Minami-Daito Island Research Articles

Population genetics of recent natural colonization by the bull-headed shrike (Lanius bucephalus; Aves) suggests the importance of recurrent immigration on remote islands

Abstract Founder effects and recurrent immigration are two major factors that might potentially contribute to genetic differentiation and population persistence in the early stage of remote island colonization. However, their relative importance remains controversial. By conducting population genetic analyses of four remote island populations of the bull-headed shrike (Lanius bucephalus) established naturally within several decades in Japan, we examined the contributions of founder effects and recurrent immigration to these island populations. Based on the standard genetic indices and population structure analyses using 15 microsatellite loci, we suggested island-specific scenarios of colonization. Notably, the founder effect strongly influenced genetic differentiation in the population on the most remote oceanic island, Chichi-jima Island, which, however, became extinct 20 years after colonization, possibly owing to a lack of recurrent immigration. In contrast, another oceanic island, Minami-Daito Island, was probably subjected to multiple recurrent immigration events from the mainland, which obscured any genetic differentiation previously established by the founder effect. Temporal samples collected over 8 years on this island confidently supported this scenario. Underlying the island-specific scenarios of colonization, we provide evidence that recurrent immigration strongly affected the population persistence, overwhelming the initial founder effects. We argue for the importance of recurrent immigration even in colonization of highly remote islands.

First Northwest Pacific Record of the Apogonid Fish, <i>Pristiapogon taeniopterus</i> (Actinopteri: Kurtiformes) from Minamidaito Island, Okinawa Prefecture, Japan

A single specimen of apogonid fish, Pristiapogon taeniopterus (Bennett, 1836) (Actinopteri: Kurtiformes) previously reported from Micronesia, Polynesia, Melanesia, the Hawaiian Islands, Madagascar and Indian Ocean, was recently caught at Minamidaito Island, Okinawa Prefecture, Japan. This specimen is the first record of the species from the Northwest Pacific including Japan and is now the most northerly record. Pristiapogon taeniopterus differs from its congeners in having developed gill rakers 15–17; circumpeduncular scales 15–18; upper-jaw length 17.9–21.5% SL; distinct dark stripes on both dorsal, anal and both lobes of caudal fin; dark vertical bar posterior to the base of caudal fin incompletely developed or interrupted at the central portion of the fin; dark band or blotch on posterior part of caudal peduncle just anterior to caudal-fin base; and pectoral-fin rays usually 13. The new standard Japanese name “Sumihiki-ishimochi” is proposed for the species.

Synergistic effect of secondary metabolites isolated from Pestalotiopsis sp. FKR-0115 in overcoming β-lactam resistance in MRSA.

Six aromatic secondary metabolites, pestalone (1), emodin (2), phomopsilactone (3), pestalachlorides B (4), C (5), and D (6), were isolated from Pestalotiopsis sp. FKR-0115, a filamentous fungus collected from white moulds growing on dead branches in Minami Daito Island. The efficacy of these secondary metabolites against methicillin-resistant Staphylococcus aureus (MRSA) with and without meropenem (β-lactam antibiotic) was evaluated using the paper disc method and broth microdilution method. The chemical structures of the isolated compounds (1-6) were characterised using spectroscopic methods, including nuclear magnetic resonance and mass spectrometry. All six isolated compounds exhibited synergistic activity with meropenem against MRSA. Among the six secondary metabolites, pestalone (1) overcame bacterial resistance in MRSA to the greatest extent.

Acoustic Characteristics of Songs in a Recently Established Population of the Japanese Bush Warbler on an Oceanic Island.

The acoustic structure of birdsongs is determined by ecological and social factors. Moreover, the founder effect can occur when a few colonizers bring a small subset of the song diversity from a source population to a newly established population, generating the acoustic features of its songs. Around 2000, the Japanese bush warbler (Cettia diphone) naturally colonized Minami-Daito, an oceanic island in the northwest Pacific. This raises the question of whether the songs in this population have changed through adaptation to the insular environment or maintained the features of songs in the mainland population. In this study, the acoustic characteristics of Japanese bush warbler songs on Minami-Daito Island at present (i.e., approximately 20 years after colonization) were compared with the songs of conspecifics on the mainland and another island. The acoustic structure of one of two basic song types on this island did not differ from that on the mainland. The other song type had a simpler structure on the island than on the mainland. Analyses of intonation structure showed that a certain pattern of frequency increase and decrease among sound elements was rare (< 10%) on the mainland but dominant on the island. The song characteristics substantially overlapped between the island and the mainland, and have not changed on the island since its colonization. These results suggest that the song characteristics on Minami-Daito Island can be explained by the founder effect. The songs on this island may change adaptively over a long period. Continuous investigation to follow the changes is required.

Random mating and the lack of sex-biased kin clustering in an island population of the bull-headed shrike, Lanius bucephalus

The encounter with relatives at the time of mating can be reduced if one or both sexes breed away from the natal site, and it is often assumed that this occurs to a lesser extent on islands where only short dispersal is possible. However, endemic insular populations may have evolved fine-tuned dispersal patterns enabling effective avoidance of inbreeding even in a small habitat, and the influence of spatial constraint should be more apparent for non-endemic than endemic populations. Recently established island populations of originally migratory birds can provide insights into the direct effect of habitat area on inbreeding risk. We studied spatial distribution of kin and inbreeding in a population of the bull-headed shrike, Lanius bucephalus, that became established on small oceanic islands separated from the mainland by > 360 km, Minami-daito and Kita-daito Islands, early in the 1970s. Sex-specific spatial genetic structure that could reduce inbreeding risk was absent within the focal Minami-daito Island. This result remained unchanged after the neighbouring Kita-daito Island was incorporated into the analysis. Average relatedness of breeding pairs was not different from that expected under random mating, and FIS was no greater than zero. The estimates of extra-pair paternity frequency were not high (4.5–16% of offspring, depending on the estimation method) compared to the mainland. Inbreeding coefficient of offspring was variable among nests, indicating some occurrence of inbreeding. We found no clear evidence of recent population bottleneck, suggesting that the purging of deleterious recessive alleles did not occur upon colonisation. These results suggest the absence of obvious inbreeding avoidance despite the potentially negative effects of inbreeding. Future studies should quantify inbreeding depression and also formally rule out the possibility that inbreeding is not avoided as an adaptive strategy as has been suggested for endemic island species.

The Effect of Experience on Parental Role Division in Ryukyu Scops Owl Otus elegans

Parental role division is common among monogamous birds. Among raptors, males typically expend more effort to feed their chicks than do females, yet few studies have focused on how each pair establishes this role division. Therefore, in this study we examined the mechanism of role division in the Ryukyu Scops Owls Otus elegans on Minami-daito Island. Pairs of this species typically remain together and use the same nest over several years, so there is a possibility that their role division improves with experience over time. To clarify this, we studied the feeding frequency and territorial usage of individuals. Males fed their chicks more frequently than females, and experienced individuals using the same nests fed their chicks more than inexperienced individuals. On comparing experienced and inexperienced pairs, we found that experienced individuals hunted closer to the nest than inexperienced individuals. Furthermore, the home ranges of the males and females of experienced pairs, overlapped less than those of inexperienced pairs. Experience of using a nest site in consecutive years may allow such individual behavioural differences to develop and be involved in role division in parental care.

Conservation paleobiology on Minami-Daito Island, Okinawa, Japan: anthropogenic extinction of cave-dwelling bats on a tropical oceanic island.

BackgroundWith strong environmental and geographic filtration, vertebrates incapable of flying and swimming are often extirpated from island ecosystems. Minami-Daito Island is an oceanic island in Okinawa, Japan that harbors the Daito flying fox (Pteropus dasymallus daitoensis), a subspecies of the fruit bat and the only extant mammal endemic to the island. However, the skeleton of a cave-dwelling bat Rhinolophus sp. and fossil guano were briefly reported in a previous study.MethodsHere, we present evidence for the anthropogenic extirpation of two species of cave-dwelling bats (Miniopterus sp. & Rhinolophus sp.) from Minami-Daito Island. Our goal is to reliably constrain the ages of the extirpated bat species by a multiproxy approach. Because skeletal materials did not preserve sufficient bone collagen for direct radiocarbon dating, we alternatively examined guano-like deposits based on SEM observation and Fourier-transform infrared spectroscopy (FTIR) along with stable carbon and nitrogen isotope analyses for possible indirect dating. We also examined stable carbon isotopes in bone apatite, assuming that an isotopic signal of C4 plants on the bat bones links to sugarcane plantation on the island based on the historical knowledge that early human settlers quickly replaced the island’s native C3 forests with sugarcane (C4 perennial grass) plantation from 1900 onward.ResultsOur cave survey documents the remains of Miniopterus sp. from the island for the first time. Based on the unique taphonomic conditions (unpermineralized bones, disarticulated skeletons closely scattered without sediment cover, various degrees of calcite crystal growth around bones) and a radiocarbon age of a humic sample, we suggest that the maximum age constraint of Miniopterus sp. and Rhinolophus sp. is 4,640 calBP. Based on a series of analyses, we conclude that the guano-like deposits are composed not of bat guano but mainly of humic substances; however, a hydroxyapatite crust associated with bat-lying stalagmites may be derived from bat feces. Stable carbon isotope analysis of bone apatite revealed C4 signals in various degrees, confirming that small populations of cave-dwelling bats persisted on Minami-Daito Island after 1900.ConclusionsThe results of this study indicate that these populations remained rather small and did not leave many generations and that the estimated ages can be bracketed from 4,640 calBP to the post-1900 (perhaps, until the 1950s). They likely faced a continuously high mortality risk due to severe anthropogenic stresses on the island, where most of the forests were turned into sugarcane plantations within a few decades in the early 20th century. A result of hearing surveys to local residents suggests the latest remnants most likely disappeared on the island concurrently with the introduction of chemical pesticides after World War II.

Relationships between Water Balance and the Growth and Yield of Ratoon Sugarcane on Minamidaito Island

Relationships between Water Balance and the Growth and Yield of Ratoon Sugarcane on Minamidaito Island

Reversed Sexual Size Dimorphism in the Ryukyu Scops Owl Otus elegans on Minami-Daito Island

Abstract One of the consequences of selections acting on body size is the difference in body size between the sexes or sexual size dimorphism. Although many hypotheses have been proposed for reversed sexual size dimorphism (RSD) in raptors, ornithologists have rarely paid attention to temporal aspects of RSD when testing their hypotheses. Because selection pressures may vary temporally, describing temporal variation in RSD is a first step towards understanding evolutionary mechanism which shape and maintain the dimorphism. Here, we describe RSD in the population of Ryukyu Scops Owls Otus elegans on Minami-daito Island using a dataset of the external measurements of 770 individuals obtained during a 17-year long-term population monitoring project. Females were larger than males in body mass, culmen length, bill depth, bill width, tail length and flattened wing length, whereas males were larger than females in tarsus and head lengths. Among these traits, the degree and direction of RSD of body mass, tarsus length, bill depth and flattened wing length varied across years. There were neither increasing nor decreasing trends in RSD. This is a rare study which addressed temporal variation in RSD in a raptor species.

Molecular phylogenetic position of Minamitalitrus zoltani elucidates a further troglobisation pattern in cave-dwelling terrestrial amphipods (Crustacea: Talitridae)

Talitrids are a highly diverse group of amphipod crustaceans that have colonized various terrestrial habitats. Three genera have successfully adapted to cave habitats on islands in the Pacific and Atlantic Oceans. However, the evolutionary origin of the Pacific troglobitic talitrids has remained unknown. We estimate the phylogenetic position of the troglobitic Minamitalitrus zoltani, which inhabits limestone caves on Minamidaito Island in the Northwestern Pacific, on the basis of the traditional multi-locus dataset. For the analyzed talitrids, we also reconstruct ancestral states of the maxilliped palp and male gnathopod 2. Our results indicate that Minamitalitrus zoltani is sister to the epigean Nipponorchestia curvatus with a deep divergence. Nipponorchestia curvatus inhabits coastal habitats in Japan, but is not indigenous to Minamidaito Island. A previous study estimated that the Atlantic troglobitic species had invaded subterranean habitats multiple times, but we provide new insight into the troglobisation history in talitrids. We also recover secondary shifts of character states of the maxilliped palp and male gnathopod 2 within the lineage composed of Minamitalitrus and its phylogenetically close genera. Our findings highlight the need for the genus-level reclassification of these genera; we split Nipponorchestia into two genera, establishing a new genus for Nipponorchestia nudiramus.

Hydrogeochemical processes and long-term effects of sea-level rise in an uplifted atoll island of Minami-Daito, Japan

Study regionAn uplifted atoll of Minami-Daito Island, Japan. Study focusMajor ions and stable isotopes (δ2H and δ18O) of groundwater at fifteen observation wells, surface water at eight representative lakes and one seawater site were measured to unravel the dominant processes controlling the chemistry of water, its spatial distribution and to identify the salinization mechanism caused by long-term sea-level rise. New hydrological insights for the regionRainfall is the main source for groundwater and lake water. Evaporation affects both the ion concentration and the stable isotopes of the lake water. Geochemical modeling suggests that freshwater-seawater mixing is the main process increasing concentrations of Na+, Cl−, Mg2+, and SO42−, whereas dissolution of calcite and dolomite increases concentrations of Ca2+, Mg2+, and HCO3− in groundwater. Fresh groundwater and lake water (i.e., Cl− < 500 mg/L) are largely distributed along a SW-NE direction, but they have been reduced since the 1970s. Sea-level rise causes an increase in the salinity of lake water by flowing through fractures being connected from lakes to the northern coast, then spreading to other lakes through the artificial channels built in the years.

First records of a blind centipede, Cryptops navis Chamberlin, 1930 (Chilopoda, Scolopendromorpha, Cryptopidae), from Japan

Eight specimens of a scolopendromorph centipede collected in Tokashiki Island and Minamidaito Island (both in the Ryukyu Islands, Japan) represent the first record of Cryptops (Cryptops) navis Chamberlin, 1930 from the islands of the Far East (i.e., Japanese Archipelago, Ryukyu Islands and Taiwan). This material also provides new details of the morphological variability of C. (C.) navis and the first data on natural habitats of C. (C.) navis, which previously was known only from soil samples from Singapore and China.

Evaluating the existence and benefit of major histocompatibility complex-based mate choice in an isolated owl population.

How mate preferences evolve in the first place has been a major conundrum for sexual selection. Some hypotheses explaining this assume fitness benefit derived from subsequent generations. Major histocompatibility complex (MHC)-based mate choice is a representative example of the mate choice that is associated with such trans-generational mechanisms. To provide evidences for fitness benefit of MHC-based mate choice, previous studies assessed the association between own MHC genotype and own fitness components. However, the association between MHC-based mate choice in the parental generation and fitness components in the resultant offspring generation has only rarely been measured in wild populations. Focusing on the isolated population of the monogamous Ryukyu Scops Owl (Otus elegans interpositus) on Minami-daito Island, Japan, we found evidence of MHC-based mate choice. However, we found no evidence of MHC-based mate choice increasing own reproductive success or offspring survival. This is a rare case study that directly examines the existence of the trans-generational indirect benefit of MHC-based mate choice for genetic compatibility from trans-generational data in a wild bird population. By investigating the fitness benefits of mate choice, this study serves to facilitate our understanding of the evolution of MHC-based mate choice.

Relative Abundance Differences of Two Invasive Toad Species on Minami-Daito and Kita-Daito Islands, Japan

Biological invasions are major threats to global biodiversity, and island ecosystems are especially vulnerable. Many previous studies have reported the presence of invasive species on islands, but relatively few studies have quantified the population status of such species. Furthermore, multiple invasive species are often introduced to the same region or island. In such case, quantifying the relative abundance of each species would provide us with significant insight into their population dynamics and potential species interactions. Rhinella marina and Bufo gargarizans miyakonis were introduced to both Minami-daito and Kita-daito Islands before 1945. Although some studies have reported their invasion and establishment, few studies have quantified their relative abundance. To reveal their relative abundance, we set 5 km line transects and counted the number of each species from 11 to 31 July 2019. We applied generalized linear models to examine the relationship between the number of individuals observed with species and environmental factors. We also applied these models to examine the relationship between the number of individuals observed with island and environmental factors. We found the relative abundance of R. marina was significantly higher than that of B. g. miyakonis on Minami-daito Island. This is the opposite pattern of 30 years ago, suggesting the population dynamics of these invasive toads is unstable on Minami-daito Island. Although B. g. miyakonis was found on both islands, no individual of R. marina was found on Kita-daito Island. The population dynamics may depend on environmental differences between islands and ecological differences between species. Future studies are needed to reveal the potential factors determining the differences in relative abundance between islands and between species.

Remigial and rectricial molt sequences in a captive Ryukyu Scops-Owl (Otus elegans)

We describe the remigial and rectricial molt sequences of a Ryukyu Scops-Owl (Otus elegans) held in captivity on Minami-daito Island, Nansei Islands, southern Japan. The owl completely shed its remiges and rectrices during its preformative molt as well as during its definitive prebasic molts in 3 consecutive years. In the preformative molt, the primaries were all shed during October, November, and December, in distal order from P1. Although all 4 groups of secondaries were shed in proximal order, the sequence may have been caused by adventitious shedding under captive conditions. During definitive prebasic molt, the primaries were shed in distal order during the period from June to September. The secondaries were shed from 3 nodes in 4 feather groups; 3 groups were shed in proximal order, but the remaining group was shed in distal order. The primary coverts were shed in exact sequence with their corresponding primaries during the definitive prebasic molt, but in a different sequence during the preformative molt. Whereas the greater coverts were shed irregularly during the early part of the primary covert molt, the median coverts were shed during the first half of the primary covert molt. The alula feathers were shed distally over a period of weeks. The right rectrices were shed in irregular order over a short period (12 days) and the left rectrices were shed over a long period (at least 82 days). The shedding sequence of the captive Ryukyu Scops-Owl basically followed the known pattern previously observed in Strigidae species. The captive individual underwent a complete molt when it was a juvenile as a result of being provided with sufficient food, and presumably because of the lower energy demands of being in a cage.

Vocalizations of the Ryukyu Scops Owl Otus elegans: individually recognizable and stable

ABSTRACTTo identify individual birds based on vocalizations, it is necessary to first confirm that individuals produce distinct vocalizations that are stable over time. Using vocal features, I was able to estimate the Ryukyu Scops Owls population on Minami-daito Island in 2003–2012 and confirmed that it is possible to identify individuals over time. I analysed 20 hoots per individual male in 2005–2007. Using 11 hoot parameters, the discriminant function analyses (DFAs) achieved a 97.4% probability of correct classification. Using 31 marked males, I examined the vocal features of 118 series of hoots over 1–6 years. Each male’s vocal features remained essentially stable over time and the calls of 14.9% (13/87) males were wrongly identified as those of other males. Individual misidentifications were the result of subtle differences in spectral features, or imperceptible differences in frequency and duration. The typological features (to visually classify variation in spectrograms according to the shape of syllables in the owl species) of the sound spectrograms of the misclassified males were found to be consistent over several years. When considering the efficacy of the DFA as a tool for identifying individuals, we rely on their vocal features, but should examine the visible typological features.

Nest and Roost Characteristics of the Ruddy-Breasted Crake Porzana fusca on Minami-Daito Island

We found five Ruddy-breasted Crake Porzana fusca nests in a swamp among emergent vegetation, and one in a dry sugarcane field more than 500 m (radius) from any swamp or pond, on Minami-daito Island. Nests were saucer-shaped. All of the nests in the swamp fledged chicks. It is assumed that nesting in such habitat renders nests safe from ground predators such as introduced weasels and feral cats. Roosts were similar in structure to nests, but lacked lining. It is inferred from one observation that males and females roost together at night before the egg-laying period.

Distinctive Features of the Skull of the Ryukyu Scops Owl from Minami-Daito Island, Revealed by Computed Tomography Scanning

Abstract Morphological differentiation of island-dwelling organisms provides model systems for studying evolution. Computed tomography (CT) scanning is an entirely non-destructive technique that provides detailed three-dimensional (3D) images of physical structures. Geometric morphometrics has been increasingly used in avian morphology studies by analyzing 3D data obtained from CT scans. We used geometric morphometrics to evaluate the morphological details of the skulls of three, genetically distinct, island populations of the Ryukyu Scops Owl Otus elegans: O. e. elegans from the northern part of the Ryukyu Archipelago, O. e. elegans from the southern part of the Ryukyu Archipelago, and O. e. interpositus from Minami-daito Island. Skulls were scanned using an X-ray CT system and the digitized 3D coordinates of 16 landmarks for each skull were analyzed in order to describe geometric morphometric features. O. e. interpositus was found to have a significantly smaller skull than either population of O. e. ele...

Holocene relative sea level history from phreatic overgrowths on speleothems (POS) on Minami Daito Island, Northern Philippine Sea

This research aims to elucidate the Holocene sea level (SL) variability on Minami Daito Island, Japan, where no previous such research has been done due to the apparent absence of any SL indicators. Here we use phreatic overgrowths on speleothems (POS) in a cave as an alternative SL indicator. The relationship between the highest modern groundwater fluctuation level (GWFL) and the highest POS level was studied, and the oldest POS calcite was dated using the radiocarbon method considering the dead carbon proportion (the “reservoir effect”). The results show that SL reached its Holocene maximum between ca. 5.1 and 4.6 ka cal BP, after which it remained more or less stable till the present day, with a possible minor sea-level drawdown of ca. 30–35 cm. Although the timing of the maximum Holocene SL coincides with mid-Holocene SL highstand observed across the tropical and subtropical Indo-Pacific, the relative stability of the SL after reaching its peak is not expected in the given subtropical setting. The SL on Minami Daito did not have a distinct highstand in the Holocene and instead followed closely the eustatic sea-level change. These findings are important in better understanding the mechanisms that control local relative SL variability and show the high value of POS as precise SL indicator.

Genetic characteristics and possible introduced origin of the paradise fish Macropodus opercularis in the Ryukyu Archipelago, Japan

It has hitherto been unknown whether the paradise fish Macropodus opercularis that inhabits the Ryukyu Archipelago, Japan, is native to the region or was introduced. This study examined the genetic identity of fish from five islands in the Ryukyu Archipelago (Okinoerabu, Okinawa, Yagaji, Kume, and Minamidaito islands) and compared it with those from Taiwan Island, mainland China, and Hainan Island. Analyses of the mtDNA control (760 bp) and cytb (660 bp) regions showed that haplotypes of specimens from the Ryukyu Archipelago were the same as or were very similar (with a 1- or 2-bp difference) to those from Taiwan. In addition, haplotypes from the Ryukyu Archipelago also showed lower genetic diversity than those from Taiwan Island, mainland China, and Hainan Island. These results suggest a high likelihood that the fish in the Ryukyu Archipelago were artificially introduced from Taiwan. However, the possibility that the fish is indigenous to the Ryukyu Archipelago cannot be completely ruled out, because some haplotypes and a clade from the Ryukyu Archipelago have not been found in the other areas. Regardless of its origin, we emphasize the importance of the conservation of the paradise fish in the Ryukyu Archipelago as an indicator of the threatened wetland environment as well as for its cultural value.