Tridacna (Chametrachea)costata ROA-QUIAOIT,KOCHZIUS,
JANTZEN, AL-ZIBDAH &RICHTER from the Red Sea,
a junior synonym of Tridacna squamosina STURANY, 1899
M. Huber*& A. Eschner**
Analysis of the material of Tridacnidae collected during the Pola Red Sea Expeditions 1895/96
and 1897/98 revealed that the recently described Tridacna (Chametrachea)costata, ROA-
QUIAOIT,KOCHZIUS,JANTZEN, AL-ZIBDAH &RICHTER in RICHTER, ROA-QUIAOIT, JANTZEN, AL-
ZIBDAH & KOCHZIUS, 2008 is identical with T. elongata var. squamosina STURANY, 1899. For
more than one hundred years the material has been kept in the Mollusca collection of the Natural
History Museum in Vienna without proper identification. During the analysis of the material it
was possible to identify the relevant specimens with the consequence that T. costata has to be
considered a junior synonym of T. squamosina. With this newly designated lectotype the south-
ernmost occurrence of T. squamosina is recorded.
Key Words: Red Sea, Bivalvia, Cardioidea, Tridacna,squamosina,costata, Sturany, Pola type
Bei Untersuchung der Tridacnidae, die während der Pola Expeditionen ins Rote Meer 1895/96
und 1897/98 aufgesammelt wurden, zeigte sich, dass die jüngst beschriebene Tridacna (Chame-
trachea)costata, ROA-QUIAOIT, KOCHZIUS, JANTZEN, AL-ZIBDAH & RICHTER in RICHTER, ROA-
QUIAOIT, JANTZEN, AL-ZIBDAH & KOCHZIUS, 2008 mit T. elongata var. squamosina STURANY,
1899 identisch ist. Über 100 Jahre war das Expeditionsmaterial in der Molluskensammlung des
Naturhistorischen Museum in Wien aufbewahrt worden, ohne eingehende Identifizierung.
Während der Bearbeitung des Materials konnten die entsprechenden Exemplare identifiziert
werden, mit der Konsequenz, dass T. costata als Juniorsynonym von T. squamosina anzusehen
ist. Mit dem neu designierten Lectotypus wird gleichzeitig auch die südlichste Verbreitung doku-
*Dr. Markus Huber, University Zürich, Zoological Museum, Winterthurerstrasse 190, 8057 Zürich,
Switzerland. – email@example.com
** Mag. Anita Eschner, Naturhistorisches Museum Wien, 3. Zoologische Abteilung, Burgring 7, 1010
Wien, Austria, – Anita.Eschner@nhm-wien.ac.at
Ann. Naturhist. Mus. Wien, B 112 153–162 Wien, März 2011
Giant clams are among the most spectacular marine bivalves. Huge size, ease of recog-
nition, peculiar feeding mechanism and shallow reef habitats are well known features of
this group. Tridacna BRUGUIÈRE, 1797 encompasses the heaviest and after Kuphus
GUETTARD, 1770 the second largest recent bivalve. Numerous tridacnids have been
described. Most modern authors consider currently seven extant Tridacna as valid; in
one case, however, the validity is disputed. For Tridacna rosewateri SIRENKO & SCARLATO,
1991 it is unclear if this species is a habitat-variant – as indicated by BENZIE & WILLIAMS
(1998), or a distinct species endemic to the Mascarene Plateau, in the sense of NEWMAN
& GOMEZ (2000). LUCAS et al. (1990, 1991) described a new species from Tonga and Fiji
which turned out to be a junior synonym of T. mbalavuana LADD, 1934 from the Upper
Tertiary of Fiji. Paleontological studies, which tried to reconstruct the evolution and
radiation of Tridacnidae implied that modern lineages originated in the Paleogene and
early Neogene of the East African-Arabian Province (see HARZHAUSER et al. 2008).
Difficulties in the morphological classification of tridacnids have led to modern DNA-
based genetic studies. These molecular genetic analyses tried to clarify the situation of
giant clam populations in the northern part of the Red Sea and concluded that there live
at least three species of Tridacna (ROA-QUIAOIT 2005; MOHAMED et al. 2006). In fact
MOHAMED et al. (2006) even indicate the possibility of five extant Red Sea species. Fur-
thermore, there are doubts, whether the species commonly identified as T. squamosa
LAMARCK, 1819 from the Red Sea is indeed Lamarck’s true T. squamosa (J.J. ter
Poorten, pers. comm., Oct. 2010). Definitely, the number and identity of the living Red
Sea tridacnids is currently unresolved. Consequently, the respective Red Sea species is
here termed T. cf. squamosa.
Recently, an 8th Tridacna species has been described from the northern Red Sea. Tri-
dacna costata ROA-QUIAOIT KOCHZIUS, JANTZEN, AL-ZIBDAH & RICHTER, 2008 presents
clear morphological, habitat and genetic diagnostics compared to the other two well
known tridacnids occurring in the Red Sea, T. maxima (RÖDING, 1798) and T. cf.
squamosa LAMARCK, 1819.
In 1899, Sturany published the results of the "Pola"-Expedition to the Red Sea. He men-
tioned three Tridacna taxa found during this expedition of which one was new.
The material from the Pola expeditions forms a significant part of the scientific collec-
tions in the Naturhistorisches Museum Wien, Austria (NHMW). In the course of 7 expe-
ditions - all carried out by SM Ship Pola - clearly defined areas of the ocean were sys-
tematically explored. Between 1890 and 1894, the eastern Mediterranean, and from
1895 to 1898, the Red Sea were sampled. Compared to larger expeditions of other coun-
tries, these expeditions by the Austro-Hungarian monarchy turned out to be highly effi-
cient and minutely planned. Thanks to the use of modern oceanographic instruments, the
eastern Mediterranean and the Red Sea became the most thoroughly explored areas of
sea at that time. For more details concerning the expeditions, their scientific results and
the history of oceanography, see SCHEFBECK (1991).
Molluscs alone collected during the Pola expeditions in the Red Sea amount to 1300
series of gastropods and bivalves (STAGL et al. 1996). As the curator responsible at the
154 Annalen des Naturhistorischen Museums in Wien, B, 112
k.u.k. Hofmuseum, Rudolf Sturany worked on the mollusc material gathered during the
Pola expeditions. Sturany published a series of papers on gastropods (STURANY 1896;
1900a, b; 1904) and bivalves (STURANY 1896; 1899). Unfortunately, his works either
remained disregarded or were inaccessible to English speaking researchers.
Sturany’s publications are mandatory reading for every Red Sea bivalve specialist. His
report on the Red Sea "Lamellibranchiaten" dating from 1899 was first published sepa-
rately as a "Sonderdruck" and 1901 reprinted as part of the "Berichte der Commission
für oceanographische Forschungen" which were published in the "Denkschriften der
mathematisch-naturwissenschaftlichen Classe der kaiserlichen Akademie der Wis-
Sturany described many rare deepwater species dredged during the Pola Expedition.
Included were also many shallow water species collected near or on shore.
The complete dry and wet material of the Pola Expeditions - including the syntypes of
T. elongata var. squamosina – was originally inventoried at the Mollusca Collection
under the number NHMW Moll. 38076.
Sturany did not individually label each specimen. However, Sturany stated the respec-
tive Red Sea stations in his report and the station data are available for each of the spec-
imens. With very few exceptions the whole wet and dry material of Sturany was avail-
able and conformed to the stations indicated.
Six T. squamosina syntypes were collected in the Gulf of Aqaba, in Dahab (4 speci-
mens) and in Sharm el Sheikh (2 specimens). The seventh and largest specimen origi-
nated in the southern Red Sea at Kamaran Island, off Yemen. Altogether Sturany studied
7squamosina specimens ranging in size from 102 to 190 mm.
Almost 30 Pola specimens conform to T. maxima. More than 30 Pola specimens con-
form to T. cf. squamosa. Contrary to the situation nowadays, T. cf. squamosa was around
1900 the most commonly collected Tridacna in the Red Sea.
Unfortunately, many authors including ROSEWATER (1965) and ZUSCHIN & OLIVER
(2003) as well as ROA-QUIAOIT et al. (2008) and very recent works of RUSMORE-VIL-
LAUME (2008) and KNOP (2009) have overlooked Sturany’s publication. Only OLIVER
(1992) mentioned his paper and he assigned Sturany’s T. rudis Rve. to T. squamosa
Sturany sorted the well over 60 tridacnids collected by the Pola expedition and recog-
nized 3 taxa. Two Red Sea species – T. maxima and T. cf. squamosa – were found com-
monly in various lots with approximately 30 specimens each, one species in just 3 lots
with 7 specimens. This is in accordance with modern findings.
STURANY (1899) briefly described the nov. var. T. elongata squamosina: "... hingegen
HUBER & ESCHNER:Tridacna costata a junior synonym of Tridacna squamosina (Bivalvia) 155
eine Varietät besonders hervorzuheben, die systematisch zur Tr. squamosa Lm. hinüber-
führt. Diese mit squamosina nov. var. zu bezeichnende Form liegt von den Localitäten
12, 14 und 43 in mehreren Exemplaren vor ...". He pointed out the characteristic "...
gegen den Unterrand blättrig aufgestellte Querschuppen der Rippen ...".
ROA-QUIAOIT (2005: table 9) elaborated precisely the characteristics of T. costata – shell
asymmetrical, hinge length < half shell length, scutes crowded and well spaced, wide
byssal orifice, and 5–6 deeply triangular radial folds (Table 1).
A close morphological comparison of Sturany’s T. squamosina with the newly described
T. costata left no doubt that these two species are identical. This assessement was con-
firmed by J. J. ter Poorten, a well known cardioid specialist. In addition, two of Stu-
rany’s three collecting stations conform to the type locality of T. costata in the Gulf of
Although Tridacna squamosina STURANY, 1899 was not found cited in other publica-
tions, Sturany’s species was validly proposed and is recognizably characterized. Seven
syntypes are unambiguously available. Sturany’s name is not preoccupied.
Consequently, a lectotype of T. squamosina is herein designated and T. costata formally
With respect to the description of the habitat, spawning, genetics of T. squamosina and
comparisons with other species of Tridacna, the works of ROA-QUIAOIT et al. (2008) and
especially the thesis of ROA-QUIAOIT (2005) provide an excellent overview. These
aspects are not discussed in this article.
However, two facts merit being added. More than 100 years ago T. squamosina was also
the species least commonly collected. The locality of station 43 further indicates that T.
squamosina may be found throughout the Red Sea and not just in its northern part.
Tridacna squamosina STURANY, 1899
Tridacna elongata var. squamosina STURANY, 1899: 283
Tridacna nov. sp. ROA-QUIAOIT, 2005: 67, 72, 75–77 (with an extended characterization of nov. sp. and a
comparison of the other Red Sea species)
Tridacna costata ROA-QUIAOIT,KOCHZIUS,JANTZEN, AL-ZIBDAH &RICHTER in RICHTER, ROA-QUIAOIT,
JANTZEN, AL-ZIBDAH & KOCHZIUS, 2008: 1349–1354 (syn. nov.)
Type material. Lectotype (Figs. 1–6) selected herein: largest specimen selected from the material Sturany
described as Tridacna elongata var. squamosina. – Red Sea, Yemen, Kamaran Island, ca. 15°17’ N; 42° 37’
E, shallow water, Pola expedition leg. F. Steindachner & F. Siebenrock 1.-3. Nov. 1897 [Loc. 43], (NHMW
Moll. 107075); paralectotypes: Egypt, Dahab, shallow water, Pola expedition leg. F. Steindachner & F.
Siebenrock 6. Apr. 1896 [Loc. 12] (4 specimens, NHMW Moll. 107076; Figs. 7, 10); Egypt, Sharm el
Sheikh [Sherm Sheik], shallow water, Pola expedition leg. F. Steindachner & F. Siebenrock 1. Apr. 1896
[Loc. 14] (2 specimens, NHMW Moll. 107077).
156 Annalen des Naturhistorischen Museums in Wien, B, 112
Comparative material. Tridacna cf. squamosa (Figs. 8, 11) – Red Sea, Egypt, Gulf of Aqaba, Nawibi,
Pola expedition leg. F. Steindachner & F. Siebenrock 9.-10. Apr. 1896 [Loc. 10] (NHMW Moll. 38076);
Tridacna maxima (Figs. 9, 12) – Red Sea, Egypt, Shadwan Island, Pola expedition leg. F. Steindachner & F.
Siebenrock 18.-20. Feb. 1896 [Loc. 16] (NHMW Moll. 38077).
Earlier workers treated tridacnids as their own superfamily (e.g. KEEN 1969;
STAROBOGATOV 1992). However, most modern analyses, including SCHNEIDER (1998),
BRALEY & HEALY (1998), SCHNEIDER & FOIGHIL (1999), GIRIBET et al. (2002) and MAT-
SUMOTO (2003) demonstrate a close relationship between tridacnids and cardiids, either
to Fragum RÖDING, 1798 or even to Nemocardium (Keenaea) HABE, 1951 (MATSUMOTO
2003). Consequently, many modern researchers attribute tridacnids subfamilial status
within Cardiidae. From an exclusive phylogenetic point of view, this placement may be
A superfamily Tridacnoidea LAMARCK, 1819 is certainly no longer valid. Tridacnids
must be understood as firm portion of Cardioidea LAMARCK, 1809, see BIELER et al.
(2010). However, the restricted biogeography of tridacnids, their peculiar habitats and
mode of life, their unique anatomy, together with their size and weight nonetheless jus-
tifies a special treatment within Cardioidea. As such, a familial treatment as Tridacnidae
LAMARCK, 1819 is here preferred and T. squamosina is understood as member of this
small bivalve family.
IREDALE (1937) started to divide Tridacna and proposed five additional genera for Aus-
tralian and Pacific species. He even created for the most variable tridacnid, T. maxima,
two distinct genera (i.e. Vulgodacna for T. fossor and Sepidacna for T. throughtoni,
which today are both considered synonymous to T. maxima). But it is doubtful if Per-
sikima is genetically distinct from Tridacna s.s. (ROA-QUIAOIT 2005; SCHNEIDER & Ó
FOIGHIL 1999). In addition, the subgenus Chametrachea MÖRCH, 1853 was earlier used
by HERRMANNSEN (1846), which is based on Chama aspera RUMPHIUS, 1705. Taking
further into account the characteristic features of each species and the small number of
HUBER & ESCHNER:Tridacna costata a junior synonym of Tridacna squamosina (Bivalvia) 157
Table 1: Diagnostic criteria in tridacnids after ROA-QUIAOIT (2005) and HUBER (2010)
T. squamosina T. squamosa T. maxima
Shell symmetry asymmetrical symmetrical asymmetrical
Hinge length < half shell length = half shell length < half shell length
Scutes crowded, well spaced large, well spaced low, crowded
Radial folds 5–6 deeply triangular 4–6 pointed to usually 5 sharply
bluntly rounded triangular
Byssal orifice wide narrow moderate to wide
Mantle patterns subdued subdued mottled colored
Incurrent tentacles distinct distinct indistinct
Maximum size 320 mm (Red Sea) 476 mm (Tonga) 500 mm (India)
Distribution Red Sea only Indo-Pacific Indo-Pacific
Habitat to 2 m, byssally to 25.5 m, to 32 m,
weakly attached byssally attached usually embedded
8 valid tridacnids, a forced differentiation of various subgenera within Tridacna adds lit-
tle benefit. More important are the real differences between species regarding habitats,
modes of life, genetics and detailed morphology. This was excellently worked out by
ROA-QUIAOIT (2005). Consequently, the subgeneric level is here abandoned and T.
squamosina is placed as Tridacna.
158 Annalen des Naturhistorischen Museums in Wien, B, 112
Figs. 1–6. Tridacna squamosina – lectotype, NHMW Moll. 107075 from Kamaran. 1, right valve
inside; 2, left valve inside; 3, right valve outside; 4, left valve outside; 5, byssal orifice; 6, radial
folds. Scalebar: 2 cm. Photographs: A. Schumacher.
STURANY (1899) recognized three Red Sea taxa. He clearly used the monograph of
REEVE (1862) on Tridacna as an identification guide. He identified elongata correctly.
Sturany’s elongata conforms to T. elongata LAMARCK, 1819. But in the 1950s and 1960s
it was recognized that Lamarck’s elongata is a junior synonym of Tridachnes maxima
RÖDING, 1798. Tridacna maxima (RÖDING, 1798) is today recognized as valid, earliest
name for this widely distributed species.
HUBER & ESCHNER:Tridacna costata a junior synonym of Tridacna squamosina (Bivalvia) 159
Figs. 7–12. Comparison of the three Tridacna species from the Red Sea in the collection of the
NHMW; top: radial folds; bottom: byssal orifice. 7, 10, Tridacna squamosina – paralectotype,
NHMW Moll. 107076a from Dahab; 8, 11, Tridacna cf. squamosa – NHMW Moll. 38076 from
Nawibi; 9, 12, Tridacna maxima – NHMW Moll. 38077 from Shadwan. Scalebar: 2 cm. Pho-
tographs: A. Schumacher.
The second common Red Sea species identified by Sturany was T. rudis. REEVE (1862)
described T. rudis from the Philippines. Indeed, Reeve’s pl. 5 fig. 4 a, b, c superficially
resembles the material identified by Sturany. Based on Rosewater’s treatment of this
family, modern authors today accept Reeve‘s rudis as a peculiar form of maxima. Con-
sequently, this second species was erroneously identified by Sturany as rudis. This was
recognized by OLIVER (1992), who identified it as Lamarck’s T. squamosa, here termed
T. cf. squamosa. It is not excluded that Tridachnes imbricata RÖDING, 1798 was the ear-
lier name for Lamarck’s famous species. Large portions of the Bolten collection, on
which Röding based his imbricata, are still to be found in Museum der Natur at Gotha,
Germany. However, a critical review of the respective material at Gotha in September
2010 led to the following conclusion: "Overall, due to missing descriptions and specific
marks in Röding, due to lack of original numbers and labels, due to the peculiar sup-
pressing and renaming method of Schmidt and finally due to the curational condition of
the collection we could not unambiguously identify any Bolten/Röding Bivalve type."
(HUBER &TER POORTEN in prep.). Thus, Tridachnes imbricata RÖDING, 1798 is best
treated as nomen dubium
The third, the least common species was described by Sturany as squamosina. ROA-
QUIAOIT (2005) stated the closest morphological affinities for T. costata are to T. max-
ima. STURANY (1899) came to the same result in considering squamosina closer to elon-
gata (=maxima) than to squamosa. From a molecular phylogeny based on mitochondrial
16 rDNA, ROA-QUIAOIT (2005) concluded that T. costata‘s closest relative is T. maxima,
whereas lower affinities were found between T. squamosa and T. crocea. For further
detailed information on T. squamosina we refer to ROA-QUIAOIT et al. (2008 as T.
costata) and especially to ROA-QUIAOIT (2005 as T. nov. sp.).
As such, Tridacna squamosina STURANY, 1899 is the 8th member of the genus Tridacna
within the family Tridacnidae. Currently, it is only known from the Red Sea, likely liv-
ing throughout. It is the least common of the described three Red Sea tridacnids. The
known habitat is very shallow within approximately 5 meters and the maximal size
reported is 320 mm.
Thanks are due to: Prof. Tony Wilson for editorial support, Dr. Peter C. Dworschak for fruitful
discussions and valuable help with the plates, Dr. Mathias Harzhauser for help with paleontological
literature and to Alice Schumacher for the photographs of the material.For their precise remarks
and additions, the authors are deeply indebted to one anonymous reviewer and J.J. ter Poorten,
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