Whale-fall Communities Research Articles

Shallow-water whale-fall communities: Evidence from the middle–late Eocene basilosaurid whale bones, Wadi El-Hitan, Fayum, Egypt

The present study examines the basilosaurid whale bone specimens collected from the Sandouk El-Borneta section, Wadi El-Hitan (Valley of Whales), Fayum, Egypt. These specimens are embedded in highly fossiliferous calcareous sandstones of the middle Eocene Gehannam and the upper Eocene Birket Qarun formations. These whale bones display some post-mortem alterations, representing good signs of at least three distinct stages in what is called the whale fall. The co-occurrence of shark, ray, and crocodile fossil specimens with the examined whale bones may indicate the first mobile-scavenger stage. Borings of the siboglinid polychaete genus Osedax represent the most common worm type that deeply bioeroded the whale bones, contributing to their rapid degradation, and representing the enrichment-opportunist stage. These fossil traces of the bone-eating worm Osedax represent the first record in the Eocene Epoch regionally and inter-regionally, filling the gap between the Late Cretaceous and the Oligocene occurrences. Subsequently, the studied whale bones served as hard substrates for some calcareous tube-dwelling encrusting polychaetes, balanoid barnacles, sheet‐like cheilostome bryozoans, and scleractinian corals. Furthermore, a bioerosion structure produced by polychaete annelids was also observed. The presence of these sclerobionts assemblage confirms the well-developed final reef stage with prolonged exposure and colonisation of these whale bones prior to final burial. Based on the recorded post-mortem alterations, together with other sedimentological and palaeontological data, the studied whale bones were deposited in a shallow open marine bay to sheltered gulf environments, which were characterised by low depositional energy, low to moderate rate of sedimentation, and high surface water productivity.

The annelid community of a natural deep-sea whale fall off eastern Australia

In the deep ocean, whale falls (deceased whales that sink to the seafloor) act as a boost of productivity in this otherwise generally food-limited setting, nourishing organisms from sharks to microbes during the various stages of their decomposition. Annelid worms are habitual colonizers of whale falls, with new species regularly reported from these settings and their systematics helping to resolve biogeographic patterns among deep-sea organic fall environments. During a 2017 expedition of the Australian research vessel RV Investigator to sample bathyal to abyssal communities off Australia’s east coast, a natural whale fall was opportunistically trawled at ~1000 m depth. In this study, we provide detailed taxonomic descriptions of the annelids associated with this whale-fall community, using both morphological and molecular techniques. From this material we describe nine new species from five families (Dorvilleidae: Ophryotrocha dahlgreni sp. nov. Ophryotrocha hanneloreae sp. nov., Ophryotrocha ravarae sp. nov.; Hesionidae: Vrijenhoekia timoharai sp. nov.; Nereididae: Neanthes adriangloveri sp. nov., Neanthes visicete sp. nov.; Orbiniidae: Orbiniella jamesi sp. nov.), including two belonging to the bone-eating genus Osedax (Siboglinidae: Osedax waadjum sp. nov., Osedax byronbayensis sp. nov.) that are the first to be described from Australian waters. We further provide systematic accounts for 10 taxa within the Ampharetidae, Amphinomidae, Microphthalmidae, Nereididae, Orbiniidae, Phyllodocidae, Protodrilidae, Sphaerodoridae and Phascolosomatidae. Our investigations uncover unique occurrences and for the first time enable the evaluation of biogeographic links between Australian whale falls and others in the western Pacific as well as worldwide.

Does substrate matter in the deep sea? A comparison of bone, wood, and carbonate rock colonizers.

Continental margins host methane seeps, animal falls and wood falls, with chemosynthetic communities that may share or exchange species. The goal of this study was to examine the existence and nature of linkages among chemosynthesis-based ecosystems by deploying organic fall mimics (bone and wood) alongside defaunated carbonate rocks within high and lesser levels of seepage activity for 7.4 years. We compared community composition, density, and trophic structure of invertebrates on these hard substrates at active methane seepage and transition (less seepage) sites at Mound 12 at ~1,000 m depth, a methane seep off the Pacific coast of Costa Rica. At transition sites, the community composition on wood and bone was characteristic of natural wood- and whale-fall community composition, which rely on decay of the organic substrates. However, at active sites, seepage activity modified the relationship between fauna and substrate, seepage activity had a stronger effect in defining and homogenizing these communities and they depend less on organic decay. In contrast to community structure, macrofaunal trophic niche overlap between substrates, based on standard ellipse areas, was greater at transition sites than at active sites, except between rock and wood. Our observations suggest that whale- and wood-fall substrates can function as stepping stones for seep fauna even at later successional stages, providing hard substrate for attachment and chemosynthetic food.

Pleistocene Shallow-Water Whale-Fall Community from the Omma Formation in Central Japan

We report a single whale bone associated with many molluscan fossils from the Omma Formation, Lower Pleistocene shallow marine deposits, along the Sai-gawa River, Kanazawa City, in central Japan. Most molluscan species which are commonly found in the Omma Formation show disarticulated and/or damaged shells, indicating semi-autochthonous or allochthonous modes of occurrence. However, the assemblage contained chemosynthetic bivalves, such as lucinid, solemyid and thyasirid bivalves, which are rare in the Omma Formation. The lucinids and solemyids show a high articulation ratio, along with some predatory and scavenging gastropods, such as naticids, nassariids and borsoniids whose well-preserved shells indicate an autochthonous mode of occurrence. In addition, most of the lucinid bivalves show an umbo-upward position similar to the life position of Recent species. Recent lucinid, solemyid and most thyasirid bivalves harbor chemosymbiotic bacteria in their gills and are well known members of the chemosynthetic community. These lines of evidence indicate that the community, mainly comprising lucinid bivalves and other autochthonous molluscan species associated with the whale bone, is an ancient whale-fall community. This shallowest fossil whale-fall community differs from deep-water cases in the dominance of infaunal bivalves, such as lucinids, and in the lack of epifaunal and semi-infaunal chemosynthetic bivalves, such as bathymodiolins and vesicomyids. This community supports a previous suggestion that the difference in characteristic species of the whale-fall communities depends on the water depth.

Relationship between geochemical environments, nutritional resources, and faunal succession in whale-fall ecosystems

Faunal succession in whale-fall communities is closely associated with the progress of decomposition of the whale carcass. The main nutritional resources supporting a whale-fall community change from whale matter to chemosynthetic products over time. To study the geochemical aspects of this nutritional succession, we sampled animals over time on and in sediments around carcasses of sperm whale Physeter macrocephalus in Sagami Bay (mobile scavenger to early sulfophilic stage) and off Cape Nomamisaki (sulfophilic stage), Japan (500 and 200-300 m water depths, respectively). In these 2 areas, stable carbon, nitrogen, and sulfur isotopes of the animal soft tissues were measured to precisely elucidate the nutritional resources for each animal. In Sagami Bay, mobile scavengers relied only on whale soft tissue. Infaunal animals at 2 wk after the deployment relied only on whale soft tissue, while infauna at 9 mo after the deployment relied on chemosynthetic products. Such changes in nutritional resources were consistent with the transition of the geochemical environment in the sediments. Off Cape Nomamisaki, vigorous microbial sulfate reduction and thioautotrophic primary production nourished the fauna around the carcasses. The fauna in this area consisted of chemosymbiotic bivalves and necrophagous animals with sulfide-tolerant metabolism. We conclude that the changes in microbial processes, biomass, and compositions in sediments influence faunal succession in whale-fall ecosystems via change in the available nutrition for the fauna.

Diversity of bone-eating Osedax worms on the deep Atlantic whale falls—bathymetric variation and inter-basin distributions

Bone-eating Osedax worms can quickly colonize exposed bones and are important ecosystem engineers in whale fall communities, contributing to cause of bone degradation. This study shows that the deep SW Atlantic margin harbors many Osedax species. Using DNA barcoding, we found four putative new lineages as well as O. frankpressi Rouse, Goffredi, and Vrijenhoek, 2004 and O. braziliensis Fujiwara, Jimi, Sumida, Kawato, & Kitazato, 2019, with assemblages varying with depth. It is probable that the bathymetric distributions of these species are controlled by different water masses and their directions of flow. The haplotype network of Atlantic and Pacific O. frankpressi populations suggests segregation between populations, as is also seen in the high FST. However, the low p distance between both populations and the few substitution sites separating haplogroups from both regions (Atlantic and Pacific) could be evidence that populations of both basins are somehow close to each other. It is likely that whale fall habitats exist between both populations analyzed, connecting both basins.

Borealodon osedax, a new stem mysticete (Mammalia, Cetacea) from the Oligocene of Washington State and its implications for fossil whale-fall communities.

Baleen whales (mysticetes) lack teeth as adults and instead filter feed using keratinous baleen plates. They do not echolocate with ultrasonic frequencies like toothed whales but are instead known for infrasonic acoustics. Both baleen and infrasonic hearing are separately considered key innovations linked to their gigantism, evolutionary success and ecological diversity. The earliest mysticetes had teeth, and the phylogenetic position of many so-called toothed mysticetes remains debated, including those belonging to the nominal taxonomic groups Llanocetidae, Mammalodontidae and Aetiocetidae. Here, we report a new stem mysticete, Borealodon osedax gen. et sp. nov., from the Oligocene of Washington State, USA. Borealodon preserves multi-cusped teeth with apical wear; microCT scans of the inner ear indicate that the minimum frequency hearing limit of Borealodon was similar to mammalodontids. Borealodon is not recovered within a monophyletic Mammalodontidae nor a monophyletic Aetiocetidae; instead, it represents an unnamed lineage of stem Mysticeti, adding to the diversity of stem mysticetes, especially across the Rupelian–Chattian boundary. Furthermore, the presence of a putative chemosynthetic bivalve along with Osedax, a bone-boring annelid, found in association with the type specimen of Borealodon, offer more insights into the evolution of deep-sea whale-fall communities.

Variation of geochemical environments associated with whale-fall biomass mineralization processes in the sediment during the mobile scavenger, enrichment opportunist, and sulfophilic stages

The succession of faunal composition of a whale-fall community is believed to depend on the progress of decomposition of the whale carcass, which itself is dependent on microbial activity in the sediment around the carcass. This means that the faunal succession could be reflected in the variation of the geochemical environment. In this study, we examined sperm whale carcasses placed in two areas of Japanese waters. In Sagami Bay at a depth of ca. 500 m, dive surveys were performed 2 weeks, 2 months, and 9 months after implantation, when mobile scavengers and Osedax worms were observed. Off Cape Nomamisaki, 12 carcasses were implanted at a depth of 200–300 m, and dive surveys were performed there 17, 29, and 41 months after implantation, and abundant chemosynthesis-based mussels were observed at these times. The recovered sediment cores from beneath and around the carcasses in Sagami Bay showed increases in 15N-enriched ammonium and 34S-depleted sulfide and decreases in 34S-enriched sulfate during the 9 months following implantation, and their stoichiometric relationship suggested that proteinaceous matter from the carcass was decomposed preferentially followed by active bacterial sulfate reduction. In comparison, relatively low ammonium but high sulfide concentrations after 41 months off Cape Nomamisaki suggest that primarily lipids from the almost skeletonized carcasses were decomposed in the sediment during bacterial sulfate reduction. Such variation of decomposed organic substrates and geochemical environments result from interaction with the animals inhabiting in and on the sediments adjacent the whale carcasses. It is possible that evolution of the geochemical environment plays an important role in faunal succession, from the enrichment opportunist to sulfophilic stages.

Trophic structure and chemosynthesis contributions to heterotrophic fauna inhabiting an abyssal whale carcass

The trophic structure and role of chemosynthesis remain unexplored in deep-sea whale-fall communities in areas other than the California margin. This gap limits the understanding of these communities and their ecological relationships with other chemosynthetic ecosystems, such as vents and seeps. Here, we studied 3 different whale skeleton microhabitats with hypothesized high, intermediate and low reducing conditions as well as the sediments surrounding an abyssal whale fall (4204 m depth, SW Atlantic Ocean). We analyzed trophic structures (δ13C and δ15N) and the contribution of chemosynthetically derived carbon to heterotrophic species. The high and intermediate reducing microhabitats harbored food webs dominated by consumers of chemosynthetic production, similar to those of diffusive areas of hydrothermal vents and seeps. Both the low reducing microhabitat and the sediments harbored food webs with greater trophic complexity, dominated by higher consumers mainly relying on whale and/or photosynthesis-derived organic matter, a type of food web commonly reported in small whale, wood and kelp falls. The main whale-fall ecosystem engineer, the bone-eating worm Osedax, appeared to produce unique food web effects not observed in other chemosynthetic habitats. We conclude that whale falls provide the deep sea with a mosaic of microhabitats that supports assemblages with different chemosynthesis reliance levels and trophic structures, similar to those found at vents and seeps. Such a mosaic allows species-rich communities with numerous trophic levels to develop in a very small area of the food-limited deep sea.

A New Miocene Whale-Fall Community Dominated by the Bathymodiolin MusselAdipicolafrom the Hobetsu Area, Hokkaido, Japan

Abstract. We report the fourth record of a fossil whale-fall community in Japan. The new material consists of a single whale bone in association mainly with small bathymodiolin mussels, Adipicola sp., found in the Karumai Formation (late middle Miocene—early late Miocene) in the Hobetsu area of Hokkaido, Japan. This association of whale bone and Adipicola sp. and its mode of occurrence resembles the description of some other ancient whale-fall communities dominated by small mussels from the Olympic Peninsula in Washington State (early Oligocene), Shosanbetsu in Hokkaido (early middle Miocene) and Carpineti in northern Italy (middle Miocene) and constitutes an example of a chemosynthesis-based community sustained by whale-fall decay in the Miocene deep sea. The new example extends the Miocene distribution of bathymodiolin-dominated whale-fall communities to the northwestern Pacific Ocean.

The early Pleistocene whale-fall community of Bargiano (Umbria, Central Italy): Paleoecological insights from benthic foraminifera and brachyuran crabs

The early Pleistocene whale-fall community of Bargiano (Umbria, Central Italy): Paleoecological insights from benthic foraminifera and brachyuran crabs

A new eyeless species of Neanthes (Annelida: Nereididae) associated with a whale-fall community from the deep Southwest Atlantic Ocean

A new whale-fall community was discovered in the abyssal SW Atlantic Ocean (4204m depth) during the Iatá-piúna expedition. Several specimens of a new nereidid were found living in sediments around and immediately below whalebones. This new species, Neanthes shinkai, is described here. The most interesting feature of the new species is the absence of eyes on the prostomium. Although three other deep-sea Neanthes species are also eyeless, the arrangement of paragnaths on the pharynx, the shape of parapodia and the type of neuropodial falcigers chaetae can distinguish N. shinkai n. sp. from these other species. In addition, interspecific comparisons using COI fragment shown a high genetic divergence (23.6–24.9% K2P) from other Neanthes species. Some nereidids have been already known to live in association with deep-sea organic falls and other reducing environments, however this is the first record and description of a Neanthes species in a deep-sea whale-fall community. Observed behavioral and carbon and nitrogen isotopes suggest that N. shinkai n. sp. is an omnivore relying mainly on whale carcass with slightly contribution of chemosynthetic bacterial mats, suggesting that it is an inhabitant of whale-falls from SW Atlantic.

Bone-eating Osedax worms (Annelida: Siboglinidae) regulate biodiversity of deep-sea whale-fall communities

Although it is well recognized the capital role of “bone-eating” Osedax worms in the degradation of vertebrate skeletons in the deep sea, very little is known about their effects on bone faunal assemblages. Here we aim to shed light on the bone colonization process and determine 1) whether Osedax degradation induces different bone epi/infaunal assemblages and 2) how biodiversity is affected by Osedax colonization. We describe and compare the epi/infaunal assemblage structures of caudal vertebrae colonized and not colonized by Osedax of an abyssal juvenile whale carcass serendipitously found at 4204m depth in the SW Atlantic Ocean by HOV Shinkai 6500. Our results show that whale skeletons are very heterogeneous habitats that harbor specific and very rich assemblages and that contrasting epi/infaunal community patterns are found depending on the presence of Osedax. Vertebrae not colonized by Osedax were both well preserved and in a highly sulfophilic stage with chemosynthetic bacterial mats and numerous epifaunal organisms that fed on them. On the contrary, vertebrae colonized by Osedax were heavily degraded and did not exhibit evidence of a sulfophilic stage, harboring a distinct epifaunal assemblage. In general, bone infaunal assemblages were dominated by nematodes, especially in vertebrae without Osedax (ca. 77%) where organisms were only found in bone outer layers, showing a colonization pattern similar to that described for bacteria. Infauna in Osedax-colonized bones were present throughout the inner-matrices and were on average three times more abundant (ca. 1800 ind. 100cm−3) and twice as rich in number of species (16 species). Here, bones had a relatively higher proportion of the polychaete Capitella iatapiuna (ca. 39%) in comparison with nematodes (ca. 52%). Besides, a higher number of rare species were present in bones with Osedax. We suggest that Osedax degradation increases water diffusion through matrices probably modifying reduced-compound fluxes and truncating the sulfophilic stage. Furthermore, it is likely that larger and distinct infaunal biodiversity is a result of an increase in bone structural complexity caused by Osedax, which also facilitates the colonization of inner-bone matrices. We conclude that Osedax is an important ecosystem engineer that enhances biodiversity in deep-sea whale-fall communities.

A New Find of the Whale-fall Lancelet <i>Asymmetron inferum</i> (Cephalochordata) near a Hydrothermal Vent at Oomuro-dashi Submarine Volcano in the Izu Islands, Pacific Coast of Japan, as a Possible Case to Prove the Stepping-stone Hypothesis

Two mature specimens of the whale-fall lancelet Asymmetron inferum Nishikawa, 2004 , were collected in 2016 from reducing gravel bottom near a hydrothermal vent, 196 m deep, at Oomuro Hole of Oomuro-dashi submarine volcano in the Izu Islands, Pacific coast of Japan by using the ROV Hyper-Dolphin during the RV Shinsei-maru KS-16-6 cruise of JAMSTEC. This species has so far been known exclusively from whale-fall communities off Cape Nomamisaki, SW Kyushu, Japan, 219–254 m deep. The new find suggests the dispersal stepping-stone hypothesis of whale-fall communities for the origin of vent and/or seep fauna.

A biogeographic network reveals evolutionary links between deep-sea hydrothermal vent and methane seep faunas.

Deep-sea hydrothermal vents and methane seeps are inhabited by members of the same higher taxa but share few species, thus scientists have long sought habitats or regions of intermediate character that would facilitate connectivity among these habitats. Here, a network analysis of 79 vent, seep, and whale-fall communities with 121 genus-level taxa identified sedimented vents as a main intermediate link between the two types of ecosystems. Sedimented vents share hot, metal-rich fluids with mid-ocean ridge-type vents and soft sediment with seeps. Such sites are common along the active continental margins of the Pacific Ocean, facilitating connectivity among vent/seep faunas in this region. By contrast, sedimented vents are rare in the Atlantic Ocean, offering an explanation for the greater distinction between its vent and seep faunas compared with those of the Pacific Ocean. The distribution of subduction zones and associated back-arc basins, where sedimented vents are common, likely plays a major role in the evolutionary and biogeographic connectivity of vent and seep faunas. The hypothesis that decaying whale carcasses are dispersal stepping stones linking these environments is not supported.

A new Capitella polychaete worm (Annelida: Capitellidae) living inside whale bones in the abyssal South Atlantic

A new species of the genus Capitella, Capitella iatapiuna sp. nov., has been found in deep sea whale-fall samples, São Paulo Ridge-Southwest Atlantic. The new species is mainly characterized by a bluntly rounded prostomium and a very distinct peristomium forming a complete ring. Ribosomal 16S sequences were obtained and used for inter-specific comparisons. This species is herein described and compared to others species of the genus. Its ecological role in the whale-fall community is also discussed.

A new species of Protodrilus (Annelida, Protodrilidae), covering bone surfaces bright red, in whale-fall ecosystems in the northwest Pacific.

There are unique ecosystems in the ocean that are driven by chemosynthesis. Whale-fall communities are one of these reducing habitats, and many unknowns are left to be resolved to understand their uniqueness. A new species of the genus Protodrilus was discovered on the exposed bones of sperm whale carcasses found in the waters off Cape Nomamisaki in the northwest Pacific. Protodrilus puniceus sp. nov. was the most abundant annelid to be found on the 2.5-y-old carcasses; the exposed bone surfaces appeared bright red due to the coloration of the innumerable worms covering them. Closer inspection revealed that this species was found in the small pores of both the internal and external surfaces of the bones. P. puniceus shows simple morphology; it has paired palps and pygidial lobes, but no eyespots, nuchal organs, statocysts, or ciliary rings dorso-anteriorly-an exceptional finding in this group. A ventral ciliary band was conspicuous, extending over each segment of the animal. The male of the species possessed paired, separated lateral organs on segments 6-11; salivary glands were inconspicuous. From morphological, biological, and ecological characteristics, P. puniceus showed differences from the already known Protodrilus group of shallow interstitial inhabitants. P. puniceus is thought to be a unique deposit feeder, acquiring nutrients by adhering to organic substances from whale carcasses. This is the first description of this group to be found in the organically enriched whale-fall environments of the deep sea below 200 m and from Japanese waters. Information on a nuclear, 18S ribosomal RNA gene sequence is included.

The dolomite nodules enclosing fossil marine vertebrates in the East Pisco Basin, Peru: Field and petrographic insights into the Lagerstätte formation

Abstract The Mio-Pliocene Pisco Formation (Peru) is a world-famous marine vertebrate Lagerstatte. Several fossil specimens are wrapped up in dolomitic nodules. Some others lie in the sediment displaying dolomite only in bone cavities (e.g., mesorostral canal and endocranium). With the aim to understand whether the precipitation of the dolomitic nodules influenced the formation of the Lagerstatte, we collected field data on a high number of fossil vertebrates and conducted petrographic and mineralogical analyses on samples representative of the variable development of concretions. Our results revealed positive relationships between size, completeness and articulation of skeletons and the presence of an external nodule. Clear evidence of chemoautotrophic communities that thrived on the carcasses is scarce. Microborings are often found in the cortical bone tissues together with iron oxides; the former are left by microorganisms feeding on the carcass, the latter are traces of former Fe sulphides, a product of organic matter degradation. We suggest that an early burial of the skeletons was a determinant factor in the development of dolomite concretions, since it allowed methanogenesis and anaerobic sulphate reduction exploiting the lipids in the bones and the organic matter dispersed in the sediments. Dolomite precipitation was driven by the same bacteria operating during the suphophilic stage of whale-fall communities. Textural observations imply that dolomite precipitated shortly after the burial of carcasses. The increase of alkalinity generated by sulphate reduction and methanogenesis caused a rapid precipitation of the dolomite within skeletal cavities and prevented the degradation of the bones and diagenetic compression of skeletons; the nodules themselves prevented erosion of fossils after exhumation. Therefore, nodule formation had a crucial role in the development of the Pisco Lagerstatte.

AN ICHTHYOSAUR CARCASS-FALL COMMUNITY FROM THE POSIDONIA SHALE (TOARCIAN) OF GERMANY

The possibility that large marine reptiles and other Mesozoic vertebrates produced nekton-fall communities similar to those of modern cetaceans is presently receiving increased attention in the literature. The author describes a rare ichthyosaur carcass-fall community from the Posidonia Shale (lower Toarcian) of Germany, which provides insights into the role played by large marine vertebrates in determining regional benthic ecology during this period. It is demonstrated here that, while more important than previously thought, there is little evidence to suggest that ichthyosaur carcasses played a substantial role in structuring the benthic ecology of the European Toarcian epeiric sea. In general, within the shallow waters of the Posidonia Shale, conditions conducive to the creation of carcass-fall communities were rare, and when present, resulted in localized magnification of background taxa and higher local biodiversity, rather than a unique community of epibiont organisms. The community which developed is ecologically similar to modern whale-fall communities, but differs in important ways, particularly with regard to the presence/absence of the chemosynthetic faunas which are most intensively described in the literature.

Dispersal Ability and Genetic Structure in Mytilid Mussels of Whale-Fall Communities

Since organic falls are ephemeral and distributed sporadically, organisms in organic-fall commun- ities must acquire high dispersal ability to migrate from one organic fall to another. However, the dispersal ability of obligate organic-fall organisms has not been investigated thoroughly and the stability of the genetic structure of their communities is unknown. In this study, in order to eluci- date the dispersal ability and genetic structure in the organic-fall communities, we carried out po- pulation genetic analyses based on sequences of mitochondrial NADH dehydrogenase subunit 4 in two mytilid mussels. Adipicola pacifica was obtained from whale and cow bones artificially settled in Japanese waters off Cape Noma (CN) and in the Nansei-Shoto Trench (NS) and Sagami Bay (SB), and Benthomodiolus geikotsucola from natural whale bones in the Torishima Seamount (TS); both species are symbiotic with chemoautotrophic bacteria. Genetic differentiation (Fst) indicated al- most no annual change in genetic structure between 2003, 2004, 2005, and 2007 collections of A. pacifica from CN (depth 225 - 229 m), although the 2010 collection had somewhat different ge- netic structure from the others. Similarly, there was not significant genetic differentiation be- tween 1993 and 2005 collections of B. geikotsucola from TS (depth 4020 m). The Fst and gene bidi- rectional mean rate of gene flow (Nm) indicated high gene flow and no significant genetic differen- tiation between A. pacifica specimens collected from CN, NS, and SB. The results suggest that the genetic structure is stable and A. pacifica has high dispersal ability. The mismatch distribution suggests that A. pacifica expanded their distribution from SB to NS via CN, as expansion time (τ = 2ut) decreased from SB to CN and NS. * Corresponding author.