3217
Author
Raines, Bret
Author
Huber, Markus
text
Zootaxa
2012
2012-02-29
3217
1
106
journal article
11755334
Thecodonta rainesi
Huber
in
Raines & Huber
sp. nov.
Figures 31 A–G
Type Material.
Holotype
: LACM 3201, single LV,
3.7 mm
.
Paratype
1: LACM 3202, single RV,
3.2 mm
, from type locality.
Paratype
2: LACM 3203, single LV,
3.6 mm
, from
EI
,
Mataveri Bay
,
50 m
.
Paratypes
3–4: Coll. MHU, 2 single valves, from
EI
, Hanga Roa,
50 m
.
Paratypes
5–6: Coll. BR, 2 single valves, from type locality.
Type
Locality.
Dredged
at
30–50 m
in fine sand, off the western coastline near
Tahai
,
Easter Island
,
27°07’20” S
,
109°26’30” W
.
Description.
A small, whitish
Thecodonta
species
, ovate, smooth, with low and strongly prosogyrate umbones. Shell small, not exceeding
3.7 mm
; ovate, gently rounded posteriorly and anteriorly; equivalve, strongly inequilateral; rather compressed; thin; white, showing spots and blotches in the substance of shell which lend to it a watered silk effect. The umbones are low and strongly prosogyrate and are situated near the anterior end. Prodissoconch, P1 typically eroded, possibly D-shaped, ca. 130 µm length by 85 µm height, P2 ovate with slender growth lines, white to tan in color, ca. 430 µm length by 369 µm height. In adult valves the dorsal line curves gently from the umbo to the posterior end. The surface is nearly smooth, marked by weak commarginal growth stops only. The elongate lateral dorsal tooth, stronger expressed in the receiving LV is marginally placed. The shorter anterior laterals are equally marginally placed. The single cardinal tubercles in either valve rest at the upper margin of the anterior lateral. The laterals are smooth not serrated. The posterior scar is narrow, elongate, rather small and marginally placed; the posterior scar is suborbicular, also small and marginally placed. The connecting, continuous pallial line is very weakly impressed, but comparatively broad and patchily interrupted. The margins are smooth.
TABLE 8.
Selected measurements from the type material.
|
T. rainesi
sp. nov.
|
Length (mm) |
Height (mm) |
P1 (L x H) (µm) |
P2 (L x H) (µm) |
| Holotype |
3.7 |
2.4 |
not visible |
425 x 375 |
| Paratype 1 |
3.2 |
1.9 |
ca. 130 x 85 |
450 x 375 |
| Paratype 2 |
3.6 |
2.3 |
not visible |
425 x 375 |
| Paratype 5 |
2.2 |
1.4 |
not visible |
420 x 350 |
Comparative diagnosis.
The very inequilateral-ovate shape and thin structure, together with a continuous pallial line, characteristic dentition, and minute size closely resembles the small
Thecodonta
-group.
In umbonal position
Bernard’s (1983: 68)
prosogyrate view for thecodontinids is still applied. Prosogyrate beaks were also diagnosed by Chavan
in
Moore (1969
: N532) for
Thecodonta
. However,
Dall, Bartsch and Rehder (1938: 148)
noted the reverse montacutinid opisthogyrate position. The as yet unknown anatomy of
Thecodonta
will be needed to decide once and for all.
Japanese authors synonymized
Dall, Bartsch & Rehder, 1938
’s genus
Kona
with
Thecodonta
and recognized only the type species as valid from
Japan
(e.g.
Higo
et al
., 1999
: N732). Possible
syntypes
of smaller and larger Japanese specimens are illustrated in
Higo
et al
. (2001
B732 and B732s).
Severns (2011: 470
, pl. 215 fig. 6) recognized only
Thecodonta symmetrica
Pilsbry, 1921
, as valid from the Hawaiian Islands, and consequently
Kona bucki
Dall, Bartsch & Rehder, 1938
as a synonym. Both views are shared.
Moreover, due to a similar morphology, the nearly identical shape changes during growth and similar maximum sizes
4.7 mm
(
T. sieboldi
) and
4 mm
(
T. symmetrica
) synonymy cannot be completely excluded, because genetic data may one day reveal these two as conspecific. Most distinct at present are their habitats, i.e. subtidal
2– 10 m
(
T. symmetrica
) and sublittoral
50–100 m
(
T. sieboldi
).
The respective data of
T. rainesi
is within
30–
50 m
. There is no commensal mode of life data recorded for either the Japanese or Hawaiian species of
Thecodonta
.
Severns (2011)
noted the habitat as silty sand, inshore of the reef, Japanese authors recorded sandy and muddy bottoms, while
T. rainesi
is found on fine sandy bottoms.
Morphologically,
T. rainesi
differs significantly in shape from its two congeners. Equal sized
T. sieboldi
and
T. symmetrica
are shorter and oblique in shape. Larger congeners approach
T. rainesi
somewhat more, but still have a stronger curved ventral margin and stronger developed teeth. The nearly smooth sculpture with watery blotches is similar to Japanese specimens, whereas all Hawaiian specimens seen are rather dull.
FIGURE 31. A–G
,
Thecodonta rainesi
Huber
in
Raines & Huber
sp. nov.
;
A
, Exterior LV, 3.7 mm in length;
B
, Exterior LV, 3.6 mm in length, SEM;
C
, Interior RV, 3.5 mm in length, SEM;
D
, Interior LV, 3.4 mm in length, SEM;
E
, Close-up of hinge, RV, SEM;
F
, Close-up of hinge, LV;
G
, Close-up of the prodissoconch, LV, SEM, arrows indicating boundaries.
Remarks.
At first this species was misidentified by the junior author. However, the senior author observed differences which finally led to a new description by the junior author.
Habitat.
Occasionally found at several locations around
EI
, dead in sand, from
30–50 m
.
Distribution.
At present
Thecodonta rainesi
is only known from Easter Island—
E1.
Etymology.
This new species honors Bret Raines’ relentless efforts during many years of his life for a better understanding of the bivalve fauna of Rapa Nui.