Revised Morphological Characteristics of Species of Two Ostracod Genera Loxoconcha Sars, 1866 (Loxoconchidae) and Xestoleberis Sars, 1866 (Xestoleberididae)


 Total 23 species of the genus Loxoconcha and 21 species of the genus Xestoleberis were presented in this study. A scanning electron microscope for observing muscle scars of the carapaces, and a stereoscopic microscope for dissecting, observing and sketching the appendages were used. The results show that most species of the genus Loxoconcha consist of four adductor scars to arrange in a curved sub-vertical row and concave anteriorly, except L. pulchra carrying five, in which the top one is divided into two. The frontal scars with C-shape, opening anteriorly are presented in most of Loxoconcha species, however, the frontal scar with bean-shape or oval-shape is discovered in five species of L. japonica group. Most of L. kosugii bear the frontal scar with C-shape, but the frontal scar with Y-shape is found in several individuals. This phenemenon shows the close phylogeny between the genus Loxoconcha and Palmoconcha. About the genus Xestoleberis, chaetotaxy of setae on all appendages, except setae of exopodite on the maxillula is a typical character of this genus. The published fossil records and the tendencies of change in the number of setae on the maxillula and mandible among three phylogenetic groups suggest that Group A or B is an ancestor of the genus Xestoleberis, and Group C is a derived group.


Introduction
Ostracods are small bivalve crustaceans and living in various aquatic habitats. Their biodiversity is very high, including many different genera and families. Loxoconcha Sars, 1866 (Loxoconchidae) and Xestoleberis Sars, 1866 (Xestoleberididae) are the most diverse ostracod genera. A total of 575 Loxoconcha species and 344 Xestoleberis species have been recorded around the world (Brandão et al. 2015). The members of these two genera are distributed in low to middle latitude areas in marine and brackish waters (Kempf 1986a(Kempf , 1986b. The body structure of ostracoda consists of carapaces ouside and soft parts inside. Carapaces of ostracods are closed by adductor muscle scars running through the central part of the body and attached to the inner surface of the calci ed outer lamellae (Fig. 1). The adductor scar pattern is another important taxonomic character, particularly useful at superfamily level. As well as a pattern of adductor scars, there may be frontal scars and a pair of "mandibular scars" which are not muscle scars but the points of attachment of chitinous rods which, together with the mandible, form a tripod with its apex butted against a fulcra point on the inside of the valve (Fig. 1) (Horne et al. 2002). In the Ostracoda, carapaces not only give protection against external impact; but also provide some important functions such as respiration and osmoregulation (Yamada et al. 2004). The soft parts of ostracoda are enveloped inside the carapace, so functions such as locomotion and feeding require that the appendages be extended outside the carapace (Tanaka and Tsukagoshi 2013). The maxillula (referred as maxilla or the rst maxilla by some authors) is the fourth head appendage of ostracods. It lies immediately behind the mandible and has two main functions, i.e., feeding and, in some groups, respiration (Fig. 2) (Athersuch et al. 1989;Horne et al. 2002). The mandibles are the third pair of appendages and in most ostracods each has a strong, heavily sclerotized coxa, provided ventrally with a number of teeth. The main functions of mandibles are for feeding, crawling or digging (Fig. 2) (Athersuch et al. 1989).
The muscle scars of carapace, maxillula and mandible of the soft parts play very important roles in classifying living ostracodas because of their signi cant taxonomic characters. Therefore, the aims of this paper are to document in comprehensive way morphological characteristics of the muscle scars of the genus Loxoconcha, and of maxillula and mandible of the genus Xestoleberis. These results are useful for taxonomic utility of the two genera.

Materials And Methods
Samplings were mainly carried out on reef slopes using SCUBA diving, on reef ats, tidal beaches and river mouths during low tide at some localities in the Okinawa Island, Okinawa Prefecture, southern Japan ( Fig. 3 and Appendix 1) and in Vietnam ( In each site (Figs. 3, 4), sediments (sand, rubble dead coral, silt…), sea weeds, sponge… were collected and put into a plastic bottle using a scoop. Super cial sediments were colected from the uppermost 5 mm of the active layer. Then, all of the collected specimens were xed in 5-10% formaldehyde neutralized with hexamethylenetetramine, before being washed through 16-mesh (# 1 mm) and 250-mesh (# 0.063 mm) sieves. Part of the washed material containing ostracods and small sediment was xed with 70-80% alcohol for later observations of the appendages, and the remaining material was dried.
All the specimens were dissected under a stereoscopic microscope in the laboratory. Appendages were used to calculate the number of setae on the maxillula, mandible and sketched using a differential interference contrast microscope with a camera lucida (BX-50, OLYMPUS). Dried valves were coated with gold using a quick auto-coater (JFC-1500, Ion Sputtering Device) and were then observed with a scanning electron microscope (SEM) (JSM-5600LV, JEOL). SEM photos were used to measure the type of pore on the carapace, the number of adductor scars, the shape of frontal scar… All the illutrated specimens are deposited in the collections of the Shizuoka University Museum (Japan) and are identi ed by numbers with the pre x SUM-CO.

Results And Discussion
Muscle scars of the genus Loxoconcha Muscle scars of a total of 23 species of the genus Loxoconcha were observed in this study (Table 1 and Appendix 2). Most of these species consist of four adductor muscle scars arranging in a weakly curved sub-vertical row or a curved sub-vertical row and concave anteriorly. These are the typical characteristics of genus Loxoconcha (Athersuch and Horne 1984;Athersuch et al. 1989). However, observation of plural individuals of the species L. pulchra showed this species carries ve adductor muscle scars, in which the top one was divided into two (Appendix 2.13). This character may be a mutation phenomenon that was already mentioned by several authors (e. g., Higashi and Tsukagoshi 2012). It also can be regarded as apomorphy in Loxoconcha. The four adductor muscle scars are normally unequal dimensions with each other, the second scar from the top is much longer than the other in some species, e. g., Loxoconcha sp. 1, L. sp. 9, L. sp. 30, L. mutsuensis, L. modesta, L. harimensis, L. sp. 8 and L. damensis (Appendix 2.2, 4, 6-8, 10-12). According to Athersuch and Horne (1984), Athersuch et al. (1989), frontal scars of the genus Loxoconcha are characterized with C-shape. In the present study, most of species of the genus Loxoconcha have the frontal scar with C-shape, opening anteriorly (Table 1 and Appendix 2). However, the frontal scar with bean-shape or oval-shape was discovered in the species of L. japonica group that was distinguished from other groups by carapace outlines, surface ornamentation patterns, hinge structures and muscle scar patterns (Tanaka and Ikeya 2002 Additionally, there was a variation about the shape of frontal scar in L. kosugii (Table 1, Appendix 2.14 and 3) in this study. Most individuals of L. kosugii bear the frontal scar with C-shape (Appendix 3.1-3), but the frontal scar with Y-shape was found in several individuals of this species (Appendix 3.4). The frontal scar with Y-shape is a typical character of the genus Palmoconcha whose phylogeny is close to the genus Loxoconcha (Ishii 2004). The mandibular scar of the Loxoconcha species generally includes two scars with a lengthened circle shape or sub-circle shape. The dimension of two mandibular scars is somewhat equal or unequal each other.
Maxillula, mandible of the genus Xestoleberis and some initial assumptions about evolutionary trend according to the subgroups of this genus The maxillula and the mandible of a total of 21 Xestoleberis species were observed in the present study, including 18 species inhabited in Japan coast (including the Okinawa Islands) and 3 species (X. sp. 7, X. vietnamensis and X. munensis) in Vietnam (Figs. 5, 6, Table 2 and Appendix 1). Also, data on the chaetotaxy of the maxillula and the mandible of 3 species was referred completely from the previous studies, of 5 species were both referred and newed, and of 13 species were showed herein for the rst time ( Table 2). The checked results of several soft appendages indicated that the number of setae of the outer rst podomere of the endopodite on the maxillula ranges from 2 to 5, of three endites on the maxillula from 11 to 14, of exopodite on the maxillula from 13 to 17 and of the third podomere of mandibular endopodite from 2 to 6 ( Table 2). The above morphological characteristics of maxillula and mandible have almost no difference compared to previous studies except the total number of setae of exopodite on the maxillula. Following to Smith et al. (2005), this number of the family Xestoleberididae is 16. Based on the combination of the morphological types of pore systems (Sato and Kamiya 2007), the 21 species of Xestoleberis were divided into three groups. The Group A has both sieve-type and lip-type pores and comprises 15 species. The Group B has only sieve-type pore, and consists of two species. The Group C has simple-type and sieve-type pores and includes four species ( Table 2). The pore types of the genus Xestoleberis were referred from Puri (1974). The number of setae of the rst podomere of maxillulan endopodite is low in the Group C (average of 2 setae) and high in the Group B (average of 4 setae) and the Group A (average of 4 setae). The number of setae of the third podomere of the endopodite on the mandible is constant among species of each group, and this number is low in the Group C (2 setae), median in the Group A (4 setae) and high in the Group B (6 setae). On the other hand, the total number of setae of three endites and of exopodite on the maxillula varies among species within each group, and there are no differences in the two characters among three groups ( Table 2).