How is a giant sperm ejaculator formed? Development of the Zenker organ after the last moult in Pseudocandona marchica (Crustacea, Ostracoda, Candonidae)

Three Loxocaudinae species (Ostracoda, Podocopida) from South Korea

For many ostracod groups in Korea, published records are missing or are very limited. Loxocaudinae is one such subfamily, with only one named species, Loxocaudaorientalis Schornikov, 2011 reported from Korea. Having fewer than 50 species, this subfamily can be considered a small ostracod group, with most of the species known only by their shell morphology. The diagnoses of genera are based on the shell characters that are often homoplastic, and soft body appendages that are difficult to observe, such as the mandibular exopodite. Because of this, the validity of the entire subfamily and some of its genera have been questioned. Here three Loxocaudinae species were collected from the marine macrobenthic assemblages from Korea. Two are new and belong to the genus Glacioloxoconcha Hartmann, 1990, previously known only from Antarctica: Glacioloxoconchajeongokensis sp. nov. and Glacioloxoconchajisepoensis sp. nov. Loxocaudaorientalis is briefly redescribed, with some of the populations having unusual morphological features. COI and 18S rRNA sequences of all three species are provided and the latter marker used to assess the position of the subfamily within the family Loxoconchidae and the superfamily Cytheroidea. The resulting tree shows that within the family Loxoconchidae, the genera Glacioloxoconcha and Loxocauda Schornikov, 1969 are the most closely related, with very shallow but well-supported branches. Polyphyletic and paraphyletic natures of several Cytheroidea families are discussed, inferred from the reconstructed phylogeny.

Morphological and molecular diversity of Lake Baikal candonid ostracods, with description of a new genus

Uncoupling between molecular and morphological evolution is common in many animal and plant lineages. This is especially frequent among groups living in ancient deep lakes, because these ecosystems promote rapid morphological diversification, and has already been demonstrated for Tanganyika cychlid fishes and Baikal amphipods. Ostracods are also very diverse in these ecosystems, with 107 candonid species described so far from Baikal, majority of them in the genera Candona Baird, 1845 and Pseudocandona Kaufmann, 1900. Here we study their morphological and molecular diversity based on four genes (two nuclear and two mitochondrial), 10 species from the lake, and 28 other species from around the world. The results of our phylogenetic analysis based on a concatenated data set, along with sequence diversity, support only two genetic lineages in the lake and indicate that a majority of the Baikal Candona and Pseudocandona species should be excluded from these genera. We describe a new genus, Mazepovacandonagen. n., to include five Baikal species, all redescribed here. We also amend the diagnosis for the endemic genus Baicalocandona Mazepova, 1972 and redescribe two species. Our study confirms an exceptional morphological diversity of Lake Baikal candonids and shows that both Baikal lineages are closely related to Candona, but only distantly to Pseudocandona.

Correlation between investment in sexual traits and valve sexual dimorphism in Cyprideis species (Ostracoda)

Assessing the long-term macroevolutionary consequences of sexual selection has been hampered by the difficulty of studying this process in the fossil record. Cytheroid ostracodes offer an excellent system to explore sexual selection in the fossil record because their readily fossilized carapaces are sexually dimorphic. Specifically, males are relatively more elongate than females in this superfamily. This sexual shape difference is thought to arise so that males carapaces can accommodate their very large copulatory apparatus, which can account for up to one-third of body volume. Here we test this widely held explanation for sexual dimorphism in cytheroid ostracodes by correlating investment in male genitalia, a trait in which sexual selection is seen as the main evolutionary driver, with sexual dimorphism of carapace in the genus Cyprideis. We analyzed specimens collected in the field (C. salebrosa, USA; C. torosa, UK) and from collections of the National Museum of Natural History, Washington, DC (C. mexicana). We digitized valve outlines in lateral view to obtain measures of size (valve area) and shape (elongation, measured as length to height ratio), and obtained several dimensions from two components of the hemipenis: the muscular basal capsule, which functions as a sperm pump, and the section that includes the intromittent organ (terminal extension). In addition to the assessment of this primary sexual trait, we also quantified two dimensions of the male secondary sexual trait—where the transformed right walking leg functions as a clasping organ during mating. We also measured linear dimensions from four limbs as indicators of overall (soft-part) body size, and assessed allometry of the soft anatomy. We observed significant correlations in males between valve size, but not elongation, and distinct structural parts of the hemipenis, even after accounting for their shared correlation with overall body size. We also found weak but significant positive correlation between valve elongation and the degree of sexual dimorphism of the walking leg, but only in C. torosa. The correlation between the hemipenis parts, especially basal capsule size and male valve size dimorphism suggests that sexual selection on sperm size, quantity, and/or efficiency of transfer may drive sexual size dimorphism in these species, although we cannot exclude other aspects of sexual and natural selection.

Removal of extracellular coat from giant sperm in female receptacle induces sperm motility in Mytilocypris mytiloides (Cyprididae, Ostracoda, Crustacea)

Previous studies of cypridoidean ostracods have noted that (1) their giant spermatozoa are immotile inside the male, (2) these spermatozoa are motile in the female seminal receptacle and (3) these receptacles are often filled with empty sperm coats. Such findings have led previous authors to hypothesize that sperm must shed their coats in the female receptacle to become motile. We present light and electron microscopy results and video recordings of mating experiments with virgin specimens of Mytilocypris mytiloides. We show that the empty sperm coats frequently found in the female receptacles are not the result of sperm molting but are the resistant inner coats of exhausted sperm not used for egg fertilization. In contrast, we show that an outer granular coating material is successively removed from the sperm while resident inside the female receptacles before first oviposition occurs. During this period, previously immotile sperm gain motility, showing strong movement shortly before first oviposition takes place. By correlation of these phenomena, we suggest that dissolution of the outer coat material is required for motility to develop.