Maxilliclausia Kim 2014, n. gen.

Maxilliclausia n. gen.

Diagnosis (based on female). Body cylindrical, consisting of cephalothorax, second to fifth pedigerous somites, genital somite, and 4 abdominal somites. Caudal ramus with 6 setae. Egg sac containing mutiserial eggs. Antennule 5- or 6-segmented; first segment with 4 setae. Antenna 4-segmented, non-prehensile, and consisting of coxa, basis, and 2-segmented endopod; terminal segment (second endopodal segment) blunt at tip, with setae, and lacking claw or spine. Labrum with elongate, digitiform posteromedian process. Mandible with extremely elongate distal spine. Maxillule lobate, with adhesion pad and setae. Maxilla, as an attachment organ, large and 2-segmented; second segment transformed to sucking disk. Maxilliped absent. Legs 1 and 2 with 2-segmented rami; coxa without inner element. Leg 3 lobate, bearing 3 setae. Leg 4 absent. Leg 5 represented by 2 setae.

Type species. Maxilliclausia propria n. sp.

Etymology. The generic name Maxilliclausia is the combination of the “maxilla”, a mouthpart and “ Clausia ”, the name of the type genus. It alludes to the possession by the new genus of a characteristic maxilla.

Remarks. With the possession of 2-segmented rami of legs 1 and 2, a rudimentary leg 3 consisting of a lobe tipped with three setae, and the absence of leg 4, the new genus Maxilliclausia can be placed near the genera Clausia Claparède, 1863, Spionicola Bjornberg and Radashevsky, 2009 and Sheaderia Kim, Sikorski, O’Reilley and Boxshall, 1013. Spionicola, in particular, is noticed, because its only species S. mystaceus Bjornberg and Radashvsky, 2009 is known as an associate of a polychaete living on molluscan shells in Brazilian waters (Bjornbeg and Radashevsky, 2009). However, Maxilliclausia n. gen. hardly can be considered to be a relative of these genera, due to a number of its autapomorphic morphological features. The most significant autapomorphy is displayed by its maxilla in which the distal segment is transformed to a sucking disk. This form of maxilla has not been reported in the Clausiidae and its related families. In the Clausiidae the key limbs for securing attachment to the host are the maxillae and/or maxillipeds, at least in females (Kim et al., 2013). In Maxilliclausia n. gen. the maxilla is transformed to function as an authentic attachment organ. In contrast, other possible attachment devices are suppressed to develop. For examples, the maxilliped is absent, the antenna does not carry any claw on distal segments, and the legs lack spines. The lacking of a maxilliped also is an autapomorphic feature, because all of known species in the Clausiidae are known of their maxillipeds, although this appendage is reduced to a setiferous lobe in some genera such as Pseudoclausia and Spinonicola (Bocquet and Stock, 1963; Bjornberg and Radashevsky, 2009) or a segmented vestige as in Pontoclausia antiqua (Kim, 2001b).

The presence of distal spine on the mandible is not uncommon in the Clausiidae. This spine is moderately long and tapering as in Mesnilia cluthae (T. and A. Scott, 1896) (see Kim et al., 2013) or short and stout as in Rhodinicola gibbosus Bresciani, 1964 (see Kim et al., 2013) and Megaclausia mirabilis O’Reilly, 1995 (see O’Reilly, 1995) or tapering and paired as in Likroclausia namhaensis Ho and Kim, 2003 (see Ho and Kim, 2003). However, unlike in these species, the spine in Maxilliclausia is markedly elongated, extending to the posterior margin of the maxilla. Therefore, the elongated distal spine of the mandible also is a characteristic feature of Maxilliclausia in consideration that in clausiids having a single distal spine on the mandible this spine is short, not longer than the proximal segment.

In summary, the new genus Maxilliclausia may be clearly defined by its following characteristic features: (1) the antenna is of an unusual form, non-prehensile, without any claw on distal segments, but with large adhesion pad on the two endopodal segments; (2) the labrum has a large, digitiform posteromedian process; (3) the mandible is armed with a very elongate distal spine; and (4) the distal segment of the maxilla is transformed to a sucking disk; and (5) the maxilliped is absent.