Zoological Science

In bryozoans (phylum Bryozoa), representative colonial animals mostly found in marine environments, some species possess different types of individuals (heterozooids) specialized in different functions such as defense or structural support for their colonies. Among them, the best-known heterozooids are the avicularia, known to function as defenders. The differentiation processes of heterozooids, including avicularia, should be important keys to understand the evolutionary significance of bryozoans. However, the developmental process of avicularium formation remains to be fully understood. In this study, therefore, in order to understand the detailed developmental process and timing of avicularium formation, extensive observations were carried out in a bryozoan species, Bugulina californica (Cheilostomata, Bugulidae), that possesses adventitious avicularia, by performing stereomicroscopy on live materials, in addition to scanning electron microscopy and histological observations. The whole process can be divided into seven stages based on developmental events. Especially notably, at the earlier stages, there are three major budding events that produce proliferating cell masses corresponding to primordial tissues: (1) budding of the peduncle cushion at the outer margin of the distal part of a young autozooid, (2) budding of the head-part primordium from the peduncle cushion, and (3) budding of the polypide inside the head part. Experimental control of temperature showed that 20°C would be the best to maintain B. californica colonies.

Many plant-sucking stinkbugs possess a specialized symbiotic organ with numerous crypts in a posterior region of the midgut. In stinkbugs of the superfamily Pentatomoidea, specific γ-proteobacteria are hosted in the crypt cavities, which are vertically transmitted through host generations and essential for normal growth and survival of the host insects. Here we report the discovery of an exceptional gut symbiotic association in the saw-toothed stinkbug, Megymenum gracilicorne (Hemiptera: Pentatomoidea: Dinidoridae), in which specific γ-proteobacterial symbionts are not transmitted vertically but acquired environmentally. Histological inspection identified a very thin and long midgut symbiotic organ with two rows of tiny crypts whose cavities harbor rod-shaped bacterial cells. Molecular phylogenetic analyses of bacterial 16S rRNA gene sequences from the symbiotic organs of field-collected insects revealed that (i) M. gracilicorne is stably associated with Pantoea-allied γ-proteobacteria within the midgut crypts, (ii) the symbiotic bacteria exhibit a considerable level of diversity across host individuals and populations, (iii) the major symbiotic bacteria represent an environmental bacterial lineage that was reported to be capable of symbiosis with the stinkbug Plautia stali, and (iv) the minor symbiotic bacteria also represent several bacterial lineages that were reported as cultivable symbionts of P. stali and other stinkbugs. The symbiotic bacteria were shown to be generally cultivable. Microbial inspection of ovipositing adult females and their eggs and nymphs uncovered the absence of stable vertical transmission of the symbiotic bacteria. Rearing experiments showed that symbiont-supplemented newborn nymphs exhibit improved survival, suggesting the beneficial nature of the symbiotic association.

Copepods in the family Dirivultidae are one of the most successful meiofauna in deep-sea hydrothermal vent fields and are abundant near venting fluid. Although vents are spatially limited ocean habitats, they are distributed widely in the Atlantic, Pacific, and Indian Oceans. However, knowledge of dirivultid biogeography and phylogeography remains limited, especially in the northwestern Pacific. Here, we obtained partial mitochondrial COI gene sequences of three dirivultids from the northwestern Pacific—Stygiopontius senokuchiae and an unidentified Chasmatopontius species from vent fields in the Izu–Bonin Arc and Stygiopontius senckenbergi associated with the squat lobster Shinkaia crosnieri in the Okinawa Trough—and analyzed them in comparison with existing data. The among-species sequence diversity exceeded 80 out of 560 bp (14% or 0.166 in Kimura 2-parameter distance), whereas the within-species diversity was less than 10 bp (2% or 0.018 in Kimura 2-parameter distance), with no genetic saturation. Each species formed a monophyletic clade and the genetic region targeted is deemed reliable for identifying species and populations for these copepods. Among the three genera targeted, only Chasmatopontius formed a monophyletic cluster, while Aphotopontius and Stygiopontius did not. Species delimitation analyses suggested the existence of cryptic species in Chasmatopontius. Subdivision among local populations was observed in Aphotopontius, but not in Stygiopontius in the same distribution, implying potential differences in dispersal ability among different genera of dirivultids. Further sampling is required, to fill the spatial gaps to elucidate the biogeography and evolution of dirivultids in the global deep ocean.

A Japanese piscivorous chub, “hasu” (Opsariichthys uncirostris), and its main prey, “ayu” (Plecoglossus altivelis), both have laterally asymmetric bodies, similar to other fishes; each population consists of righty morphs and lefty morphs. This antisymmetric dimorphism has a genetic basis. Temporal changes in the ratios of laterality (i.e., frequency of righty morphs in a population) of these predator and prey fish species were investigated for a 20-year period at a pelagic site in the southwestern area of Lake Biwa, Japan. The dimorphism of each species was maintained dynamically throughout the period, and the ratio of laterality was found to change periodically in a semi-synchronized manner. Direct inspection of the relationship between the ratios of laterality of the two species indicated that the ratio of ayu followed that of hasu, suggesting that the predator–prey interaction was responsible for the semi-synchronized change. Stomach contents analysis of each hasu revealed that cross-predation, in which righty predators catch lefty prey and lefty predators catch righty prey, occurred more frequently than the reverse combination (parallel-predation). This differential predation is presumed to cause frequency-dependent selection on the two morphs of the predator and prey, and to drive semi-synchronized changes in the laterality of the two species. Some discussion pertaining to the atypical form of the semi-synchronized change in laterality found in this study is presented from the viewpoint of predator–prey interaction in fishes.

Growth-retarded (grt) mice display primary congenital hypothyroidism due to the hyporesponsiveness of their thyroid glands to thyroid-stimulating hormone (TSH). We examined somatic growth, anterior pituitary development, and hormonal profiles in female grt mice and normal ones. Although growth in grt females was suppressed 2 weeks after birth, the measured growth parameters and organ weights gradually increased and finally reached close to the normal levels. Grt mice exhibited delayed eye and vaginal openings and remained in a state of persistent diestrus thereafter, plasma estrogen levels being lower than those in normal mice. Grt mice that received normal-donor thyroids showed accelerated growth and their body weights increased up to the sham-normal levels, indicating the importance of early thyroid hormone supplementation. In the anterior pituitary, there were fewer growth hormone (GH) and prolactin (PRL) cells in grt mice than in normal mice as examined at 12 weeks after birth, but the numbers of these cells did not differ from those in normal mice after 24 weeks. Grt mice had more TSH cells than normal mice until 48 weeks. Plasma GH levels in grt mice were lower than those in normal mice at 2 weeks, but did not differ substantially after 5 weeks. Compared with normal mice, grt mice had significantly lower plasma PRL and thyroxine levels, but notably higher TSH levels until 48 weeks. These findings indicate that thyroid hormone deficiency in grt mice causes delayed development and growth, and inappropriate development of GH, PRL and TSH cells, followed by the abnormal secretion of hormones by these pituitary cells.

Hybridization is one of the negative outcomes for the introduction of non-native species, which can lead to rapid displacement and genetic extinction of native species. Salmonid fishes have been widely introduced outside of their native ranges for food supply and recreational fishing. Here, we investigate the occurrence of introgressive hybridization among native Dolly Varden (Salvelinus curilus (syn. Salvelinus malma)), white-spotted charr (Salvelinus leucomaenis), and introduced brook trout (Salvelinus fontinalis), in streams of the Nishibetsu River, Hokkaido, Japan. Microsatellite DNA analysis detected five hybrids between native Dolly Varden and introduced brook trout. This is the first evidence for hybridization between native Dolly Varden and introduced brook trout, while the latter has been known to hybridize with many other salmonids. Furthermore, incongruence between mitochondrial DNA and microsatellite DNA analyses suggested introgression among the three Salvelinus species. Further studies to estimate the hybrid fitness are necessary to understand how hybridization among the three species affects the native species.

Holothuria atra is a black sea cucumber commonly found on the sandy bottom of Okinawan coral reefs. The body surface of H. atra is usually covered with sand; however, sand never covers the body of another black congener, Holothuria leucospilota, which is sympatrically distributed with H. atra. The epidermal structures were examined in these two species by means of transmission electron microscopy to determine how sand adheres to the surface of H. atra. While the epidermis was basically composed of support cells bearing microvilli and vacuolated cells probably corresponding to mucus cells, two types of granular cells, type 1 and 2, were also found at the tip of the tube feet. These granular cells were closely similar in structure to secretory cells that have been supposed to secrete adhesive substances in other holothurians. Type 1 granular cells were also found in the dorsal epidermis of H. atra but not in H. leucospilota. Therefore, adhesive secretion by type 1 granular cells probably enables the attachment of sand to the H. atra body.

The Tsushima salamander, Hynobius tsuensis, is a lotic-breeding species endemic to Tsushima Island, Japan. We investigated genetic variation in this species using samples covering all known localities on the island. Phylogenetic analyses based on mitochondrial cytochrome b gene sequences divided H. tsuensis populations into two genetic groups: true H. tsuensis and an unknown species (Hynobius sp.) showing a closer relationship to Hynobius nebulosus than to H. tsuensis. The genetic distance between the two groups was greater than the genetic distances between different species of Hynobius. Both groups were found to breed in mountain streams, sometimes in the same streams syntopically. Population structure analysis based on several nuclear microsatellite loci strongly indicated reproductive isolation between the two groups in the syntopic localities. This is the first report confirming the syntopic occurrence of two lotic salamanders of Hynobius on a small island. We propose that the lotic breeding habits of the two salamanders were independently acquired.

We present a complete mitochondrial genomic sequence for the tanaidacean Arctotanais alascensis (Richardson, 1899); this is the first complete mitogenome reported from the order Tanaidacea. The mitogenome is 13,988 bp long and contains 13 protein coding and two ribosomal RNA genes (as is typical for animal mitogenomes), and 21 of 22 transfer RNAs; we did not detect an isoleucine transfer RNA (trnI) gene. The gene order differed markedly from the hypothetical ground pattern for Pancrustacea; only four clusters (trnM + nad2; trnC + trnY + cox1 + trnL2 + cox2; trnD + atp8 + atp6 + cox3; trnH + nad4 + nad4l) ancestrally present were retained. In a malacostracan phylogenetic tree reconstructed from mitogenome data, basal relationships were marginally supported or incongruent with the traditional morphology-based classification and the latest phylogenetic reconstructions from large transcriptomic datasets. Relationships involving more recent divergences were better supported in our tree, suggesting that complete mitogenome sequences are more suitable for phylogenetic analyses within malacostracan orders, presumably including Tanaidacea.

In this paper, two new brackish-water species of the macrostomid turbellarian genus Macrostomum, Macrostomum pseudosinense sp. nov. and Macrostomum taurinum sp. nov., collected from coastal water at Shenzhen, Guangdong Province, China, are described based on morphological, histological, and molecular phylogenetic analyses. Macrostomum pseudosinense sp. nov. differs from similar species within the genus in the length of the stylet (152 ± 15.0 µm), diameter of stylet opening (20 ± 4.0 µm proximally; 7 ± 0.5 µm distally), two bends of the stylet, and the non-spiral end of the stylet. Macrostomum taurinum sp. nov. differs from its congeners in the length of the stylet (81 ± 7.4 µm), the stylet bending position and angle (50% and 60°), diameter of stylet proximal opening (15 ± 3.0 µm), sperm with bristles and brush, and the smooth-walled ovaries. Phylogenetic analyses inferred from nuclear 18S and 28S rRNA genes support the establishments of these two new species. In addition, reciprocal mating behavior of M. pseudosinense sp. nov. was observed and documented.

We describe the cypridoidean ostracod Heterocypris spadix sp. nov. from brackish water on Okinawa Island, Japan. The species closely resembles Heterocypris salina ( Brady, 1868 ) but differs in that (1) the marginal infolds on valves are less developed, (2) the tubercles on the anterior margin of the right valve are completely covered by the selvage and invisible in inner view, and (3) the calcified inner lamella on the ventral margin of the left and right valves is scarcely evident in inner view, as the ventral margins of the valves bend inwardly. We determined partial sequences for the cytochrome c oxidase subunit I (COI; cox1) and 18S rRNA genes in H. spadix for future DNA barcoding and phylogenetic analyses. Our sample contained only females. A breeding experiment revealed that H. spadix females reproduce parthenogenetically. Another experiment showed that H. spadix has low tolerance to desiccation, with all individuals at 25°C dying between 1–2 hours after removal from water. We amplified and sequenced a partial 16S rRNA sequence for the endosymbiotic bacterium Cardinium from H. spadix. Infection by Cardinium may be related to the parthenogenetic reproductive mode we observed in H. spadix.