Thalassina Latreille, 1806

Hyžný, Matúš & Angeli, Antonio De, 2022, Mud lobster Thalassina Latreille, 1806 (Decapoda: Gebiidea: Thalassinidae), its Cenozoic occurrences in Italy and palaeobiogeography, Geodiversitas 44 (13), pp. 417-425 : 419-420

Genus Thalassina Latreille, 1806

TYPE SPECIES. — Thalassina scorpionides Latreille, 1806 , a junior synonym of Cancer (Astacus) anomalus Herbst, 1804 , by monotypy.

INCLUDED SPECIES. — See Table 1.

DIAGNOSIS. — Carapace laterally compressed with ocular and antennal spines on anterior margin. Rostrum moderately large; cervical groove distinct. Linea thalassinica extending whole length of carapace, convergent posteriorly, two horizontal and oblique suture posterior to cervical groove. Posterior margin of carapace with median concavity and dorsomedian process. Pleon longer than carapace, cylindrical, slender, pleonal pleura bordered with tubercles or denticles; pleonal sternites 2-5 with tubercles on transverse ridge between pleopods; telson longer than wide, unarmed. Pereiopods 1 and 2 subchelate, pereiopod 1 propodus with mesial and lateral tuberculate or spinuous carinae; pereiopod 2 ovate; pereiopods 3-5 simple (emended from Ngoc-Ho & de Saint Laurent 2009: 122).

COMMENTS

It has long been believed that the genus includes two species, T. anomala (Herbst, 1804) and T. squamifera De Man, 1915 , with several other nominal taxa considered as junior synonyms of T. anomala ( Holthuis 1991: 229) . More recent revisions based on morphological characters (Ngoc-Ho & de Saint Laurent 2009; Sakai & Türkay 2012) have shown an unexpected diversity of the genus. The genus currently consists of thirteen valid species ( Table 1 ).

Thalassina is characterised by subchelate pereiopods 1 and 2, with the pereiopod 1 propodus having tuberculate or spinous longitudinal carinae (ridges sensu Moh & Chong 2009) extending across it (Ngoc-Ho & de Saint Laurent 2009; Sakai & Türkay 2012). These carinae have a fairly good potential to be preserved in the fossil record and can be compared directly with the extant material as demonstrated recently by Ando et al. (2016).

The diagnosis of Thalassina presented above highlights the characters with good fossilization potential. In this respect, pereiopods 1 are of particular interest. They are often unequal in males, sometimes in females, and have similar morphology in both except for a stouter propodus and a larger proximal lower tooth on the dactylus of the major cheliped (Ngoc-Ho & de Saint Laurent 2009). The dactylus with longitudinal rows of tubercles is rather stout and is always at least twice longer than the fixed finger (Ngoc-Ho & de Saint Laurent 2009); such a condition is present only rarely in decapod crustaceans and together with the outline of distal elements of pereiopod 1 can be considered as a reliable character for attributing fossil material to the genus.

Importantly for palaeontological practice, the characters on pereiopods 1 (elements with the highest fossilization potential) have been recognised as taxonomically important on the species level (Ngoc-Ho & de Saint Laurent 2009; Sakai & Türkay 2012; Lin et al. 2016). For distinguishing species, a combination of characters on pereiopods 1 (e.g., the development of carinae) is always needed (Ngoc-Ho & de Saint Laurent 2009: table 2; Ando et al. 2016: table 2; Ando et al. 2019: table 1). The outline of the propodus appears not to be sufficient for attribution on the species level due to intraspecific variation (Ngoc-Ho & de Saint Laurent 2009). In general, two basic morphotypes can be recognized, the robust one and the slender one; in the slender morphotype the palm is three or more times longer than high (e.g., in Thalassina pratas Lin, Komai & Chan, 2016 ; T. tsuyamensis Ando & Kishimoto in Ando, Kishimoto & Kawano, 2016; or T. emerii Bell, 1844 ). It must be noted, however, that these forms represent two extremes of a continuous range of variability rather than a stable pattern. Consequently, the propodus outline cannot be considered as taxonomically important if evaluated on a handful of specimens because both, sexual dimorphism and intraspecific variation, may have an impact on the morphology of pereiopods 1 ( Ngoc-Ho & de Saint Laurent 2009; Sakai & Türkay 2012).