Small, rare and little known: new records and species of Cardiomya (Bivalvia: Cuspidariidae) from Brazil

Abstract. This paper describes rare Cardiomya species from Brazil which have been hitherto misidentifi ed as Cardiomya cleryana (d’Orbigny, 1842) in literature or museum collections. Cardiomya minerva sp. nov. is proposed as new species and is characterized by its quadrangular shell, short and truncated rostrum, and external ornamentation composed of six radial ribs on the posterior half of the shell fl ank. Cardiomya striolata (Locard, 1897) described from the Mediterranean Sea and northwestern Atlantic Ocean, is reported from Brazil for the fi rst time; although previously regarded as a junior synonym of Cardiomya costellata (Deshayes, 1835), it is herein considered as a full species and redescribed. This species is characterized by its trapezoidal shell fl ank, elongated rostrum, tapering towards the tip, and external ornamentation composed of 18–53 radial ribs, the 3–4 posterior ones being the strongest and more widely spaced. Other three previously unknown species are illustrated but not formally named due to the lack of well-preserved articulated shells.


Introduction
Cuspidariidae Dall, 1886 is a family of marine carnivorous bivalves (Allen & Morgan 1981;Morton 1981). Most species are less than 10 mm in length, and many of them are rare, only known by their original descriptions and few subsequent records. This rarity plus the fragility of the cuspidariid shells led to the scarcity of available specimens in scientifi c collections, where most species are represented only by few and usually broken shells.
Due to the numerous misidentifi cations in literature and in museum collections, many species are still not described or even recognized as new to science. A more complex problem of identifi cation appears in Cardiomya A. Adams, 1864, where species are very similar, most of them with shells displaying few and variable characters.
Nevertheless, there are rare and little known Cardiomya still hidden on museums' shelves. Remarkable, most of them have been lumped with C. cleryana. The present study aims at reviewing these species.

Material and methods
We examined all available Cardiomya material from the Atlantic ocean found in the following institutions: Specimens were compared with the type material and/or original descriptions. For the unknown species, only that with well preserved and articulated specimens are proposed as new.

CMPHRM-
Literature records were critically assessed and an updated geographical distribution of the species considered is provided. Records originating from checklists, unvouchered material, and non fi gured or properly described specimens are not included. Each species is compared with the most similar species or to those species with which they have been confused in the literature and/or in malacological collections. A pictogram illustrates the terminology used for the shell structures ( Fig. 1). Higher taxonomy follows Gofas (2015). Species descriptions are based on original description and type/ordinary material examined. Table 1 summarizes the characters of the Cardiomya species studied here.

Description
Shell small (maximum length 10.5 mm; maximum height 8 mm). Shell fl ank triangular, umbones blunt. Anterodorsal margin descending, anterior margin rounded little extended, ventral margin rounded, crenulated. Rostrum very short to short, posteroventral sinuation shallow, posterodorsal margin slightly curved upwards, not extending above the umbones. External sculpture of 17-25 almost equally spaced radial ribs of varied expression on the shell fl ank; rostrum with one to three radial lines or only commarginal growth lines.

Remarks
Cardiomya ornatissima differs from C. cleryana by its straight posterodorsal margin by having only and 6-7 strong radial ribs, which may be followed or not by incomplete ribs. Cardiomya perrostrata can be distinguished from C. cleryana by its long rostrum, rounded shell fl ank, and the presence of a shoulder. Cardiomya striata can be distinguished from C. cleryana by its quadrangular shell fl ank, well marked shoulder, and radial ribs covering the entire shell, including the rostrum.
The record of C. simillima to Malvinas/Falkland Islands was uncritically replicated by many authors (e.g., Powell 1951;Figueiras & Sicardi 1970;Rios 2009;Barroso et al. 2016;Machado et al. 2016). Nevertheless, subsequent records from Malvinas/Falkland Islands were never provided. As has happened to others species records from the Terra Nova expedition (for details see Scarabino 2003b;Signorelli et al. 2019), it is possible that a labeling error occurred in the record of C. simillima from station #38 (Malvinas/Falkland Islands, 229 m). Taking into account the evidence of tag scrambling and also the biogeographical implications of such an unusual distribution, the record of station #38 from Terra Nova expedition to C. simillima (= C. cleryana) should be viewed with caution.

Description
Shell small (maximum length 9.3 mm; maximum height 6.3 mm). Shell fl ank trapezoidal, umbones blunt. Anterodorsal margin forming a shoulder, anterior margin little extended, ventral margin rounded, crenulated. Rostrum short, tapering towards the tip, posteroventral sinuation unobtrusive, posterodorsal margin curved upwards, extending above the umbones. External sculpture of 18-35 radial ribs, weak and closely spaced anteriorly; becoming stronger posteriorly, the last three to four being the strongest and most widely spaced, the posterior most rib separated from the previous ones; commarginal growth lines well marked; rostrum with one to three radial lines.

Remarks
Cardiomya striolata (Locard, 1897), was usually regarded as a junior-synonym of Cardiomya costellata (Deshayes, 1835), with more strongly ribbed ornamentation (see Dall 1886;Salas 1996;Porcheddu et al. 1997). However, in our opinion C. striolata is a distinct, full species, which differs from C. costellata by its tapering rostrum which extends above the umbones, trapezoidal shell fl ank, anterodorsal margin forming a shoulder, and unobtrusive posteroventral sinuation. Cardiomya costellata was described based on fossil specimens from the Tertiary of Greece (Fig. 4I). Since its original description, several incorrect records of C. costellata have been made in a series of erroneous taxonomic identifi cations of different species (e.g., Philippi 1844;Abbott 1974;Abbott & Morris 1995;Redfern 2001;Ardovini & Cossignani 2004;Lee 2009;Morton 2016;Oliver et al. 2016;Lamy & Pointier 2018). Literature records of true specimens of C. striolata could be confi rmed in references of C. costellata accompanied by fi gures, detailed descriptions or cross-referenced with vouchers in collections. Additionally, many specimens of C. striolata labeled as C. costellata were found among museums holdings.
Considering the specimens from Brazil, Cardiomya striolata has often been misidentifi ed in the literature: [1] part of the specimens listed as C. cleryana in Barroso et al. (2016: 6), as well as the fi gured specimen in Barroso et al. (2016: 7, fi g. 5).
Cardiomya striata can be distinguished from C. striolata by its rectangular shell fl ank and entire shell (including the rostrum) ornamented with radial ribs.

Etymology
This species is named after the character Minerva McGonagall, from the Harry Potter series of books written by the British J.K. Rowling. The name is employed as a noun in apposition.

Description
Shell small (length 4.3 mm; height 3.2 mm). Shell fl ank quadrangular, umbones raised. Anterodorsal margin forming a shoulder, anterior and ventral and margin rounded, little angulated. Rostrum very short, truncated, posteroventral sinuation shallow, posterodorsal margin curved upwards, extending above the umbones. External sculpture of six radial ribs on the posterior half of the shell fl ank; rostrum with commarginal growth lines only.

Geographic distribution
Only known from the type locality.

Remarks
The most similar species to Cardiomya minerva sp. nov. are Cardiomya surinamensis van Regteren Altena, 1971 (Fig. 5G-H) and Cardiomya cadiziana Huber, 2010 (Fig. 5I-J). These two species can be distinguished from Cardiomya minerva sp. nov. by their trapezoidal shell fl ank, external sculpture composed of incomplete radial ribs on anterior region of the shell fl ank. Additionally, C. cadiziana exhibits a shorter rostrum, with several faint radial lines, and straight posterodorsal margin, not extending above the umbones.

Description
Shell small (length 7.8 mm; height 6.1 mm). Shell fl ank elliptical, umbones blunt. Anterodorsal margin forming a shoulder, anterior margin truncated, ventral margin rounded, crenulated. Rostrum very short, truncated, posteroventral sinuation shallow, posterodorsal margin curved upwards, extending a little above the umbones. External sculpture of 22 radial ribs, less spaced towards the anterior region, the four strongest and most widely spaced ribs at the posterior end of the shell fl ank; rostrum with four radial lines.

Description
Shell small (length 5.8 mm; height 4.1 mm). Shell fl ank triangular, umbones blunt. Anterodorsal margin descending, anterior margin extended, ventral margin rounded, crenulated. Rostrum very short, posteroventral sinuation unobtrusive, posterodorsal margin slightly curved upwards, not extending much above the umbones. External sculpture of 17 radial ribs, many incomplete on the middle of the shell fl ank, followed by three strongest and most widely spaced complete radial ribs; rostrum with commarginal growth lines only.

Description
Shell small (length 6.2 mm; height 4.2 mm). Shell fl ank triangular, umbones blunt. Anterodorsal margin descending, anterior margin angulated, ventral margin rounded. Rostrum short, posteroventral sinuation shallow, posterodorsal margin slightly curved upwards, not extending much above the umbones. External sculpture of seven weak and incomplete radial ribs, which vanish towards ventral margin, followed by six complete ribs on the posterior half of the shell fl ank; rostrum with commarginal growth lines only.

Discussion
The discovery of undescribed Cardiomya species hidden among the museum shelves in Brazilian collections reinforces the need for more taxonomic studies in this area. The combination of a reduced number of specialists, rarity of species and high cost of fi eld expeditions hampers the improvement of our knowledge on marine biodiversity. It is not surprising that a large number of Cardiomya are still waiting proper description. But these limitations, especially the rarity of some species, should not prevent our attempts to understand and recognize them. Describing such species and their unique characteristics is the fi rst step to understand their importance as components of natural ecosystems. It is worth noting that we do not comply with 'taxonomic splitting', which can lead to a rise in unwanted invalid names. We are aware that larger number of specimens allows for a more precise description of the full phenotypical variation for a species. Nevertheless, the cost of describing a rare species based on few specimens with noticeable diagnostic features seems lower than the cost of not knowing it at all. With predictions of massive extinctions in the near future, the next generation of taxonomists may be describing extinct species, only known from material preserved on the shelves (Fontaine et al. 2012).