Amphithyris cavernicola n. sp.

(Fig. 7–9; Table 6)

Etymology. From Latin caverna ("cave") and incolere ("inhabiting"). The name was chosen for the species as all individuals were found in a reef cave.

Type material. Holotype: QM G333507, Paratypes: QM G333508; ZMB Bra 2144; ZMB Bra 2175-2180; ZMB Bra 2182-2186; ZMB Bra 2187.

Type locality. Holmes Reefs, Coral Sea, Australia. Attached to piece of coral substrate removed from cave ceiling, at a depth of 8m.

Diagnosis. Dorsal septum absent. Ventral valve with low median septum and muscle impressions visible on each side, exterior surface with tubercles and capillae.

Description. Very small species of Amphithyris (length 1.2mm; width 1.3mm), oval to round, transparent (Fig. 7 A–B). Outline and shells often asymmetrical due to substrate, ventribiconvex. Short pedicle, dorsal valve grows very close to substrate (Fig. 7 A). Dorsal valve slightly convex (Fig. 8 A). Exterior surface smooth with growth lines (Fig. 8 B). Dorsal septum absent, interior surface endopunctate, punctae variable in size (Fig. 8 C and D). Interior with U-shaped amphithyrid foramen framed by pair of short socket ridges at lateral margins that slightly extend beyond posterior margin (Fig. 8 E). Oval hinge sockets (Fig. 8 E). Schizolophous lophophore (Fig. 7 B, 8F).

Ventral valve more convex than dorsal valve (Fig. 8 A). Interior with very low median septum extending from anterior edge of V-shaped delthyrium to third of valve length (Fig. 9 A–C). Oval muscle impressions visible on each side of septum, anterior rounded margin of muscle impressions on the same level as anterior end of septum (Fig. 9 B). Hinge line almost straight, small emergent hinge teeth, not parallel to hinge line (Fig. 9 B). Dental plates absent. Protegulum in all specimens with one wrinkled middle ridge and two very low ridges on each side (Fig. 9 E and F). Weak capillae on exterior side of ventral valve (Fig. 9 I). Brush of endopunctae visible on exterior valve surface (Fig. 9 J), majority of endopunctae accompanied by single tubercle at distal end of brush (Fig. 9 K and L). Tubercles always pointing posteriorly. Distribution and form of tubercles irregular and absent in smooth umbonal area (Fig. 9 D, G and H).

Population density. Thirty-two individuals of A. cavernicola n. sp. have been counted on the coral rock (see Fig. 1). The distribution of the specimens across the rock surface is irregular and most of the individuals are concentrated on the bottom area. The highest density found is thirteen individuals in six square centimeters, with no more than two specimens per any square centimeter. The shortest distance between two individuals is approximately 0.4 cm and the longest 5.5 cm. The animals never touch each other nor do they attach to conspecifics. All specimens are attached to the substrate by a short pedicle with some distance to the neighboring individual of the same species.

Remarks. A. cavernicola n. sp. is clearly distinguished from the other species within the genus Amphithyris by the tubercles covering the surface of the ventral valve, the low ventral median septum, and the visible muscle impressions inside the ventral valve. A. cavernicola n. sp. resembles A. parva as both share a very small size and the absence of a dorsal median septum. A. cavernicola n. sp. represents the first record of the genus from the Coral Sea, Australia.

DV length VV length Average Width

Holotype QM G333507 1.3 - 1.4

Paratype ZMB Bra 2144 1.3 - 1.2

Paratype ZMB Bra 2176 - 1.3 1.4

Paratype ZMB Bra 2177 1.0 1.1 1.2

Paratype ZMB Bra 2180 1.3 1.4 1.5

Paratype QM G333508 - 1.3 1.4

Paratype ZMB Bra 2183 1.3 1.4 1.4

Paratype ZMB Bra 2187 1.1 1.2 1.4

Ϭ = 1.2 ± 0.1 Ϭ = 1.2 ± 0.1 Ϭ = 1.3 ± 0.2