Izu Islands Research Articles

Data-driven proactive prediction of pumice drifting patterns using similarity search of the Kuroshio current axis

Pumice drifting poses substantial risks to maritime navigation and coastal communities. While traditional ocean-current-based simulations effectively predict drifting patterns, they are resource-intensive and unsuitable for real-time use following abrupt eruptions. This study proposes a data-driven framework that enables rapid, low-cost pumice drift prediction, leveraging daily-reported Kuroshio Current axis (KCA) patterns and conducting similarity searches on pre-existing simulation datasets. Focusing on eruptions at Bayonnaise Rocks in the Izu Islands, Japan, we demonstrate that the dynamic time warping distance, a measure of similarity between the current KCA pattern and historical KCA patterns, accurately predicts drifting scenarios within the critical first 10 days post-eruption. This method reliably identifies high-risk cases, including those affecting coastal regions, without requiring new simulations. By refining simulation datasets and enhancing prediction accuracy, this framework can become a practical tool for hazard assessments, offering a scalable solution for proactive disaster-risk management in response to unpredictable pumice eruptions.

Surface‐Wave Relocation and Characterization of the October 2023 Tsunamigenic Seismic Unrest Near Sofugan Volcano, Izu Islands, Japan

AbstractA moderate‐magnitude earthquake swarm occurred in the remote Izu Islands region of Japan between October 1 and 8, 2023. The swarm included 151 shallow earthquakes cataloged by the U.S. Geological Survey, which notably included a roughly 2.5‐hr episode of 15 successive magnitude (M) < 5.5 earthquakes. Origin times were coincident with regionally recorded tsunami waves, but tsunamigenesis for moderate‐magnitude earthquakes is uncommon, indicating that volcanic activity generated the ocean displacements. Leveraging a surface‐wave relative relocation approach, we estimate precise epicentroid locations for the remote swarm. Final epicentroids and caldera analogs indicate a three‐stage model to explain swarm activity: (a) caldera pressurization due to magma intrusion, (b) depressurization via dike propagation away from the caldera, and (c) eruption corresponding with caldera reactivation either by collapse or additional intrusion.

Diversity of invertebrates within the burrows of the streaked shearwater (Calonectris leucomelas) on Toshima Island, Izu Islands, Japan

Diversity of invertebrates within the burrows of the streaked shearwater (Calonectris leucomelas) on Toshima Island, Izu Islands, Japan

Insular environment-dependent introgression from an arid-grassland orchid to a wetland orchid on an oceanic island.

Adaptive introgression plays a vital role in allowing recipient species to adapt and colonize new environments. However, our understanding of such environment-dependent introgressions is primarily limited to specific plant taxa in particular settings. In Japan, two related orchid species, the autonomously self-pollinating Pogonia minor and the outcrossing Pogonia japonica, typically inhabit dry grasslands and wetlands, respectively. Intriguingly, an island ecotype of P. japonica exists in arid, wind-swept, open sites on volcanic mountain slopes on Kozu Island, in the oceanic Izu Islands. To investigate potential introgression and its implications between P. japonica and P. minor on Kozu Island, we applied a comprehensive approach that included examining morphological traits, genome-wide SNP data, and plastid DNA sequences. We also examined the breeding systems of these species on Kozu Island through artificial pollination experiments to determine if introgression from P. minor has endowed the P. japonica ecotype with selfing capabilities. Extensive sampling on Kozu Island revealed that all P. japonica specimens exhibit signs of introgression from P. minor, suggesting the absence of pure P. japonica populations on the island. Furthermore, the chloroplast haplotypes of the insular P. japonica ecotype consistently match those of P. minor, indicating a predominantly asymmetrical initial hybridization with P. minor acting mainly as the maternal parent in the formation of F1 hybrids. Despite the advantages of self-fertilization in isolated environments, the insular P. japonica does not exhibit autogamy. Consequently, the scarcity of moist habitats, rather than selection pressure for selfing, likely contributes to the observed widespread introgression. Our study strongly suggests that the arid-environment-adapted P. minor has introgressed into the insular ecotype of P. japonica, enabling its successful colonization of arid volcanic mountain slopes of the oceanic island.

Morphological and functional evolution of gametophytes in epilithic Hymenasplenium murakami-hatanakae (Aspleniaceae): The fifth family capable of producing the independent gametophytes

The fern independent gametophytes that can maintain populations by vegetative reproduction without conspecific sporophytes have been considered an unusual phenomenon found in some epiphytic or epilithic species of Hymenophyllaceae, Pteridaceae, Lomariopsidaceae, and Polypodiaceae. By chance, the discovery of mysterious strap-like gametophytes on Izu-Oshima Island, Japan, has led to the hypothesis that Hymenasplenium murakami-hatanakae, a fern species belonging to Aspleniaceae, can also form independent gametophytes. Our investigation revealed gametophyte populations of H. murakami-hatanakae on three islands in the Izu Islands. Based on chloroplast DNA analysis of the gametophyte and sporophyte populations, the gametophytes were found to be maintained by vegetative reproduction without a new supply of spores from sporophytes. A comparison of the surrounding vegetation at the collection sites showed that environmental factors such as light and humidity may influence the maintenance of gametophyte populations. These results clearly show that H. murakami-hatanakae is one of the ferns capable of forming independent gametophytes. This is the first report of independent gametophytes from the suborder Aspleniineae (eupolypod II). The discovery of the independent gametophyte within a phylogenetic lineage previously thought not to form independent gametophytes will provide important insights into the morphological and functional evolution of gametophytes in ferns.

Community structure of parasitoids attacking Schizomyia sasakii (Diptera: Cecidomyiidae) and local adaptation of the gall midge in the Izu region

AbstractInsect community structures and biological interactions vary with the distance from the mainland to islands. Gall inducers are key organisms in local arthropod communities because their galls harbor diverse arthropods. We investigated the parasitoid community of a gall‐inducing cecidomyiid Schizomyia sasakii on the Izu Peninsula and the Izu Islands, Japan. We examined relationships between parasitism by Inostemma sp. and Torymus hirtipennis, and gall characteristics, and analyzed the directional selection on gall characteristics induced by S. sasakii in each locality. The species richness of parasitoids on the Izu Islands decreased with the distance from the Izu Peninsula to each island, and area of respective islands. Inostemma sp. preferably attacked large galls on Ohshima Island, and T. hirtipennis tended to attack relatively small galls in the Izu Peninsula. Directional selection on gall characteristics of S. sasakii favored the induction of larger galls and thicker tissues by S. sasakii on Ohshima Island. In contrast, no directional selection was detected in the gall characteristics on Hachijojima Island. The number of alternative host species of parasitoid may affect variation in parasitoid richness of S. sasakii on distant and small islands, because the species richness of gall midges is necessary to maintain local parasitoid populations. Our study supported the ovipositor limitation hypothesis for the torymid species, related to gall size. The directional selection of parasitoid attacks on the Izu Islands may act to favor the induction of large and hypertrophic galls by S. sasakii to avoid the parasitoids.

Two new species of the genus Agramma (Hemiptera, Heteroptera, Tingidae) from small islands of Japan, with an illustrated key to the Japanese species of the genus

The present study describes two new species of the monocotyledon-feeding lace bugs of the genusAgrammaStephens, 1829 (Hemiptera, Heteroptera, Tingidae, Tinginae, Tingini) from small islands of Japan. The first is A. (A.) izuensesp. nov., which was recorded as A. (A.) japonicum (Drake, 1948) from Hachijo Island, the Izu Islands, in a previous study, and is considered an independent species here based on morphological characteristics and molecular data. The second is A. (A.) keramensesp. nov., which has a remarkable spineless head and was discovered from Aka and Geruma islands, Kerama Group, the Ryukyu Islands. Consequently, the following four species ofAgrammawere recognized in Japan: A. (A.) abruptifrons Golub, 1990, A. (A.) izuensesp. nov., A. (A.) japonicum, and A. (A.) keramensesp. nov.Only dozens of submacropterous morphs were confirmed in these two species in the present study, suggesting that both new species are flightless. In addition, an illustrated key for the identification of the four species from Japan and the host plant relationships of the two new species are provided.

Enigmatic Tsunami Waves Amplified by Repetitive Source Events Near Sofugan Volcano, Japan

AbstractOn 8 October 2023, mysterious tsunamis with a maximum wave height of 60 cm were observed in Izu Islands and southwestern Japan, although only seismic events with body‐wave magnitudes mb 4–5 have been documented to the west of Sofugan volcano. To investigate the source process, we analyze tsunami waveforms recorded by an array network of ocean bottom pressure gauges. Stacked waveforms of pressure gauge records suggest recurrent arrivals of multiple wave trains. Deconvolution of the stacked waveforms by tsunami waveforms from an earlier event revealed over 10 source events that intermittently generated tsunamis for ∼1.5 hr. The temporal history of this sequence corresponds to the origin times of T‐phases estimated by an ocean bottom seismometer and of the seismic swarm, implying a common origin. Larger events later in the sequence occurred at intervals comparable to the tsunami wave period, causing amplification of later phases of the tsunami waves.

Distribution pattern of Armadillidae species (Crustacea: Isopoda: Oniscidea) on the Izu Islands, Japan

Faunal studies of oceanic islands provide unique opportunities to study ecology and evolution. Four Armadillidae taxa were recorded from the Izu Islands, a group of oceanic islands off the coast of Japan. The distributions of these taxa and a phylogenetic tree are based on molecular data for 249 specimens from 88 sites. The Spherillo russoi (Arcangeli, 1927) species complex is distributed widely throughout these islands, and dispersal may have occurred via oceanic currents. On the nearby Izuoshima and Toshima Islands, Spherillo dorsalis (Iwamoto, 1943) may have recently migrated from the Izu Peninsula either via currents or been transported by human activity. In the middle islands, Shikinejima, Kozushima, Miyakejima, and Mikurajima, Venezillo hasegawai (Nunomura, 1991) was found, which may be derived from Spherillo obscurus (Budde-Lund, 1885) distributed in the Kanto region on the mainland. On the most southern and distant Hachijojima Island, Spherillo punctatus Nunomura, 2007 was found, and it does not differ genetically from specimens from Izenajima and Tokunoshima Islands in the Nansei Island Group, despite an approximate 1,000-km distance between them; this species may have been transferred by human activity. Our study showed that the Armadillidae assemblages in the Izu Islands were formed by several migration routes.

Environmental Influence on Distribution of Bare Land on Shikinejima Island, the Izu Islands

Environmental Influence on Distribution of Bare Land on Shikinejima Island, the Izu Islands

Potential Volcanic Origin of the 2023 Short-period Tsunami in the Izu Islands, Japan

On October 8, 2023, at 21:40 UTC (6:40 on October 9 local time), a tsunami warning was issued for the Izu Islands and southwest Japan. This tsunami was initially considered to be associated with the Mw 4.7 earthquake at 20:25 UTC (5:25 JST). However, we know events of this magnitude are far too small to generate observed tsunamis from coseismic deformation alone. In this study, we analyzed the ocean-bottom pressure records of DONET and S-net, real-time cabled observation networks on the Pacific coast of Japan. We find that the dominant period of this tsunami was relatively short, 250 sec, and that the largest tsunami occurred at 21:13 (6:13 JST) near Sofu-gan volcano. In addition, T waves, or the ocean-acoustic waves, were clearly observed by DONET – we posit these correspond to a vigorous swarm-like seismic event at the same region of the tsunami source. We formally invert for the tsunami source and find that several tsunami sources with an interval of about 4 min are necessary to reproduce the observed records. These most likely correspond to volcanic eruptions.

The absence of bumblebees on an oceanic island blurs the species boundary of two closely related orchids.

Oceanic islands offer valuable natural laboratories for studying evolution. The Izu Islands, with their recent geological origin, provide an exceptional opportunity to explore the initial evolution on oceanic islands. Another noteworthy aspect is the absence of bumblebee species on most Izu Islands. We used ecological, morphological, and molecular data to investigate the impact of bumblebee absence on the evolution of two closely related orchid species, Goodyera henryi and Goodyera similis, focusing on Kozu Island, the Izu Islands. Our investigation revealed that while G. henryi exclusively relies on a bumblebee species for pollination on the mainland, G. similis is pollinated by scoliid wasps on both the mainland and the island. Intriguingly, all specimens initially categorized as G. henryi on Kozu Island are hybrids of G. henryi and G. similis, leading to the absence of pure G. henryi distribution on the island. These hybrids are pollinated by the scoliid wasp species that also pollinates G. similis on the island. The absence of bumblebees might result in sporadic and inefficient pollination of G. henryi by scoliid wasps, consequently promoting hybrid proliferation on the island. Our findings suggest that the absence of bumblebees can blur plant species boundaries.

Relationship between microorganisms and volatile components in each fermentation process in the kusaya gravy that plays an important role in the manufacturing of kusaya, a traditional Japanese fermented fish product.

The relationship between the microbiota and volatile components of kusaya gravy involved in the manufacturing of kusaya, a traditional Japanese fermented fish product, in the Izu Islands (Niijima and Hachijojima) and the fermentation processes are not clear. In this study, we aimed to investigate the relationship between the microbiota and volatile compounds involved in the manufacturing and management of kusaya gravy. 16S ribosomal RNA (rRNA) gene-based amplicon sequencing revealed that the microbiota in kusaya gravy was significantly different between the two islands, and the microbiota hardly changed during each fermentation process. Gas chromatography-mass spectrometry analysis also revealed that the volatile components were strongly related to the microbiota in kusaya gravy, with Hachijojima samples containing sulfur-containing compounds and Niijima samples containing short-chain fatty acids. Therefore, our findings suggest that kusaya gravy is a characteristic fermented gravy with a stable microbiota, and the fermented pickling gravy is fermented by microorganisms.

Cover

On the Cover: Centre: Pinus cembra L. is an enigmatic closed-cone pine of the timberline ecotone in the European Alps and the Carpathian Mountains; photograph by Felix Gugerli; pp. 1049–1062. Top right: Feeding aggregation of coney (Cephalopholis fulva, Epinephelidae) in tropical rocky reefs of Fernando de Noronha Archipelago, northeast Brazil; photograph by João Paulo Krajewski; pp. 1163–1176. Top left: Euhadra peliomphala simodae (Jay, 1856) inhabit the southern part of the Izu Peninsula and the Izu Islands in Japan; their great diversity of shell colour is attributed to differences in island elevation; photograph by Shun Ito; pp. 1103–1116. Bottom left: The biodiversity of an aquatic ecosystem is encompassed in the transient traces of environmental DNA left behind by the inhabitants; illustration by Laura L. Hauck; pp. 1151–1162. Bottom right: This circular logo graced the cover of the first issue (March 1974) and initial 14 volumes of the Journal of Biogeography; the golden text celebrates the journal's five decades and 50th anniversary.

Confirmation of Neoporphyra cf. dentata on Shikinejima, Izu Islands, southcentral Japan, and comparison with co‐occurring Neoporphyra haitanensis

SUMMARYThe Izu Islands of southcentral Japan are thought to fall within the distribution range of Neoporphyra dentata. However, the gametophytic blades of Bangiales collected from Shikinejima and Hachijojima, Izu Islands, were identified as Neoporphyra haitanensis in our previous study. Thus, it became uncertain whether N. dentata is distributed in the Izu Islands, including Shikinejima. To clarify whether N. dentata grows on Shikinejima, we conducted a further distribution survey of N. dentata on the island. The morphological features of the blade samples collected from an additional sampling site on Shikinejima were more similar to those of N. dentata than to those of N. haitanensis: the blade thickness and the division formula of spermatangia resembled those of the former species rather than the latter species. However, the division formula of zygotosporangia was different from those of either species. The phylogenetic analyses of the rbcL gene indicated that the samples were resolved in a clade including N. dentata collected from Shirahama, Chiba Prefecture, and Enoshima, Kanagawa Prefecture, Honshu, Japan. The p‐distances of the chloroplast rbcL gene and nuclear 18S rRNA also supported identification of the samples as N. dentata. The results demonstrated that N. dentata is also distributed on Shikinejima with co‐occurring N. haitanensis, and that the island materials of the two species are genetically different from other materials of the two species, respectively.

Evolutionary history of inshore oceanic island land snails diversified in shell colour

AbstractAimOceanic islands provide an excellent opportunity to study the mode and tempo of phenotypic evolution of terrestrial organisms. Many studies have focused on oceanic islands far from the mainland. Oceanic islands near the mainland may provide distinct perspectives on phenotypic evolution, but a comprehensive understanding is still lacking. To address this gap, this study aimed to reveal when a land snail species inhabiting a volcanic archipelago within 40 km of the mainland diverged and how their shell colours evolved.LocationSouthern Izu Peninsula and five Izu Islands (Oshima, Toshima, Niijima, Shikine and Kozu), Japan.Taxon Euhadra peliomphala simodae. MethodsDouble‐digest restriction site‐associated DNA sequencing (ddRAD‐seq) was conducted using 117 individual snails. Molecular phylogenetic and population genomic analyses were performed, including approximation Bayesian computation (ABC). We then examined whether the island area, elevation, distance from the surrounding landmasses and historical event (bottleneck) explain the shell colour diversity on each island using a phylogenetic generalised linear mixed model (PGLMM).ResultsSnails could be genetically categorised according to the island they inhabited, and on Niijima, their genetic structure was further divided within the island. The divergence times among the extant populations of the snail dated back to 2.1 million years ago (Ma), which is older than that of other animals occurring in this region. Additionally, island elevation positively affected shell colour diversity, and populations with similar shell colour profiles were phylogenetically different.Main conclusionsIsland land snails diversified early in the ecosystem of the Izu Islands. This suggests that proximity to the mainland, immobility and the physiological tolerance of passive dispersal in land snails were the main causes of early diversification. Moreover, our study proposes that environments covarying with elevation would determine shell colour diversity on each island through natural selection.

Genetic structure of the Japanese Robin (Larvivora akahige) endemic to East Asian islands

Geographical isolation is a key factor in allopatric speciation although divergence with gene flow has been detected in an increasing number of studies, even in island systems. To understand the divergence in island taxa, it is necessary to examine historical gene flow with mainland sister groups, which largely differ due to the various geological and ecological characteristics of each region. The Izu Islands are a chain of young and active volcanic islands in a warm‐temperate climate zone that branch off from the middle of the Japanese mainland. Despite the chain's relatively limited isolation, given its proximity to the mainland, the Izu Islands feature endemic bird species and subspecies. The Japanese Robin Larvivora akahige is an endemic breeder on East Asian islands, and there is also a subspecies on the Izu Islands that is different in feather coloration and ecological traits. The population genetic structure of this robin was investigated using nine nuclear microsatellite markers and mitochondrial cytochrome b region sequences. Distinct and recent differentiation between the groups from the Izu Islands and mainland was detected from nuclear loci, although the genetic structure within the mainland group was unclear over the 1900‐km habitat chain. The mitochondrial haplotypes were divided into two distant clades, one dominated throughout the robin's range and the other coexisting only on the Izu Islands as a minor type. Those clades were inferred to have diverged independent of the recent differentiation of the two nuclear clusters, although their origins and the mechanism underlying their distribution remain unclear. The geographical characteristics of the Izu Islands may have formed a moderate but long‐lasting barrier for gene flow and promoted speciation, as well as serving as refugia for the preservation of relict lineages, particularly for migratory species, which usually move along the mainland chain.

Ecological Determinants of Inter-Island Distributions through Occasional Dispersal of Two Closely Related Species, Varied Tit and Cinereous Tit, in the Volcanic Izu Archipelago, Japan

Inspired by ecologists R. H. MacArthur and E. O. Wilson, who pioneered the use of islands as ideal model systems for researching the formation and maintenance of biodiversity, we studied two sympatric passerine bird species on a volcanic archipelago, in Japan. We focused on the occupancy and co-occurrence of Varied Tit Sittiparus varius and Cinereous Tit Parus cinereus, both members of the Parus guild, on the Izu Islands. Their populations range from being stable, and co-occurring, to being temporally unstable (with local extinctions, invasions, and species replacement) and even totally absent. Using results from several surveys carried out on the islands over the last 60 years, and our own contemporary surveys, we tested the ecological drivers of inter-island occupancy through occasional dispersal of these two species. We found that both species had positive relationships with island size and vegetation diversity, while Cinereous Tit exhibited a negative associating with distance to the mainland (Honshu, Japan). Cinereous Tit was also negatively correlated with Varied Tit, but their co-occurrence was positively associated with island size, connectivity, and vegetation diversity. We suspect that resource constraints play a significant role in the distributions of the two species on the islands. Varied Tit is dominant over Cinereous Tit, but Cinereous Tit is able to utilize a wider range of habitats and resources. We also discuss the potential for a competition-colonization trade-off for the two species on the islands.

Mothocya kaorui n. sp. (Crustacea: Isopoda: Cymothoidae), a fish-parasitic isopod with unique antennules from the Izu Islands, Japan.

A new species of fish-parasitic isopod in the family Cymothoidae is described from the Izu Islands, Japan. Mothocya kaorui n. sp. is reported from the gill cavities of the keeled needlefish, Platybelone argalus platyura (Bennett). Despite its unique morphological characters, such as completely article-fused antennules, mitochondrial DNA analysis indicated that it belongs to Mothocya Costa. The new species is clearly distinguished from all other species of Mothocya by having completely fused, stout antennules and partially fused, slender antennae; maxilla mesial lobe with 3 or 4 recurved robust setae, lateral lobe with 4-6 recurved robust setae; maxilliped with 5-8 robust setae on article 3; coxae 2 and 3 wide; black subtriangular pleotelson; and black uropods.

Evidence of historical pairing between two cryptic species of Short-tailed Albatross

When secondary contact occurs between allopatric sister species, several evolutionary consequences are expected, such as reinforcement of reproductive isolation, hybrid speciation, de-speciation, introgressive hybridization, or formation of a stable hybrid zone. The Short-tailed Albatross (<em>Phoebastria albatrus</em>) is a vulnerable seabird that breeds mainly in Torishima, the Izu Islands, and two islets in the Senkaku Islands in the western North Pacific. Recent studies revealed that Short-tailed Albatross comprises two cryptic species (Senkaku-type and Torishima-type) that breed sympatrically on Torishima. Ringed (hatched in Torishima) and unringed (probably hatched in the Senkaku Islands) birds mate in a mutually assortative manner at the Hatsunezaki colony (artificially established in 1995) on Torishima. However, observations of some ringed–unringed pairs suggest possible hybridization between the two cryptic species. To clarify the degree of hybridization, we analyzed microsatellite DNA and mitochondrial DNA control region 2 (CR2) sequences of chicks from Hatsunezaki and Tsubamezaki (original colony discovered in 1951) colonies and of unringed birds from Hatsunezaki. In general, both CR2 sequences and microsatellites revealed genetic differentiation between immigrants from the Senkaku Islands (unringed birds) and chicks hatched in Tsubamezaki. These findings support the existence of two cryptic species. Each chick obtained from four ringed–unringed parent pairs at Hatsunezaki displayed a high proportion of alleles from just a single population. In contrast, some chicks in Tsubamezaki had a medium proportion of alleles from both populations. Breeding unringed subadult plumage birds, which were probable immigrants from the Senkaku Islands, were observed in Hatsunezaki but not in Tsubamezaki. Therefore, we propose that interspecific pairing occurred in the past but infrequently in recent generations on Torishima, suggesting historical reinforcement of reproductive isolation. Further microsatellite DNA studies of chicks from Hatsunezaki are required to confirm whether reinforcement of reproductive isolation is achieved. Alternatively, nearly complete pre-mating isolation between the two species was established in the past, but the scarcity of Senkaku-type birds in Torishima has facilitated hybridization.