, characters
relevant in identification of O. lyellii are described and illustrated for the first
time in scanning electron micrographs.
RÉSUMÉ
Chitons (Mollusca, Polyplacophora) de Rapa, l’île la plus méridionale de Polynésie.
L’atelier RAPA 2002 a permis de récolter la première collection complète de
Polyplacophora de l’île de Rapa (archipel des Australes). Trois genres et trois
ZOOSYSTEMA • 2006 • 28 (3) © Publications Scientifiques du Muséum national d’Histoire naturelle, Paris.
www.zoosystema.com
617
Schwabe E. & Lozouet P.
MOTS CLÉS
Mollusca,
Polyplacophora,
Polynésie française,
Rapa,
espèce incubatrice,
espèces nouvelles.
espèces, dont deux nouvelles pour la science, sont représentés. L’isolement de
l’île de Rapa peut expliquer l’originalité de cette petite faune de Polyplacophora.
Les deux nouvelles espèces sont : Leptochiton hiriensis n. sp. et Chiton (Rhyssoplax)
rapaitiensis n. sp. Chiton (Rhyssoplax) rapaitiensis n. sp., l’espèce la plus commune
(83 % des spécimens), est incubatrice. Leptochiton hiriensis n. sp. diffère de ses
congénères de la même taille par la sculpture des valves, qui comporte des stries
de croissances concentriques dans la zone centrale et des nodules disposés de façon
concentrique dans la valve de tête, la région postmucronale de la valve de queue,
et les zones latérales. Chiton (Rhyssoplax) rapaitiensis n. sp. est comparé aux espèces
voisines C. (R.) linsleyi (Burghardt, 1973), C. (R.) rhynchotus (de Rochebrune,
1884), et C. (R.) maldivensis (E. A. Smith in Gardiner, 1903). En plus de sa petite
taille, la nouvelle espèce diffère par la sculpture tegmentaire et par l’ornementation
de l’écaille de la ceinture dorsale. Onithochiton lyellii (Sowerby, 1832) est connue
des îles Gambier et Pitcairn. Les radulas de Chiton rapaitiensis n. sp. et O. lyellii
sont décrites et figurées au microscope électronique à balayage.
INTRODUCTION
he species presented herein were collected during
a workshop on Rapa (27°37’S, 144°20’W), the
most isolated island of French Polynesia. his small
island (40 km²; Fig. 1) is the southernmost of the
Austral Islands, an archipelago situated south of
the Society Islands. he island, of volcanic origin,
is estimated to be 5-3 Myr old (Brousse & Gelugne
1987). he RAPA 2002 workshop formed part of
a concerted research program “Fauna and Flora of
Rapa” involving the Université de Polynésie française,
École pratique des hautes Études (EPHE), Institut
de Recherche pour le Développement (IRD) and
MNHN. It took place between October 27 and
December 10, 2002 (Lozouet et al. 2004, 2005).
his is the first study of chitons from Rapa. Previously, just a single record of Onithochiton lyelli
(Sowerby, 1832) had been provided by Salvat &
Rives (1975: 187). hus, although a relatively small
number of chiton species was collected, the material
is of utmost interest.
MATERIAL AND METHODS
During RAPA 2002 a total of 92 stations were
sampled between high tide mark and 52 m depth.
Of these, 21 stations yielded specimens of Poly618
placophora. he material was collected by intertidal
collecting, scuba diving and dredging. During the
dives, different techniques were deployed, including
hand picking, collecting bottom samples by suction
sampling, and brushing stones and dead corals (see
Bouchet et al. 2002).
he chitons were fixed and preserved in 75%
ethanol. Preparation of SEM material followed the
procedure described by Schwabe & Ruthensteiner
(2001). Eggs were prepared following the method
described by Laforsch & Tollrian (2000).
Most specimens are in MNHN, two specimens
of each species are in ZSM.
ABBREVIATIONS
MNHN Muséum national d’Histoire naturelle, Paris;
NHM Natural History Museum, London (formerly
British Museum [Natural History]);
ZSM
Zoologische Staatssammlung München,
Munich;
pd
partly disarticulated (specimens used for scanning electronic microscopy);
spm(s) specimen(s);
stn(s) station(s).
SYSTEMATICS
Class POLYPLACOPHORA Gray, 1821
Order NEOLORICATA Bergenhayn, 1955
Family LEPTOCHITONIDAE Dall, 1889
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
FIG. 1. — Map of Rapa Island showing the stations cited in the text. Symbols:
Genus Leptochiton Gray, 1847
TYPE SPECIES. — Chiton cinereus sensu Montagu, 1803
(non Linnaeus, 1767) = Leptochiton asellus (Gmelin,
1791), by subsequent designation (Gray 1847).
DISTRIBUTION. — Members of the genus distributed
worldwide, Eocene-Recent.
Leptochiton hiriensis n. sp.
(Figs 2-4; 10A)
TYPE MATERIAL. — Holotype: type locality (MNHN).
ZOOSYSTEMA • 2006 • 28 (3)
, intertidal; ★, scuba diving and dredging.
Paratypes: type locality, 7 paratypes (MNHN). — Stns
16 and 30, 2 paratypes (ZSM).
TYPE LOCALITY. — French Polynesia, archipel des Australes, Rapa, Baie de Hiri, RAPA 2002, stn 9, 27°37.3’S,
144°22.2’W, 3-24 m, coral bottom.
ETYMOLOGY. — From the type locality.
MATERIAL EXAMINED. — A total of 25 specimens (3 lots).
French Polynesia. Rapa Island, Baie de Hiri, 27°37.3’S,
144°22.2’W, RAPA 2002, stn 9, 3-24 m, coral bottom, 8
spms. — W of Tauna islet, 27°36.3’S, 144°18.4’W, RAPA
2002, stn 16, 5 m, dead corals, 13 spms. — Pointe Mei,
27°38.2’S, 144°18.2’W, RAPA 2002, stn 30, 16-20 m,
dead corals, 4 spms.
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Schwabe E. & Lozouet P.
FIG. 2. — Leptochiton hiriensis n. sp., Rapa Island, RAPA 2002: A-C, holotype, stn 9, 5.5 mm; A, complete specimen in dorsal view,
anterior region on the right side; B, detail of valve ii, left side; C, sutural region of valves vi-vii, left side; D-F, paratype, stn 16, 1.3 mm;
D, head and second valve in situ, dorsal view; E, valve vii, dorsal view; F, tail valve, dorsal view. Scale bars: A, 400 µm; B, C, 10 µm;
D-F, 100 µm.
DIAGNOSIS. — Elongate-oval animal, minute. Tegmentum
strongly sculptured with comarginal ridges. Dorsal girdle
with strongly ribbed scales and long needles. Ventral
girdle with smooth, elongate-rectangular scales. Radula
620
with c. 80 rows, about 365 µm in length. Central tooth
broad rectangular with a simple inwardly directed blade,
second lateral with an elongate-rectangular, tridentate
head. Ctenidia unknown.
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
FIG. 3. — Leptochiton hiriensis n. sp., paratype, Rapa Island, RAPA 2002, stn 16, 1.3 mm: A, anterior part of the radula; B, tail valve,
left lateral view. Scale bars: A, 10 µm; B, 30 µm.
DESCRIPTION
Minute, to 1.4 mm in length, with an elongateoval outline. Dorsum evenly convex. Head valve
comparatively large, posterior margin widely
V-shaped with a shallow notch in the centre.
Intermediate valves rectangular, without apex,
posterior margin concave. Tail valve semicircular
with a central, slightly elevated, posteriorly directed
mucro. Postmucronal slope fl at and straight.
Lateral areas of intermediate valves moderately
elevated.
Tegmentum sculptured with relatively prominent, comarginal ridges (12 in the head valve of
the holotype), which are stronger towards the outer
margin and lateral areas. Central part of the valves
more or less distinctly sculptured and appears partly
smooth, except for concentric granulated growth
marks. Ridges bearing inconspicuous granules.
Each granule with a central macropore of 3.5 µm
in diameter. Micropores seem to be present among
the granules, on their bases.
Articulamentum builds thin apophyses, triangular in intermediate valves, rectangular in tail valve.
hey are widely separated and not connected by a
sutural lamina.
Perinotum (Fig. 2) very narrow. Dorsally covered
with strongly ribbed (with up to 15 ribs), imbricated,
rectangular scales, which are 34.5 × 20 µm in size
(in situ). here are also long, smooth needles (78.3 ×
8.4 µm), which are irregularly arranged among
ZOOSYSTEMA • 2006 • 28 (3)
the scales. Suturally, there are longitudinal-ribbed
(probably up to 12 ribs) needles, which reach c. 50 ×
8.3 µm. Ventral girdle scales (Fig. 4E) smooth,
rectangular to oval (24-30 × 11.3-12.7 µm) with
a small nipple-like tip. Marginal girdle elements
consist of: finely ribbed (up to 12 ribs), rectangular
scales (57.3 × 18.7 µm) with a sharply pointed distal
end; and long, slender, smooth needles, attaining
48.5 × 3.8 µm.
Radula of a 1.3 mm specimen from stn 16 about
365 µm in length, consists of c. 80 rows. Central
tooth rectangular, 5.6 × 5.2 µm, with a simple inwardly directed blade. Second lateral tooth about
12.4 µm in length, with an elongate-rectangular
tridentate head, measuring 7.2 × 4 µm. Central
denticle sharp-pointed and longest. Radula cartilage
104 µm in length.
Due to the condition and the small size of the
specimens the number of ctenidia could not be
determined.
REMARKS
he species was found at 3-24 m depth on corals,
but probably reaches its highest density at about 5 m.
he high number of growth marks, the constant size
and the relatively “thick” valves lead us to assume
that this species is full-grown. he minute size,
in combination with the strong sculpture make
it easy to distinguish it from other Indo-Pacific
shallow-water species so far known.
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Schwabe E. & Lozouet P.
FIG. 4. — Leptochiton hiriensis n. sp., Rapa Island, RAPA 2002: A, holotype, stn 9, 5.5 mm, dorsal girdle scales, next to the suture;
B-F, specimen, stn 30, 1.2 mm; B, ribbed marginal scales; C, dorsal girdle needle; D, F, marginal girdle needle, among ventral girdle
scales; E, ventral girdle scales. Scale bars: A, C-F, 10 µm; B, 20 µm.
Family CHITONIDAE Rafinesque, 1815
Genus Chiton Linnaeus, 1758
TYPE
622
SPECIES.
— Chiton tuberculatus Linnaeus, 1758,
by subsequent designation (Dall 1879).
DISTRIBUTION. — Members of the genus are distributed worldwide, mainly in tropical waters, CretaceousRecent.
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
FIG. 5. — Chiton (Rhyssoplax) rapaitiensis n. sp., holotype, Rapa Island, RAPA 2002, stn 10, 4.7 mm: A, head valve, dorsal view; B,
valve ii, dorsal view; C, tail valve, dorsal view; D, tail valve, lateral view; E, dorsal girdle scales and marginal elements; F, ventral girdle
scales. Scale bars: A-D, 400 µm; E, F, 80 µm.
Subgenus Rhyssoplax hiele, 1893
TYPE SPECIES. — Chiton affinis Issel, 1869, by subsequent
designation (hiele 1909). Members of the subgenus are
distributed in the Indo-Pacific (except of the East Pacific)
and East Atlantic, Oligocene-Recent.
ZOOSYSTEMA • 2006 • 28 (3)
Chiton (Rhyssoplax) rapaitiensis n. sp.
(Figs 5-7; 10C, D)
TYPE MATERIAL. — Holotype: type locality, pd (MNHN).
Paratypes: type locality and stns 25 and 30, 47 paratypes
(MNHN). — Stns 25 and 30, 2 paratypes (ZSM).
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Schwabe E. & Lozouet P.
HABITAT. — he species was found on stones from the
intertidal zone down to 30 m depth, with a preference
for the upper zone (2-5 m).
FIG. 6. — Chiton (Rhyssoplax) rapaitiensis n. sp., holotype, Rapa Island,
RAPA 2002, stn 10, radula, anterior part. Scale bar: 500 µm.
TYPE LOCALITY. — French Polynesia, archipel des
Australes, Rapa, Pointe Komiré, RAPA 2002, stn 10,
27°34.8’S, 144°22.8’W, 16-18 m.
ETYMOLOGY. — From Rapa Iti (little Rapa), one of the
Polynesian names of Rapa, as opposed to Rapa Nui (big
Rapa) applied to Easter Island.
MATERIAL EXAMINED. — A total of 228 specimens (16
lots).
French Polynesia. Rapa Island, Baie de Hiri, 27°37.3’S,
144°22.2’W, RAPA 2002, stn 9, 3-24 m, corals, 44
spms. — Pointe Komiré, 27°34.8’S, 144°22.8’W, RAPA
2002, stn 10, 16-18 m, boulders covered by brown algae,
1 spm (MNHN) (Figs 5; 6). — W of Pointe Aukura,
27°38.1’S, 144°21.1’W, RAPA 2002, stn 15, 20 m, boulders
on sand bottom, 1 spm. — W of Tauna islet, 27°36.3’S,
144°18.4’W, RAPA 2002, stn 16, 5 m, dead corals, 67
spms. — E of Baie Tupuaki, 27°34.2’S, 144°20.6’W,
RAPA 2002, stn 21, 5 m, sand bottom associated with
dead corals, 11 spms. — Baie Anatakuri Nako (N of Rapa
Iti islet), 27°38.4’S, 144°18.9’W, RAPA 2002, stn 25,
3 m, 42 spms. — SW of Pointe Gotenaonao, 27°38.7’S,
144°19.2’W, RAPA 2002, stn 27, 6 m, boulders covered by
algae, 1 spm. — Pointe Taekateke, 27°38.4’S, 144°20.6’W,
RAPA 2002, stn 28, 30 m, boulders covered by algae,
3 spms. — N of Aturapa islet, 27°34.3’S, 144°21.0’W,
RAPA 2002, stn 29, 2-4 m, dead corals, 30 spms. — Pointe
Mei, 27°38.2’S, 144°18.2’W, RAPA 2002, stn 30, 16-20 m,
dead corals, 7 spms. — Entrance of submarine cave, SE
of Pointe Tematatu, 27°34.8’S, 144°19.0’W, RAPA 2002,
stn 35, 2 m, stones, 2 spms. — West of Area, 27°36.8’S,
144°19.9’W, RAPA 2002, stn 52, 23-25 m, mud bottom
associated with corals, 2 spms. — Far end of Baie de Haurei,
27°36.6’S, 144°20.5’W, RAPA 2002, stn 62, 20 m, mud
bottom associated with corals, 2 spms. — BaieTupuaki,
Pointe Kotuaie, 27°34.6’S, 144°20.6’W, RAPA 2002, stn
93, intertidal zone, 2 spms. — Pointe Komire, 27°34.8’S,
144°22.8’W, RAPA 2002, stn 98, 16-18 m, 3 spms.
624
DIAGNOSIS. — Animal elongate oval, moderately highly
elevated, with a subcarinated back. Coloration highly
variable. Tegmentum surface smooth, appearing pitted to
the naked eye. Pleural and antemucronal areas with very
faint longitudinal grooves, which occur from diagonal
ridge towards the anterior valve margin. Growth marks
present, distinctly elevated. Mucro situated centrally.
Dorsal girdle scales imbricated, sculptured with 11-14
rather coarse longitudinal ribs. Ventrally with rectangular,
smooth scales. Radula with a bidentate head of the major
lateral tooth.
Ctenidia arranged holobranchially and adanally.
DESCRIPTION
Holotype (pd) 4.7 × 3.2 mm, elongate oval. Head
valve semicircular, posterior margin widely V-shaped,
unnotched in centre. Intermediate valves broadly rectangular, posterior margin straight, on either side of the
slightly protruding apex. Lateral areas clearly elevated.
Jugal area not defined. Tail valve semicircular, anterior
margin straight, mucro central, slightly elevated.
Postmucronal area straight, moderately steep.
Tegmentum smooth, with microscopic perforations.
Comarginal growth marks on terminal valves and
lateral areas. Pleural areas with four or five fine, longitudinal ribs, reaching from diagonal ridges to anterior
valve margin. Antemucronal area with only two of
these ribs. Lateral areas, head valve and postmucronal
area sculpturless, except for growth marks.
Articulamentum rather thin, with short triangular
apophyses on valve ii and large trapezoid on tail valve.
Insertion plates short, with deep slits. Slit formula:
9 (2 undeveloped close to posterior margin)/1-1/11.
Slit rays present in all valves. Insertion teeth broad,
with pectinate sharpened outer edges.
Perinotum dorsally covered with imbricate, elongate, convex scales, each measuring 84.2-100 ×
42-52.6 µm (Fig. 5 E). hey are sculptured with
10-12 convex, rather flat, radial ribs, the distal end
of which is less distinctly ribbed then the proximal
end. Ventral girdle scales elongate-rectangular, 30.439 × 13 µm in size (Fig. 5 F). Girdle margin with
short, straight needles, which measure 47.4-52.6 ×
15.8 µm, and are distorted distally.
Radula of holotype (Fig. 6) c. 1.5 mm long, with
42 teeth rows, of which 33 are mineralized. he
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
FIG. 7. — Chiton (Rhyssoplax) rapaitiensis n. sp., paratype, Rapa Island, RAPA 2002, stn 25, 2.7 mm: A, cluster of eggs, HMDS dried,
from the right mantle cavity side; B, dorsal surface of an egg (marked by an arrow in A). Scale bars: A, 100 µm; B, 10 µm.
central tooth is relatively short (214.3 µm), tulipshaped, with a single, slightly inward-directed blade.
he first lateral tooth has a slender upper half and
a roundish wide extension on the basal half. he
second lateral has a long (643 µm), slender, and
slightly keeled shaft. he head of the second lateral
is nearly square and has two strong denticles, of
which the inner is the longer. Major uncinal tooth
spoon-like and 476 µm long.
Ctenidia of holotype arranged holobranchially and
adanally, 18 on the left and 17 on the right side.
broad, close-packed ribs, and has insertion plates
that appear smooth. he Indian Ocean species
C. (R.) maldivensis (E. A. Smith in Gardiner, 1903)
is also similar, but is 3-4 times larger, the pleural
areas are sculptured with up to 12 rather wide and
deep grooves, which do not reach the anterior valve
margin, and has dorsal girdle scales with up to 20
mostly very faint grooves.
REMARKS
In one specimen from stn 25, eggs were found in
the mantle cavity (Fig. 10D). According to Strack
(1987: 184, 185), the only other Pacific species of
this subgenus that broods its eggs is C. (R.) linsleyi
(Burghardt, 1973) (as C. (R.) perviridis Carpenter,
1865 – which Bullock 1988 [pp. 688-689] has
shown is a distinct species). he larger size (up to
16 mm) and the distinctive tegmentum sculpture,
namely the large number (up to 15) of longitudinal
grooves in the central areas, the grooved lateral areas,
head valve and tail valve, and smooth girdle scales
differentiate C. (R.) linsleyi from Chiton (Rhyssoplax)
rapaitiensis n. sp.
he new species is close to C. (R.) rhynchotus (de
Rochebrune, 1884) from New Caledonia. he latter is up to 16 mm long, has up to 12 ribs in the
pleural areas, has dorsal girdle scales with up to 21
TYPE SPECIES. — Chiton undulatus Quoy & Gaimard,
1835, by subsequent designation (Gray 1847) (non Chiton
undulatus G. Fischer, 1807) = Onithochiton neglectus de Rochebrune, 1881, replacement name after Iredale (1914).
ZOOSYSTEMA • 2006 • 28 (3)
Genus Onithochiton Gray, 1847
DISTRIBUTION. — Members of the genus are distributed in the Indo-Pacific (except of the East Pacific),
Pleistocene-Recent.
Onithochiton lyellii
(Sowerby in Broderip & Sowerby, 1832)
(Figs 8; 9; 10B)
Chiton lyellii Sowerby in Broderip & Sowerby, 1832:
26. — Sowerby 1833: fig. 7, p. 5 species n° 60. — Reeve
1847: pl. 3, fig. and species 12.
Onithochiton lyelli [sic] – Nierstrasz 1905a: 106. — Ashby
1922: 582. — Dautzenberg & Bouge 1933: 419. — Bergenhayn 1933: 34. — Leloup 1974: 7.
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Schwabe E. & Lozouet P.
Onithochiton (Chiton) lyelli – Ashby 1924: 333 (partly).
Anisochiton (Enoplochiton [section Ornithochiton (sic)])
lyelli – Fischer 1885: 882.
Onithochiton cf. lyellii – Salvat & Rives 1975: 187,
text fig.
Onithochiton lyellii – Adams & Angas 1858: 476. — Tryon
1883: 345, pl. 86, fig. 82. — Pilsbry 1893: 247, 248
(partly), pl. 55, figs 1-7. — Cox 1894: 710. — Iredale
1911: 105. — Leloup 1981: 4 (partly) . — Kaas & Van
Belle 1980: 78; 1998: 115. — Strack 1993: 22, pl. 5, fig. 3
(lectotype designated and figured). — Slieker 2000: 96,
pl. 36, fig. 25 (lectotype figured). — Gowlett-Holmes
2001: 41. — Schwabe 2004: 3160.
Chiton (Onithochiton) lyelli (sic) – Chenu 1859: 382, fig.
2874 (7-valved). — Clessin 1903: 48, pl. 17, fig. 7.
Chiton (Onitochiton [sic]) lyelli (sic) – Couturier 1907:
175.
?Onithochiton cf. societatis – Preece 1995: 351.
TYPE MATERIAL. — Lectotype (designated by Strack 1993:
22), 3 paralectotypes, dried (NHM 1985064).
TYPE LOCALITY. — Polynesia, Pitcairn Island; in small
round hollows formed by sea urchins in exposed situations
at low water mark.
MATERIAL EXAMINED. — A total of 21 specimens
(5 lots).
French Polynesia. Rapa Island, Pointe Autea (station not
numbered), 27°35.2’S, 144°18.4’W, RAPA 2002, intertidal
zone, on rocks, 4 spms. — N of Pointe Pukitarava,
27°35.8’S, 144°18.5’W, RAPA 2002, stn 58, 2-3 m,
sand bottom with boulders and corals, 1 spm. — Baie
Ahurei (Tauna islet), 27°36.5’S, 144°18.2’W, RAPA
2002, stn 80, intertidal zone, on rocks, 7 spms. — Baie
Akatanui, 27°35.9’S, 144°18.5’W, RAPA 2002, stn
81, intertidal zone, on rocks, 3 spms. — Baie Anarua,
27°36.4’S, 144°22.6’W, RAPA 2002, stn 87, intertidal
zone, on rocks, 6 spms.
DISTRIBUTION . — Rapa (Salvat & Rives 1975); Rikitea,
Makapou (both Gambier Islands), Pitcairn Island (Couturier 1907).
HABITAT. — Onithochiton lyellii is an intertidal species,
inhabiting crevices and holes on solid rocks.
DESCRIPTION
Animal moderately large (largest specimen examined
is 33 mm long), elongate-oval and surrounded by
a rather thick and wide perinotum, which appears
626
velvety. Tegmentum coloration variable, ranging from
blackish olive to bright green. Uneroded animals
with a pattern of longitudinal undulated lines, and
with fine brownish lines along the growth ridges.
Jugal area wedge-shaped, generally white with a
smaller, darker triangle within. Dorsally the animal
is slightly subcarinated. Dorsal elevation of valve iv
0.38 (5.2 mm high/13.6 mm wide) in a specimen
(29.7 × 15.4 mm) from Pointe Autea.
Large ocelli present in head valve, lateral areas
and postmucronal area, with a diameter of 38 µm,
surrounded by small apical pores with caps 14 µm
in diameter. Subsidiary pores occur randomly
around the other pores, with highest concentration around ocelli. Head valve semicircular with
posterior margin straight and unnotched. Intermediate valves (excluding the second) wider than
long with protruring apex. Second valve largest.
Lateral areas slightly elevated and clearly separated
from central areas by sculpture. Fine, comarginally
directed, wavy growth ridges also occur along part
of the diagonal ridge in the pleural areas. General
tegmentum surface smooth, except for the growth
ridges and pits in the jugal-pleural areas. Sculpture
of lateral areas homologous with that of head valve
and postmucronal area. Tail valve triangular, width
more than twice length; mucro terminal, slightly
elevated, forward-directed. Postmucronal slope
steep and very short.
Articulamentum thick, shiny, with dark brown
coloration in central area and white margins. Apophyses wide, long, triangular in valve ii to trapezoidal in
tail valve, widely separated but connected by a short
lamina, which shows minute teeth. Slits between
them correspond to rows of transverse holes. Insertion plates rather short, with long, sharp, sawed teeth,
roughly pectinate on outside. Slit formula: 8/1-1/0
(callus). Slitrays present in all valves, except for tail
valve. hey consist of transverse to round (innermost)
holes. Callus with series of large transverse holes on
its inner side. A very conspicuous thickening of the
articulamentum results in two diagonal bridges.
Cuticle of perinotum thick, dorsally beset with
very fine spicules (121-147.4 µm × 47-53 µm),
giving the whole girdle a velvety appearance. Spicules sculptured with about nine longitudinal ribs,
which are less distinct in the basal part. Among
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
FIG. 8. — Onithochiton lyellii (Sowerby, 1832), specimen, Rapa Island, RAPA 2002, stn 80, 17.4 mm: A, head valve dorsal view; B, valve ii,
dorsal view; C, valve ii, left side, ventral view, detail of slit; D, valve ii, detail of sutural laminae, ventral view; E, tail valve, dorsal view;
F, tail valve, lateral view. Scale bars: A, B, E, F, 800 µm; C, D, 400 µm.
the spicules, smooth, sharp-pointed needles were
found, 95 × 16 µm in size. Ventrally there are radial
rows of squarish to rectangular scales, measuring
24-30 µm. hey are obtusely pointed and a little
concave. here are about 7-10 shallow, longitudinal
grooves on their visible side.
ZOOSYSTEMA • 2006 • 28 (3)
Radula of pd specimen (17.4 × 10.2 mm) consists
of 48 teeth rows, 40 of them already mineralized.
Central tooth slender, 80 × 40 µm, with a single inward-curved blade, which is slightly wider
in the upper part. First lateral tooth (142 µm
long) has a thickened head with a simple edge.
627
Schwabe E. & Lozouet P.
FIG. 9. — Onithochiton lyellii (Sowerby, 1832), specimen, Rapa Island, RAPA 2002, stn 80, 17.4 mm: A, detail of area lateralis, valve ii;
B, radula, c. rows 7-10; C, detail of the major lateral tooth head showing a denticleless blade, second row; D, radula, rows 4-6; E, dorsal
girdle elements; F, ventral girdle scales. Scale bars: A, F, 40 µm; B, 100 µm; C, 20 µm; D, 60 µm; E, 80 µm.
Shaft of second lateral tooth slightly keeled, with
nearly square head (125 × 100 µm). Blade simple, without denticles in the anterior part of the
radula. A few rows later, cutting edges have four
finger-like denticles. Innermost extremely short,
628
outermost triangular with a wide edge. Major
uncinal tooth, 192 µm long, spoon-like, upper
part 58 µm wide.
hirty-seven holobranchial, adanal ctenidia both
sides of the foot in the pd specimen.
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
FIG. 10. — A, Leptochiton hiriensis n. sp., paratype, 1 mm, Rapa Island, RAPA 2002, stn 9, dorsal view; B, D, Chiton (Rhyssoplax) rapaitiensis
n. sp.; B, specimen, 4.1 mm, Rapa Island, RAPA 2002, stn 15, dorsal view; D, Rapa Island, RAPA 2002, stn 25, paratype with eggs in the mantle
cavity 2.7 mm, ventral view; C, Onithochiton lyellii (Sowerby, 1832), Rapa Island, RAPA 2002, stn 80, specimen, 19.2 mm, dorsal view.
REMARKS
Although often cited in the literature our knowledge of this species is very limited. A summary
ZOOSYSTEMA • 2006 • 28 (3)
of probable misidentifications is given herein.
Iredale (1911: 104) noted that the records of
Onithochiton lyellii from Australian waters most
629
Schwabe E. & Lozouet P.
TABLE 1. — Overview of the number of specimens of Polyplacophora
collected per station during RAPA 2002, Rapa Island.
Leptochiton Chiton Onithochiton
lyellii
hiriensis rapaitiensis
n. sp.
n. sp.
Pointe Autea
4
9
8
44
10
1
11
10
15
1
16
13
67
21
11
25
42
27
1
28
3
29
30
30
4
7
35
2
52
2
58
1
62
2
80
7
81
3
87
6
93
2
98
3
Total
25
228
21
Station
Total
4
52
1
10
1
80
11
42
1
3
30
11
2
2
1
2
7
3
6
2
3
274
probably refer to O. quercinus (Gould, 1846), as
he showed for the report of Pilsbry (1893: 248
partly). Following Iredale it seems logical that the
records from Australia of Nierstrasz (1905b: 156, pl.
10, fig. 19), Ashby (1924: 333) and Smith (1960:
167, figs 42, 2a-c) may also refer to this species.
Specimens cited from the Indonesian, Philippine
and Papuan region by Nierstrasz (1905b: 156, pl.
10, fig. 19), Ang (1967: 410, pl. 10, figs 1-4), Way
& Purchon (1981: 313) and Leloup (1981: 4) are
probably misidentifications of a new species, which
is currently under study by Hermann L. Strack (he
Netherlands, pers. comm.). Ashby (1931: 50, as
O. lyalli [sic]) pointed out that Odhner (1919: 40)
misused O. lyellii for O. literatus (Krauss, 1848).
After Strack (1993: 21), the specimens cited by
Leloup (1960: 42, figs 7, 9-13) belong to several
species and Leloup (1980: 12) misinterpreted
O. lyellii for O. erythraeus hiele, 1910. As no
detailed study of its congeneric species is available,
a restricted Polynesian distribution is assumed for
O. lyellii. It would be interesting to know if the
630
species reported as O. quercinus from the Fiji Islands
by Cernohorsky (1978) is correctly identified, or
if it also belongs to O. lyellii.
DISCUSSION
A moderately high number (274) of specimens
was collected of the three species reported herein
(Table 1). Table 1 shows that Onithochiton lyellii
inhabits the intertidal zone only, whereas the
other two species were found down to 24-30 m.
he predominant species is Chiton (Rhyssoplax)
rapaitiensis n. sp. It was found at nearly all stations
where Polyplacophora were found, but the number
of stations investigated in total is about 4 times
higher. Locally it was found syntopically with
Leptochiton hiriensis n. sp., which seems rather
uncommon. Chiton (Rhyssoplax) rapaitiensis n. sp.
is shown to be a brooding species and represents
the second known species in this subgenus with
this reproduction type. he two new species are
known only from Rapa Island and are considered
to be endemic. Onithochiton lyellii is for the time
being considered to be restricted to Polynesia.
Creese (1986) has shown that the endemic New
Zealand species Onithochiton neglectus broods its
eggs within the mantle cavity. he settlement of
the trochophores occurs within a few hours and
does not allow broad dispersal, as the presence of
a suitable substratum (in general all kinds of hard
substratum) is essential. If one infers the same larval
development for Onithochiton lyellii and excludes a
non-feeding, free-swimming larval stage, it explains
why this species is restricted to Polynesia. he fact
that two of the three species of Polyplacophora
collected are new confirms the unique nature of
the marine fauna of Rapa, suspected from previous
knowledge of other molluscs. hus, preliminary
observations of the protoconchs of caenogastropods collected at Rapa show that a relatively high
proportion of the species has non-planktotrophic
development, indicating low dispersal abilities.
Among these a large part probably consists of
endemic species (Lozouet et al. 2004). In view of
the isolated location of Rapa, the endemicity found
in the Polyplacophora is not surprising. At Easter
ZOOSYSTEMA • 2006 • 28 (3)
Chitons (Mollusca, Polyplacophora) from Rapa
Island (Rapa Nui), where a high level of endemism
is reported (42%; Rehder 1980), Dell’Angelo et al.
(2004) reported three species of Polyplacophora,
namely Plaxiphora mercatoris Leloup, 1936, Rapanuia disalvoi Dell’Angelo, Raines & Bonfitto,
2004, and Weedingia cf. mooreana Kaas, 1988, of
which the first two are also considered endemic.
his contrasts with the low level of endemism
estimated in the marine molluscs of the Pitcairn
Islands (about 2%) where only one polyplacophoran
(Onithochiton lyellii) is known (Preece 1995, as
Onithochiton cf. societatis hiele, 1910).
Compared to the Gastropoda or Bivalvia, the Polyplacophora is a very minor class in terms of species
richness, which makes it difficult to detect any clear
trends in diversity. Even in some well-documented
mollusc faunas the proportion of Polyplacophora
is always extremely low. At Rapa (Lozouet et al.
2005) Polyplacophora accounted for 0.6% of species diversity; 0.6% in New Caledonia at Koumac
(Bouchet et al. 2002), 0.3% at Enewetak Atoll
(Kay & Scott 1987), 0.4% at Hawaii and 0.9%
at Okinawa (Kay & Johnson 1987). Obviously,
the Polyplacopha is not an appropriate group for
estimating species richness gradients in the West
Pacific. Indeed, more information is needed on
the species richness of Polyplacophora within the
Polynesian region alone (a study is in progress by
the first author) before any attempt at evaluating
such trends could be made.
Acknowledgements
We thank all colleagues who participated in “RAPA
2002” and especially the mollusc group: R. von
Cosel, V. Héros, A. Le Goff, P. Maestrati, J.-L.
Menou, S. Schiaparelli & J. Tröndlé. Many thanks
also to Claude Payri (University of Papeete, Tahiti),
who organized the logistics with the French Navy
and the mayor and local council of Rapa. hanks
are also due to Philippe Bouchet (MNHN), Boris
Sirenko (Zoological Institute, Academy of Science, St. Petersburg), Michael Schrödl (ZSM),
and Alan Beu (Institue of Geological & Nuclear
Sciences, Lower Hutt, New Zealand) for helpful
comments on the manuscript. We thank R. Cowie
(University of Hawaii) for linguistic polishing. he
ZOOSYSTEMA • 2006 • 28 (3)
Total Foundation funded the expedition within its
“Coral Reef Biodiversity Programme”.
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633