Tosa Bay Research Articles

July 2020 heavy rainfall in Japan: effect of real-time river discharge on ocean circulation based on a coupled river-ocean model

In July 2020, a stationary atmospheric front over Japan caused persistent, nearly continuous rain for most of the month that resulted in new historical highest rainfall records in several areas and caused serious river floods, landslides, and debris-flow events. An existing hindcast and forecast ocean circulation model that includes climatological discharge information of major rivers failed to represent the extreme river discharge under heavy rainfall. New experiments were conducted using real-time river discharge information based on Today's Earth CaMa-Flood simulation that includes 368 rivers in Japan. The inclusion of real-time river discharge improved the salinity bias in the near-surface waters. The differences were significant compared to the observations in the heavy rain region (i.e., Ariake Bay and Tosa Bay), and insignificant at offshore stations. The ensemble experiments of real-time river discharge cases suggested the difference between the climatological and the real-time river discharge experiments was not random, but was robust. The freshening water changed the shelf circulation, and its far-reaching effect appeared hundreds of kilometers away from the shore. Passive particle tracking was conducted for examining the cross-shelf exchange. More particles released from Bungo Channel went offshore in near-surface water when the real-time river discharge was used compared to using the climatology discharge. Particles released in Tosa Bay, Seto Inland Sea, and Kii Channel showed the opposite tendency. The real-time river discharge not only changed the modeled coastal salinity distribution, but also the coastal and offshore currents. The role of the real-time river discharge on modeling normal flow periods or drought events, and its influence on a longer time scale model simulation remain to be explored.

Differences in fish herbivory among tropical and temperate seaweeds and annual patterns in kelp consumption influence the tropicalisation of temperate reefs.

Climate change is leading to novel species interactions and profoundly altering ecosystems. In marine systems, tropical and subtropical species are increasing in higher latitudes. This has been linked to the deforestation of temperate coastlines, as direct effects of ocean warming combine with increased herbivory from tropical and sub-tropical fishes and lead to the decline of canopy-forming kelp. Here, we tested the hypothesis that this deforestation may be facilitated by greater palatability of temperate kelp and other canopy seaweeds compared to tropical taxa. We used multiple-choice filmed feeding field experiments and chemical analyses to measure the palatability of temperate and tropical seaweeds from Tosa Bay (southeastern Japan) and we used single-species feeding assays to measure changes in consumption of the kelp Ecklonia cava throughout the year. We found no evidence that temperate seaweeds are more palatable to herbivorous fish. In the multiple-choice assays, consumption was concentrated on both tropical and temperate Sargassum species, which are ephemeral and peak in abundance in the spring/early summer. Consumption of the kelp Ecklonia cava peaked during the autumn, when Sargassum species are absent. The highest levels of kelp herbivory coincide with the reproductive season for E. cava and may contribute to the long-term decline of these kelp forests in southern Japan.

Variation in reproductive parameters of round herring in the Pacific coastal waters of Japan

This study assessed potential variability in batch fecundity and the spawning fraction of round herring Etrumeus micropus in the Pacific coastal waters of Japan. Fish were caught by a research vessel in the Kumano Nada (Kumano Sea) and Tosa Bay in February 2016, and other fish caught by commercial vessels in the Kumano Sea were collected between January 2019 and February 2020. The appearance of hydrated oocytes in the 2016 samples indicated that the fish spawned between midday and evening in the Kumano Sea, whereas they spawned from daytime to nightfall in Tosa Bay. The spawning fraction was estimated using the 2016 samples and was based on presence of post-ovulatory follicles and oocytes during oocyte maturation. The spawning fraction and spawning interval, respectively, were similar between the two areas: 0.17–0.26 and 3.9–5.9 d in the Kumano Sea, and 0.14–0.53 and 1.9–7.0 d in Tosa Bay. Batch fecundity was estimated by counting oocytes during oocyte maturation. The relationships of batch fecundity to body length varied significantly among the monthly samples. Batch fecundity at a given body length increased from January to May. This indicates that the acceleration in individual egg production would have contributed to the increase in egg abundance during the main spawning period. Relative batch fecundity (RBF) ranged from 76 to 142 eggs g−1 of ovary-free female. The 2016 samples showed that RBF was higher in Tosa Bay than in the Kumano Sea. The RBF increased from January to April in the Kumano Sea. A comparison of RBF values for the same month (February) differed significantly between samples in the three years, suggesting inter-annual variation in RBF. Our data provide essential information for understanding the spawning ecology of round herring and for allowing the egg production method to be applied to estimates of stock biomass.

A new snailfish of the genus Careproctus (Cottoidei: Liparidae) from the Pacific coast of southern Japan

A new snailfish, Careproctus tomiyamai, is described on the basis of four specimens collected from Suruga Bay, Tosa Bay, and the Hyuga-nada Sea, southern Japan (600–808 m depth). It is distinguished from all currently recognized congeners by the following combination of characters: total vertebrae 56–58; dorsal-fin rays 51 or 52; anal-fin rays 44–46; pectoral-fin rays 30–32; pyloric caeca 9–13; body slender, maximum depth 15.6–22.8% standard length (SL); teeth on both jaws strongly trilobed; pectoral fin shallowly notched, longest lower lobe ray 9.8–14.3% SL [46.0–60.4% head length (HL)]; proximal pectoral radials 4 (3 + 1), upper portion of 1st and 3rd radials, and lower portion of 2nd radial notched; fenestrae in pectoral girdle 2, between scapula and 1st proximal radial, and 2nd and 3rd proximal radials; pelvic disk oval, wider than long, length 3.4–4.3% SL (14.2–19.3% HL), moderately to deeply cupped; peritoneum black in preserved specimens.

Octorhopalona saltatrix, a New Genus and Species (Hydrozoa, Anthoathecata) from Japanese Waters

Simple SummaryIn this study, we describe a new genus and species of hydromedusa belonging to the family Halimedusidae (Hydrozoa, Anthoathecata) that is found off Oarai, Sagami Bay, and Tosa Bay, Japan. This family comprises four species in three genera: Halimedusa, Tiaricodon, and Urashimea. The new genus and species can be distinguished from all other Halimedusidae species by genetic sequences and morphological characteristics. A comparative table of the primary diagnostic characteristics of the genus Halimedusa is provided. In addition, the diagnosis of Halimedusidae was modified. Jellyfish blooms cause serious problems in fishing, industry and public health. This paper contributed to the understanding of ecology and diversity of jellyfish on our planet.Approximately 300 species of cnidarian jellyfish have been reported in Japanese waters. However, many specimens remain unidentified. In this study, taxonomic investigations, including morphological observations and molecular 16S phylogenetic analyses, were conducted on unknown specimens collected off Oarai, Sagami Bay, and Tosa Bay, Japan. The specimens have the following morphological characteristics: distinct peaks in jelly above the base of the manubrium, a red band on the manubrium, and cylindrical marginal bulbs, each with an abaxial ocellus that is common to the family Halimedusidae. However, the specimens can be distinguished from other Halimedusidae species by their eight radial canals, eight tentacles with numerous stalked nematocyst knobs, and eight nematocyst tracks on the exumbrella. Moreover, molecular phylogenetic analyses revealed that the Kimura two-parameter distance between the specimens and other Halimedusa species was 0.066–0.099, which is considered to represent intergeneric variability. Based on this result, we described it as a new species and established a new genus for taxonomic stabilization. We also emended the diagnostic characters of the family Halimedusidae owing to the establishment of the new genus. Halimedusidae comprises five species in four genera. This paper provides taxonomic keys for the identification of species in the family Halimedusidae.

Measuring the historical conservation status of dragonet fishes in Tosa Bay, Southwestern Japan: ecological and genetic approach

Measuring the historical conservation status of dragonet fishes in Tosa Bay, Southwestern Japan: ecological and genetic approach

Oxygen Isotope Equilibrium of the Shallow-Water Benthic Foraminifer Hanzawaia nipponica Asano in Tosa Bay, Southwest Japan

Oxygen isotopic compositions (δ18O) of benthic foraminifer tests are widely used for reconstructing paleoceanographic changes, such as global ice volumes during glacial–interglacial cycles. Although deep-sea benthic foraminifers have been well characterized and are considered reliable indicators, little attention has been paid to the geochemistry of shallow-water benthic foraminifers. In this study we evaluated δ18O in the shallow-water benthic foraminifer Hanzawaia nipponica Asano, which lives in surface sediments on continental shelves and upper slopes under the influence of two warm currents, the Kuroshio and Tsushima currents, in the East China Sea, northwest Pacific, and southwestern Japan Sea. To evaluate oxygen isotope equilibrium, we analyzed δ18O of H. nipponica and ambient seawater on the continental shelf in Tosa Bay, southwest Japan. Seawater δ18O and salinity in Tosa Bay are similar to those of surface and subsurface waters in the Kuroshio region in the Okinawa Trough and the northwest Pacific. Vertical profiles of seawater δ18O show no variation with water depth (0–200 m) in Tosa Bay. However, tests of living H. nipponica (as determined by staining with Rose Bengal) and fossil (non-stained) H. nipponica, picked from samples of the top centimeter of seafloor sediment, yielded carbonate δ18O values that clearly increase with water depth, suggesting a temperature-dependent relationship. A comparison of carbonate δ18O values in living H. nipponica and those predicted on the basis of seawater δ18O and annual mean bottom temperature shows that H. nipponica tests are in oxygen isotopic equilibrium with ambient seawater. We determined the linear equations of δ18O–temperature relationship, and the slope of −5.26 (0.19‰°C−1) for living and −4.50 (0.22‰°C−1) for the fossil H. nipponica, respectively. The carbon isotopic compositions (δ13C) of H. nipponica also closely match seawater δ13C. Thus, we propose that the carbonate δ18O and δ13C of H. nipponica are useful proxies to reconstruct shallow-water paleoenvironmental changes in the northwest Pacific and its marginal seas.

日本初記録種を含む土佐湾産トゲシャコ属（口脚目：シャコ科）

日本初記録種を含む土佐湾産トゲシャコ属（口脚目：シャコ科）

The first complete mitochondrial genome of the Northern Pacific deep-sea goniasterid sea star Ceramaster japonicus (Sladen, 1889) determined using NGS-based shotgun sequencing

The full mitogenome of an ethanol-preserved museum specimen of Ceramaster japonicus was determined using the NGS Illumina MiSeq platform. The specimen was collected from Tosa Bay, Japan, facing the Pacific Ocean (33.0781 N 134.0601 E), at 700 m depth in 2011. The mitogenome shows a typical metazoan genomic structure, with all of the 37 genes included in its 16,370 base-long mitogenome. We conducted phylogenetic analyses using a data set including 18 publicly available asteroids rooted against five ophiuroids as outgroups. The result confirms the position of C. japonicus in the order Valvatida. The complete mitogenome of C. japonicus reported here is the first reported for the family Goniasteridae Forbes, 1841.

Differences in the parasitic effects of a bopyrid isopod and rhizocephalan barnacle on the portunid crab, Charybdis bimaculata.

Bopyrid isopods and rhizocephalan barnacles are obligate parasite crustaceans which harm their decapod hosts. However, to the best of our knowledge, studies have not compared which of these parasites has a greater parasitic effect on its hosts. Here, the parasitic effect of the bopyrid isopod, Allokepon hendersoni, and an unidentified sacculinid rhizocephalan species, infesting the same population of portunid crabs, Charybdis bimaculata, was investigated and compared for the first time. Samples were collected from the bycatch of a trawl fishery in Tosa Bay, Japan. A total of 2601 crabs were collected, of which 14 (0.55%) were parasitized by the bopyrid and 21 (0.82%) by the rhizocephalan. One of the two female crabs parasitized by the bopyrid was ovigerous (with much fewer eggs than unparasitized females). No ovigerous crab was found from the eight females parasitized by the rhizocephalan. Because only two female crabs were parasitized by the bopyrid, the following analyses were made using the male crabs. Both parasites reduced the wet weight (crab condition) and the cheliped size (secondary growth) of C. bimaculata, but the impact of the parasitism did not differ between the parasite species. The size of the abdominal flap of male hosts was reduced by the bopyrid infestation; however, rhizocephalan infestation caused enlargement of the abdominal flap, which is an indication of feminization. The present study provides information on how the effect of these two parasitic castrators on the same host crab varies. A moderate decrease in crab condition and cheliped development was common among the parasites, suggesting that the degree of impact might be favorable for the survival of the two parasites species.

Records of three species of the Leiognathidae from Tosa Bay, Japan

Records of three species of the Leiognathidae from Tosa Bay, Japan

A new species of snailfish of the genus Paraliparis (Liparidae) from the western North Pacific, with a redescription of the poorly known species Paraliparis mandibularis.

A new snailfish, Paraliparis flammeus, is described on the basis of 18 specimens collected off the Pacific coast of Tohoku District, northern Japan at depths of 422–890 m. The new species is distinguished from 28 species of Paraliparis described from the North Pacific by the following combination of characters: mouth oblique; uppermost pectoral-fin base below horizontal through posterior margin of maxillary; 60–63 vertebrae, 54–58 dorsal-fin rays, 50 or 51 anal-fin rays, six principal caudal-fin rays, and 17–20 pectoral-fin rays. A maximum likelihood tree based on 106 COI gene sequences (492 bp) of Paraliparis recovered a monophyletic group comprising P. flammeus, Paraliparis cephalus, and Paraliparis dipterus. Paraliparis cephalus is similar to P. flammeus in having an oblique mouth, but it has four caudal-fin rays (vs six rays) and the uppermost pectoral-fin base above a horizontal through the maxillary posterior margin. Paraliparis dipterus differs from P. flammeus in having a horizontal mouth, 12–14 pectoral-fin rays, and lacking pyloric caeca (present in P. flammeus). Paraliparis flammeus is most similar to the eastern North Pacific Paraliparis mento in having an oblique mouth and the uppermost pectoral-fin base below a horizontal through the posterior margin of the maxillary. However, P. flammeus differs from P. mento in having six caudal-fin rays (vs five rays) and greater preanal length (29.9–35.3% SL vs 26.7–28.5% SL). A poorly known species, Paraliparis mandibularis, previously known from only two specimens collected from Tosa Bay, southern Japan, is redescribed based on the holotype and seven newly collected specimens. It is also similar to the new species but has 27–30 pectoral-fin rays and a shorter pectoral-fin lower lobe (13.8–15.9% SL in P. mandibularis vs 16.7–23.4% SL in P. flammeus).

Growth of larval and juvenile deepsea smelt Glossanodon semifasciatus in Tosa Bay, on the Pacific coast of Japan

Growth of larval and juvenile deepsea smelt <i>Glossanodon semifasciatus</i> in Tosa Bay, on the Pacific coast of Japan

Replacement of Ulva ohnoi in the type locality under rapid ocean warming in southwestern Japan

Tosa Bay in southwestern Japan is experiencing dramatic community phase shifts where temperate seaweed beds of brown algae have been tropicalized, a trend that has coincided with the most significant increases in surface seawater temperature in the world. Here, we report that blooms of green seaweed (genus Ulva) are also undergoing species replacement in this region, coincident with the tropicalization. Ulva green tides have been reported in Uranouchi Inlet, central Tosa Bay, since the late 1970s. The causative species was described as a new species, Ulva ohnoi, in 2004. However, the present investigation using a DNA marker-based identification technique showed that U. ohnoi did not form blooms but sparsely produced thallus fragments in the inlet during the period 2017–2018. Instead, the cosmopolitan species Ulva reticulata which has a distribution centered in more tropical waters was repeatedly found to dominate and form dense drifts in the inner end of the inlet.

Unpredictable extreme cold events: a threat to range-shifting tropical reef fishes in temperate waters

Persistent ocean warming has caused the rapid poleward shift of various tropical marine organisms. However, extreme cold events have been reported to have greater impacts on corals, yet no reports have elaborated how such cold events affect range-shifting tropical reef fishes in temperate waters. This study assessed benthic cover and assemblage structure of tropical reef fishes (Pomacentridae, Chaetodontidae, Scaridae, and Acanthuridae) on two reefs, dominated by tabulate Acropora corals, in Tosa Bay (33°N, 133°E), southwestern Japan. The study was conducted during winter and summer within 2 years (2017–2018), fortuitously covering periods before, during, and after the occurrence of an extreme cold event in early 2018. The event resulted in an approximately 2-month extremely low sea surface temperature (SST) of 90% corals. About 80% of the fish species richness and > 80% of their abundance declined during the winter of 2018, with a slow increase in their populations during the summer of 2018 as a consequence of the coral loss. The extremely low SST negatively affected non-established and breeding fish populations, while the massive coral loss severely affected corallivorous fishes. This study demonstrates the potential impact of extreme cold events on the persistent establishment of tropical reef fishes in temperate waters. Thus, our study highlights the potential instability of temperate waters as a refuge for range-shifting tropical reef fishes since extreme climatic events are predicted to increase amidst climate change. This may potentially impair benefits from the tropicalized temperate reef ecosystem services (e.g., local tourism and fishery industries).

First insight of bathymetric patterns among deep-sea harpacticoid diversity and composition on landward slopes of subduction zones along the Japanese island arc

Universal classification schemes have been proposed for the vertical zonation of sessile benthos on intertidal rocky shores; however, generalized patterns among deep-sea meiofauna of different regions remain rare. We conducted a preliminary, family-level study of deep-sea harpacticoid diversity and community composition off Tosa Bay, Shikoku island, Japan, along a transect on a landward continental slope of the Nankai Trough. We then compared these data to those of other subduction zones along the Japanese island arc, including the Kuril, Japan, and Ryukyu trenches. Common bathymetric trends in harpacticoid composition were observed among these subduction zones from upper bathyal to abyssal depths (400–5800 m). Ectinosomatidae was the most abundant family within this depth interval; however, its frequency decreased with increasing depth, whereas the abundance of Pseudotachidiidae was highest at abyssal depths. Harpacticoid composition differed significantly between landward slopes of the Ryukyu Trench and those of the Japan and Kuril trenches. However, harpacticoid composition on the landward slope of the Nankai Trough, between the Ryukyu and Japan trenches, was similar to that around the neighboring subduction zones. These findings imply that patterns of deep-sea harpacticoid community composition are similar along the continental slope of the Japanese island arc, and that the Izu–Ogasawara arc, which subdivides the subduction zones, has not functioned as a physical barrier to harpacticoid expansion between zones.

Vertical distribution of eggs and larvae of &lt;i&gt;Maurolicus japonicus&lt;/i&gt; (Sternoptychidae, Pisces) in Tosa Bay, Japan

Vertical distribution of eggs and larvae of &lt;i&gt;Maurolicus japonicus&lt;/i&gt; (Sternoptychidae, Pisces) in Tosa Bay, Japan

A new genus for "Neocallichirus" grandis Karasawa amp; Goda, 1996, a ghost shrimp species (Decapoda: Axiidea: Callianassidae) heretofore known only by fossil materials.

A new genus of the family Callianassidae, Laticallichirus, is established for "Neocallichirus" grandis Karasawa Goda, 1996, a ghost shrimp species heretofore represented only by fossil materials from the middle to upper Pleistocene of Japan. The newly collected specimens came from two Japanese localities: Numazu, Shizuoka Prefecture (Suruga Bay) and Tosa, Kochi Prefecture (Tosa Bay), included within the known geographical range derived from fossil records. The species identification was based on the close agreement in the cheliped morphology and the known fossil distributions. The new genus is assigned to the subfamily Callichirinae Manning Felder, 1991, and compared with seven genera, Balsscallichirus Sakai, 2011, Callichirus Stimpson, 1866, Callichiropsis Sakai, 2010, Grynaminna Poore, 2000, Michaelcallianassa Sakai, 2002, Lepidophthalmus Holmes, 1904 and Podocallichirus Sakai, 1999, all characterized by the antennular peduncle being longer and stouter than the antennal peduncle. The new genus is characteristic in combination of the following characters: rostrum spiniform; maxilliped 3 devoid of exopod; ischium-merus of maxilliped 3 broadly operculiform; maxilliped 3 ischium devoid of crista dentata; major cheliped devoid of meral hook or associated lobe-like structure; tergites of the pleomeres 3-5 without conspicuous ornamentation; male pleopod 2 biramous, its endopod devoid of appendices interna and masculina; uropodal endopod suboval in shape; telson distinctly wider than long, with shallowly concave posterior margin. The genetic analysis using the mitochondrial 16S rRNA places the new genus as sister to Callichirus Stimpson, 1866, although bootstrap support for major branches is generally low. The newly collected specimens were found in burrows in soft sediments of shallow subtidal zone being accessible at low tide.

Description of a New Species of Pelagic Eelpouts Melanostigma japonicum sp. nova (Zoarcidae) from the Pacific Coast of South Japan with a Key for the Species of the Genus Melanostigma from the Pacific Ocean and the Adjacent Waters of the Indian and Southern Oceans

A new species of Melanostigma japonicum sp. n. is described from the Tosa Bay (Southern Japan). The holotype of the species, previously identified as M. orientale, differs from the latter and other Melanostigma species by a unique structure of jaws with very large anteriorly protruding front teeth, an original combination of a number of meristic characters, coloration, and body proportions. M. flaccidum, previously placed in the synonymy of M. gelatinosum, is restored as the valid species. The issues of origin and historical dispersal of Melanostigma species are discussed. A key for identifying the species of this genus in the Pacific Ocean and in the adjacent waters of the Indian and Southern Oceans is provided.

Zenopsis filamentosa (Zeidae), a new mirror dory from the western Pacific Ocean, with redescription of Zenopsis nebulosa

A new mirror dory, Zenopsis filamentosa, is described from specimens collected off the Pacific coast of southern Japan (Mie and Tosa Bay), the Kyushu-Palau Ridge, Fiji, and Tonga. Within Zenopsis, the new species is most similar to Zenopsis nebulosa, widely distributed in the Pacific Ocean, in having nine dorsal-fin spines and similar buckler numbers along the anal-fin base. However, it can be distinguished from the latter in having 24–27 (modally 26) dorsal-fin soft rays (vs. 25–28, modally 27 in Z. nebulosa), 24–25 (usually 25) anal-fin rays (vs. 25–27, usually 26), filamentous membranes extending from the spinous portion of the dorsal fin, of length more than twice SL (vs. moderately elongated, not exceeding the caudal-fin posterior margin when adpressed); 6–9 (modally 7) bucklers between the pelvic and anus (vs. 7–10, modally 8), no bucklers above the anal-fin spines (vs. a small buckler present), and the pelvic fin with three to five dark stripes of all sizes (vs. lacking distinct markings in specimens >ca. 100 mm SL). Sequence variations on the mitochondrial DNA COI region demonstrated the validity of each species. Zenopsis nebulosa is redescribed, on the basis of western and Central Pacific Ocean specimens.