Monthly occurrence and feeding habits of larval and juvenile Ryukyu-ayu <Emphasis Type="Italic">Plecoglossus altivelis ryukyuensis</Emphasis> in an estuarine lake and coastal area of the Kawauchi River,Amami-oshima Island,southern Japan

We examined monthly occurrence patterns and feeding habits of larvae and juveniles of the critically endangered Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the lower reach of the Kawauchi River, Amami-oshima Island, southern Japan, for two successive years. The study area was classified into a brackish lake (Lake Uchiumi) and a coastal zone (Sumiyo Bay) based on monthly variations in water temperature and salinity. Larvae and juveniles occurred from December to April in the first year and from January to February in the second year, respectively. Smaller-sized and earlier developmental-staged P. altivelis ryukyuensis individuals tended to emerge in larger numbers in Lake Uchiumi than in Sumiyo Bay. The present data suggest that Lake Uchiumi, where the physical environment is characterized by lower temperature and salinity than in Sumiyo Bay, would be suitable as a nursery ground for this fish, especially for earlier developmental-staged larvae. No individuals ≥25.0 mm body length, which is the size at which they begin their upstream migration, occurred in Lake Uchiumi or Sumiyo Bay in April or May, when upstream migration is assumed to reach a peak based on previous studies, in either year. The peak of upstream migration, therefore, may have occurred several months earlier in the years covered in this study due to higher water temperature than in typical years. To assess the feeding habits of this fish, the gut contents were observed. In both areas, copepods were abundant in P. altivelis ryukyuensis diet with calanoids and Oithona spp. being the most abundant. The diet composition differed between Lake Uchiumi and Sumiyo Bay and clearer ontogenetic variation in the composition of the diet was observed in Lake Uchiumi. Small individuals fed on tintinnid ciliates, but the proportion of these organisms in the diet gradually decreased with growth. Large individuals fed exclusively on harpacticoid copepods and insects including demersal species. Overall, larval and juvenile P. altivelis ryukyuensis exhibited a generalist feeding habit that tended to increase with growth. It is indispensable to adequately manage the environmental conditions in the lower reach of the Kawauchi River, especially Lake Uchiumi, to conserve P. altivelis ryukyuensis population.     

Genetic characteristics of the amphidromous fish Ayu Plecoglossus altivelis altivelis (Osmeriformes: Plecoglossidae) on Yaku-shima Island in Japan,the southernmost population of the subspecies

Genetica - The Ayu (Plecoglossus altivelis altivelis) population on Yaku-shima Island, Kagoshima Prefecture, Japan represents the southernmost population of the subspecies and is considered to be...     

Geographical variation in echolocation call and body size of the Okinawan least horseshoe bat, Rhinolophus pumilus (Mammalia: Rhinolophidae), on Okinawa-jima Island, Ryukyu Archipelago, Japan

The Okinawan least horseshoe bat, Rhinolophus pumilus, is a cave-dwelling species endemic to the central and southern Ryukyus, Japan. We analyzed variation in the constant frequency (CF) of the echolocation call and in forearm length (FAL) of this species on Okinawa-jima Island on the basis of data for 479 individuals from 11 caves scattered over the island. CF values in samples from six caves, all located in the southwestern half of Okinawa-jima, were significantly higher than those in samples from five caves in the northeastern half of the island. Also, FAL was significantly greater in the latter group than in the former group, although the ranges of variation in this character substantially overlapped between the two groups. These results suggest substantial differentiation between R. pumilus populations on Okinawa-jima. The implications of our findings for the conservation of this endangered bat species are briefly discussed.     

Adherence of Flavobacterium psychrophilum on the body surface of the ayu Plecoglossus altivelis

Bacterial cold water disease in the ayu Plecoglossus altivelis caused by Flavobacterium psychrophilum is a serious problem in the Japanese freshwater culture industry. The distribution and activity of this bacterium on the body surface of the ayu in the infection process was investigated. The survival of F. psychrophilum in tap water showed that this bacterium might sustain its infectivity for 24 h. In an experimental infection, juvenile ayu were immersed in water containing 10(8.9) CFU/ml F. psychrophilum, and the progressing infection was followed by scanning electron microscopy during a 24-h period. This bacterium was observed in the ayu for 24 h adhering to the lower jaw and caudal peduncle, where the epidermis tissue was collapsed. This study showed that bacterial suspension in water sustains the activity of this bacterium. F. psychrophilum attaches especially to the jaw and caudal peduncle, growing at these sites, collapsing the dermal structure and invading the tissues.     

Patterns of coexistence of six anemonefish species around subtropical Okinawa-jima Island,Japan

Coral Reefs - The species diversity of anemonefish in tropical regions has been explained by the niche differentiation hypothesis or cohabitation hypothesis. However, anemonefish community...     

Development of free and canal neuromasts and their directions of maximum sensitivity in the larvae of ayu,Plecoglossus altivelis

Morphological changes in free neuromasts are reported from larvae of the Ayu,Plecoglossus altivelis. In newly-hatched larvae, free neuromasts were already recognizable in both the head and trunk. During larval growth, the number of free neuromasts increased, and the number of its sensory cells 2 days after hatching was constant. In the trunk, two types of free neuromasts, one with maximum sensitivity in the antero-posterior direction and the other with maximum sensitivity in the dorso-ventral direction, were observed. The former type predominated. In the head, free neuromasts were located around the eye and nose, their directions of maximum sensitivity forming lines tangential to concentric circles about the eye and nose. Distinct changes in free neuromasts occurred during the formation of the canal organ. The canal organ was first observed in the head region 64 days after hatching and in the trunk region 100 days after hatching. Concomitant with the formation of the canal organ, the profile of the cupulae of the free neuromasts changed from a flat bar to semispherical. Sensory cells in the canal neuromasts did not differ morphologically from those in the free neuromasts. It is considered that there is a close relationship between the sensitivity of the neuromast and the shape of the cupula, i.e., that the free neuromasts are adapted to slow water flow, as in lakes and the sea, while the neuromasts in the canal organ are adapted to rapid water flow.     

Seasonal dynamics of the fish parasite Neoplagioporus ayu (Digenea) in its definitive host, Plecoglossus altivelis, in the Chikugo River, Kyushu, Japan

Seasonal dynamics of the fish parasite Neoplagioporus ayu (Digenea; Opecoelidae; Plagioporinae) in its definitive host, the ayu Plecoglossus altivelis, in the Chikugo River, Kyushu, Japan, was examined. Natural ayu are amphidromous. In the upper Chikugo River, ayu fry raised in hatcheries are released into the river every spring because the migration of ayu is completely blocked by a dam that has no fish-pass channel. Ayu collected in April 2003 (within 1 month after release) harbored no N. ayu, but the prevalence of N. ayu rose to about 80% in May. The prevalence, abundance, and maturity of N. ayu were maintained at high levels from June to September. These finding indicate that the life cycle of N. ayu is completed in a freshwater area, although its intermediate hosts have not yet been identified despite the intense examination of invertebrates.     

Utilization of α-starch in ayu, Plecoglossus altivelis, relating to growth and body composition

To examine the possibility of dietary α‐starch in reducing feed costs in a practical diet, α‐starch was supplemented at 10, 20, 30 and 40% in a composed diet having the same protein level. The four diets were fed to ayu, Plecoglossus altivelis (initial weight 9.1 g) for 43 days. Growth and feed efficiency increased with the supplement, with values highest in the 30–40%α‐starch diet. The level of dietary α‐starch did not affect the proximate muscle composition; although the hepatosomatic index was not affected, liver glycogen increased with increasing dietary α‐starch. The dietary α‐starch did not influence evacuation time from the gut, and was well digested through passage in the gut, mainly between the stomach and the anterior part of the intestine. Ayu have an ability to adapt their metabolism to high dietary α‐starch, and can digest 40% or more in a composed diet. Although the muscle lipid content did not change, the fatty acid composition was influenced by dietary starch. With the elevation of dietary starch, a decrease of C18:2n‐6 and an increase of C22:6n‐3 occurred. These results indicate that at least 40%α‐starch can be used in practical diets for ayu.     

Variation in growth performance of Ryukyu-ayu, <Emphasis Type="Italic">Plecoglossus altivelis ryukyuensis</Emphasis>, inferred from otolith analysis

Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) is an amphidromous fish species that migrates between the sea and rivers over its one-year life span. Although growth performance during the early marine stage may affect growth in the later riverine stage of this species’ life cycle, no studies have specifically examined this relationship in P. a. ryukyuensis. In the present study, we reconstructed the growth trajectories of P. a. ryukyuensis individuals collected from the Yakugachi River, Amami-Oshima Island, Japan in 2016 (n?=?47) throughout their growth period in both the sea and river by using otolith analysis. Using this, we determined the age and body size of individuals at the time of their upstream migration, as well as their growth rates during the marine and riverine stages. Results showed that body size at upstream migration significantly affected body size at the riverine stage, indicating that juveniles with larger body size in the sea had better growth performance in the river. Individuals with higher growth rates during the marine stage tended to enter the river younger and at larger body sizes than those with lower marine growth rates. Our results demonstrated the close linkage between the growth performance in the sea and in rivers of P. a. ryukyuensis. This information will contribute to better understanding variations in growth patterns of this endangered species and potentially aid in its conservation.     

Geographical distribution of Metagonimus yokogawai and M. miyatai in Shizuoka Prefecture, Japan, and their site preferences in the sweetfish, Plecoglossus altivelis, and hamsters

Sweetfish, Plecoglossus altivelis, obtained from 18 rivers in Shizuoka Prefecture were examined for metacercarial infection of 2 flukes, Metagonimus yokogawai and Metagonimus miyatai. The infection rate and density of metacercariae in the fish were higher in eastern and western regions than in central region of the prefecture. After infection of hamsters with metacercariae derived from the scale, 98.7% of the adult worms obtained from the intestine was found to be M. miyatai. Conversely, from infection with metacercariae from the flesh, 90.0% of the worms was M. yokogawai. Since the worms had no exclusivity in the tissues, we conclude that the flukes have location preference with the former primarily preferring the scale, and the latter the flesh. Fish from two rivers located in adjacent areas in the western region had relatively a higher ratio of M. yokogawai in the scale relative to other rivers, suggesting an intraspecific genetic variation due to geographical isolation. On examination of adult worms in the hamster's intestine, M. yokogawai was mainly located towards the anterior part of the intestine, unlike M. miyatai, suggesting that in mammalian host too, the parasites have site preference.     

Effects of photoperiod and temperature on testicular development in male ayu,Plecoglossus altivelis

Synopsis Growth, gonadosomatic index and plasma steroid profiles in male ayu,Plecoglossus altivelis, cultured under short/long photoperiods and cool/warm temperatures were determined. Juvenile males were assigned to each of four different photoperiod/temperature regimes (16 L/18°C, 16 L/24°C, 8 L/18°C and 8 L/24°C) at random. Fish were killed and examined bi-weekly over the following 16 weeks. Mean body weight in the 16 L/18°C treated fish was the highest among four treated groups. No significant differences between body weights of the 16 L/24°C, 8 L/18°C and 8 L/24°C treated groups were observed. Ayu in the 8 L/18°C treated group had the highest values of gonadosomatic index, plasma testosterone (T) and 17-hydroxyprogesterone (17-OH P). No significant differences of plasma E₂ were observed among the treated groups. In the 8L/18°C and 8L/24°C groups, peak levels of 17-OH P occurred after 12 and 14 weeks of treatment, respectively. No peak levels of plasma T and 17-OH P were observed in 16 L/18°C or 16 L/24°C treated ayu. Spermiation occurred only in ayu with 8 L/18°C treatment. The data suggest that testicular development in ayu is temperature and photoperiod dependent: short photoperiod and cool temperature favour gonadal development.     

Molecular cloning and characterization of rhodopsin in a teleost (Plecoglossus altivelis,Osmeridae)

Amplified fragments encoding exon-4 of opsin cDNAs were cloned from the retina of landlocked ayu (Plecoglossus altivelis), and sequenced. On the basis of the sequence homology to previously characterized fish visual pigments, one clone was identified as rod opsin (AYU-Rh), and two clones as green (AYU-G1, -G2), one as red (AYU-R) and two as ultraviolet (AYU-UV1, -UV2) cone opsins. The 335-amino acid sequence deduced from the full-length cDNA of AYU-Rh included residues highly conserved in vertebrate rhodopsins and showed the greatest degree (88%) of similarity with salmon rhodopsin. Southern blotting analysis indicated that ayu possess two rhodopsin genes, one encoding visual rhodopsin (AYU-Rh) and the other non-visual extra-ocular rhodopsin (AYU-ExoRh). RT-PCR experiments revealed that AYU-Rh was expressed in the retina and AYU-ExoRh in the pineal gland. In situ hybridization experiments showed that the mRNA of AYU-Rh was localized only in rod cells not in cone cells. Lake and river type landlocked ayu having different amounts of retinal and 3-hydroxyretinal in their retinas expressed a rhodopsin (AYU-Rh) of identical amino acid sequence.     

Pattern of synaptic connections in the pineal organ of the ayu,Plecoglossus altivelis (Teleostei)

Summary Synaptic connections were studied by means of electron microscopy in the sensory pineal organ of the ayu, Plecoglossus altivelis, a highly photosensitive teleost species. Three types of specific contacts were observed in the pineal end-vesicle: 1) symmetrically organized gap junctions between the basal processes of adjacent photoreceptor cells; 2) sensory synapses endowed with synaptic ribbons, formed by basal processes of photoreceptor cells and dendrites of pineal neurons; 3) conventional synapses between pineal neurons, containing both clear and dense-core vesicles at the presynaptic site. Based on these findings, the following interpretations are given: (i) The gap junctions may be involved in an enhancement of electric communication and signal encoding between pineal photoreceptor cells. (ii) The sensory synapses transmit photic signals from the photoreceptor cells to pineal nerve cells. (iii) The conventional synapses are assumed to be involved in a lateral interaction and/or summation of information in the sensory pineal organ. A concept of synaptic relationships among the sensory and neuronal elements in the pineal organ of the ayu is presented.Fellow of the Alexander von Humboldt Foundation, Federal Republic of Germany     

Oral ontogeny of the Ayu, Plecoglossus altivelis and comparisons with the jaws of other salmoniform fishes

The Japanese Ayu, Plecoglossus altivelis Temminck & Schlegel is the sole representative of the salmoniform family Plecoglossidae. The Ayu is remarkable for its dentition which in adults comprises groups of diagonally arranged comb-like teeth in the outer tissue of the jaws. In juveniles (below 63 mm SL) the teeth are attached normally to the jaws. The transition of tooth form is correlated with a switch from zooplanktivory to algal or aufwuchs grazing. The present study follows the development of the teeth, jaws, oral cavity ethmoid and suspensorial elements in specimens ranging in size from 41–70 mm SL. The possible mode of function of the adult dentition is discussed. Comparisons are made with the jaws of other salmoniform fishes and a suite of supposed apomorphic characters are identified which are also shared with certain genera of the family Osmeridae, thus supporting the ideas of others that the Osmeridae is a paraphyletic assemblage.     

Habitat use, diet and body size of Heard Island weevils

Habitat use, diet and body-size variation are examined in weevils from Heard Island, with specific attention being given to the Ectemnorhinus viridis species complex. E. viridis shows marked altitudinal variation in body size and vestiture, but there are no consistent associations between body size and diet, nor are there consistent among-individual differences in conventional taxonomic characters. Thus, the status of E. viridis as a single, variable species is maintained. This species occurs from sea level to 600 m and it feeds on vascular plants and bryophytes. Canonopsis sericeus also feeds on bryophytes and vascular plants and occurs over a narrower altitudinal range. Palirhoeus eatoni is restricted to the surpralittoral zone where it feeds on marine algae and lichens. Bothrometopus brevis and B. gracilipes both feed on cryptogams, with the former species occurring from sea level to 450 m, and the latter from 50 to 550 m above sea level. In all species, males are smaller than females and there is a size cline such that populations from higher elevations are smaller than those at lower altitudes. This cline is the reverse of that found on the Prince Edward Islands which, unlike Heard Island, lie to the north of the Antarctic Polar Frontal Zone. This difference in body-size clines between weevils on the two island groups is ascribed to the shorter growing season on the colder Heard Island. The information presented here supports previous ideas regarding the evolution of the Ectemnorhinus-group of weevils on the South Indian Ocean Province Islands, although it suggests that subsequent tests of these hypotheses would profit from the inclusion of molecular systematic work.     

Desorptive behavior of phosphate in the subtropical Miyara River, Ishigaki Island, Japan

The desorption of phosphate-phosphorus (PO₄) from riverine particles with a salinity increase in Miyara River water, Ishigaki Island, Japan, was examined in laboratory experiments involving mixing of unfiltered riverine water with coastal seawater filtered through a 0.4-μm Nuclepore filter and the addition of sodium chloride (NaCl) into unfiltered riverine water. The mixing experiment yielded a convex pattern in the dissolved PO₄–salinity plot, and the dissolved riverine PO₄ concentration increased with the NaCl addition. These results suggested PO₄ desorption from riverine particles with a salinity increase. The pattern of desorption could vary according to the sample conditions, such as the concentrations of dissolved PO₄ and suspended solids. However, the obtained results seemed to simulate the convex distribution pattern in the dissolved PO₄–salinity plot observed at the mouth of the Miyara River in general.