Cardiomya, Adams, 1864

Pacheco, Leonel I., Teso, Valeria & Pastorino, Guido, 2024, Use of traditional tools and micro-computed tomography for the taxonomy of carnivorous bivalves from the deep waters of Southwestern Atlantic, Zoological Journal of the Linnean Society 202, pp. 1-23 : 3

Cardiomya

Type species: Neaera gouldiana Hinds, 1843: 77 , by monotypy.

Description

Shell: Inflated, globular to ovate. Inequivalve; less valve overlaps right ventrally, and right valve overlaps less in the posterodorsal margin. Inequilateral, posterior margin terminating in an elongated rostrum. Surface covered by thin periostracum, thickened in the dorsal margin, and ornamented by numerous radial ribs or riblets strongly defined on the posterior side of the disc. Hinge of less valve edentulous; right with a posterior lateral tooth that fits under a small ridge in less valve. Resilifer triangular, sunken into the interior (condrophore) housing an internal ligament with lithodesma. Inside with radial grooves that correspond to external ribs or riblets. Posterior adductor scar conspicuous.

Remarks

The following description of the soss parts of Cardiomya is based on the information taken from the specimens of Cardiomya cleryana and Cardiomya fragilissima studied, and the bibliography ( Allen and Morgan 1981, Morton 2015, Machado et al. 2016).

The body is completely sealed except for two siphonal and one pedal gape. The pedal gape runs from the anterior muscular adductor muscle to one-third of the ventral margin, leaving enough space for the foot to extend outside the body. Internally, the palial cavity is compartmentalized into an infraseptal and supraseptal cavity ( Fig. 1A, C ) by the muscular septum and connected through four pairs of septal pores ( Fig. 1B ) markedly distributed, (two lateral to the septal pedal gape and two posterior) which remain open when the septum is relaxed. The infraseptal cavity communicates with the exterior through the inhalant siphon, with a sphincter that regulates the size of the material captured ( Fig. 1B, G ). The supraseptal cavity communicates with the exterior through the eversible exhalant siphon.

The septum is attached to the shell anteriorly by anterior septal retractor muscles, posteriorly by posterior septal retractor muscles, and laterally by lateral septal muscles organized in a continuous row of fascicles along each side. A small extra lateral septal muscle is present between the lateral septal muscles and the siphonal sheath ( Fig. 1E ). Besides the anterior, posterior, and lateral septal muscles, the septum also contains an inner longitudinal muscle ( Fig. 1B ). The longitudinal muscle runs along the wall of the septum, limiting the septal pedal gape, where the foot can extend into the infraseptal cavity and attach to the shell along with the septal muscles. Foot linguiform with byssal groove, attached to the shell through one pair of anterior retractor muscles and one posterior muscle that bifurcates close to the attachment site.

Both siphons are encased inside a siphonal sheath, generated by the fusion of two mantle folds, and separated from one another through an extension of the muscular septum called the intersiphonal septum ( Fig. 1G ). The siphonal sheath contains longitudinal muscle bundles called siphonal retractor muscles that, when contracted, displace the siphons inside the rostrum. Seven club-shaped siphonal tentacles are present: three dorsal to the exhalant siphon and four lateral and lateroventral to the inhalant siphon.

The ventrally directed mouth has a funnel shape and is surrounded by labial palps type 1 (palps not well developed, anterior horn shaped, and posterior reduced to thickened pads), of which the anterior palps are fused to the mantle margin ( Fig. 1F ), and continues with a muscular oesophagus, which leads posterodorsally to the anterodorsal wall of the stomach. The stomach is covered anterodorsally by the digestive diverticula and posterodorsally by the gonads ( Fig. 1A, C, D ). Two groups of short muscle bundles (one on each side) can be observed, located dorsal and internal to the anterior septal muscles, and ventral and external to the anterior pedal muscles. No previous reference to them was found, and we decided to call them ‘visceral muscles’, because their function appears to be attaching the stomach to the shell ( Fig. 1C ).

Figures

Figure 1. Anatomy of Cardiomya cleryana and Cardiomya fragilissima through micro-computed tomography. A–E, Cardiomya cleryana (MACN-In 43783): A, lateral view of a virtual dissection in the sagittal plane; B, ventral view of the septum with a virtual dissection in the frontal plane; C, anterolateral view of a virtual dissection in the sagittal plane; D, E, posterolateral view showing the general distribution of the structures (C) and highlighting the septal musculature. F, G, Cardiomya fragilissima (MACN-In 44323):F, close view of the mouth and labial palps; G, virtual dissection of the siphonal sheath in sagittal plane.Scale bars: 1 mm in A–G.