Shell
The three Parabrachidontes species share similar small shells (<30 mm) with subterminal umbones (usually subterminal in position), but exhibit distinct external features, with their shape varying from mytiliform (mussel-like) to modioliform (elongate-oval). Their surface may be smooth or feature radial ribs of varying prominence, often accompanied by fine, closely spaced commarginal growth lines. P. amnicus displays dark brown shells with greenish tones, characterised by prominent radial ribs separated by deep grooves. P. leucostictus shows weaker, dark brown or black radial ribs defined by faint greenish grooves. P. cochinensis lacks radial ribs entirely, presenting instead a speckled surface with only concentric growth lines.
All three species share several internal shell features:
- A short series of small teeth both anterior and posterior to the ligament;
- Subterminal umbones positioned slightly behind the shell's anterior edge;
- Consistent shell microstructure consisting of:
- A thin (10-15 μm) outer homogeneous layer (likely calcite);
- A thicker (250-300 μm) middle nacreous layer (aragonite);
- An inner simple prismatic myostracum (aragonite).
Mineralogical analysis shows the shells are predominantly aragonite (86-99%), with minor calcite components (0.8-3.0%) present in all species.
Anatomy
The shells of Parabrachidontes species exhibit an iridescent interior with distinctive dental features: approximately twelve slender, closely spaced teeth anterior to the ligament and seven papillate crenules posterior to it. The ligament is internal and narrow, lacking resilial pits, while the posterior adductor muscle scar merges with a single, narrow posterior byssal retractor scar. Shell microstructure consists of three layers: a thin outer calcitic layer (10-15 μm), a thick middle nacreous layer (250-300 μm), and an inner prismatic myostracum. [5]
All species display identical gill morphology, with the ascending lamella of the outer demibranch being shorter than the descending lamella and fused to the adjacent mantle lobe. The inner demibranch's ascending lamella may also attach to the visceral mass in some specimens. Notably, plicate glands are absent across the genus. The pericardium consistently occupies the space between the anterior and posterior muscle groups of the posterior byssal retractor complex (Category 3 morphology). All three species possess pigmented exhalant siphon tips, though pigmentation intensity varies considerably in P. leucostictus and P. cochinensis.
Parabrachidontes amnicus is uniquely characterized by up to ten simple guard papillae lining its inhalant siphon margin, a feature entirely absent in its congeners. However, the exhalant siphon morphology of living P. amnicus specimens remains undocumented. Shell surface patterns provide additional diagnostic characters: P. amnicus shows prominent radial ribs, P. leucostictus displays faint ribbing, while P. cochinensis lacks ribs entirely, bearing only commarginal striae.
