Although sexual segregation (sexual difference in resource use) is versatile in many animal taxa, studies have mostly concentrated on vertebrates. To observe sexual segregation in marine invertebrates, I focused on the sexual differences in shell utilization patterns of hermit crabs. I first reviewed hermit crab biology mainly relating to shell use, including shell acquisition, competition, shell preferences, and influences on fitness components (growth, survival, and fecundity).Then, I considered ecological and evolutionary factors for different shell utilization patterns between sexes observed in Pagurus filholi inhabiting Kattoshi, Hakodate Bay. I integrated the reports on shell species preferences, competitive abilities, and the effects of different shell species on fitness components in P. filholi. Competitive ability for the shells showed no sexual difference. The different shell preferences between sexes were consistent with the field pattern of shell use of the sexes. Females preferred shells which provided them with high fecundity and survival from predators, whereas males preferred shells with high growth potential. As body size is important to males to acquire female mates, high growth is significant for male fitness. Conversely, female fitness greatly depends on the number of eggs produced during their lifetime. In summary, the sexual difference in shell use of P. filholi is mainly determined by different shell preferences by sex, which are ultimately driven from different priorities between sexes to maximize fitness.
The anisogammarid amphipod Jesogammarus(Jesogammarus)hinumensis Morino, 1993 has been reported from south of Iwate Prefecture in Japan and Jeju Island in Korea. In this study, one male and one female of J.(J.)hinumensis were collected from the Imaizumi River flowing into the Lake Jusan, in the Tsugaru Peninsula of Aomori Prefecture in northern Honshu, Japan. This is the first record of J.(J.)hinumensis from the prefecture, representing the northernmost record. At the site, J.(J.)hinumensis was found under fallen leaves deposited at the roots of Phragmites australis
The species composition and prevalence of symbionts attached to the body surface of the burrowing shrimp Upogebia major(De Haan, 1841) were examined at three sites (Kobe, Arao, and Kuma-gawa) in the Ariake and Yatsushiro Seas of Kyushu, Japan, in spring and summer in 2017. At each sampling time per site, 15–46 U. major individuals were collected individually by the traditional hand-fishing method of inserting a brush into each burrow opening to lure the shrimp. The prevalence and intensity of the infection were examined for each symbiont. Our results at Kobe were compared with previous data from 1998. Six species of symbionts were collected. The scale worm Hesperonoe japonensis Hong, Lee & Sato, 2017(Polynoidae) was found at the two sites in the Ariake Sea, with prevalences much lower in 2017(2–6%) than the prevalence at Kobe in July 1998(89%).The bivalve Peregrinamor ohshimai Shoji, 1938(Galeommatoidea) was not found at any site in 2017, whereas it occurred with a prevalence of 7% at Kobe in 1998. The cyclopoid copepod Hemicyclops gomsoensis Ho & Kim, 1991(Clausidiidae) was found in all sites with the highest prevalence (100%) at Kobe and Arao in July 2017. An unidentified species of harpacticoid copepod (Peltidiidae) occurred at the two sites in the Ariake Sea, with the highest prevalence (59%) at Kobe. In addition to this, two parasitic bopyrid isopods, Gyge ovalis(Shiino, 1939) and Procepon insolitum(Shiino, 1937) also occurred with the highest prevalences of 6% and 5%, respectively.
To investigate the population dynamics of Donax semigranosus, its shell length was measured each month (March 2006 to March 2007) at Kujukuri Beach, Chiba Prefecture. To examine the effects of predation of the foot part by the sanderling Calidris alba, I conducted an experiment to test whether individuals whose foot parts were preyed upon could still burrow into the sand. The results showed that D. semigranosus began growing from March onwards, reproduced in the summer, and died in the winter. Therefore, the lifespan of the bivalves at the study site was estimated to be 1–1.5 years, similar to previous reports. Additionally, successive recruitments occur between August and October and overwintering occurs from December to March with a shell length of 6.9±1.5 mm (mean±SD, n=2,308). Unlike reports from the Sea of Japan, bivalves were common in the swash zone during winter (average of 10,701 ind./m2 from December to March). The experiment showed that most of the individuals (21 of 25 ind.) whose foot parts had been preyed upon were unable to burrow into the sand. Therefore, bivalves might be stranded due to wave action and eventually die on the beach. If the foot parts were less damaged, they could burrow into the sand and be able to regenerate the foot. Thus, foraging of foot parts by sanderlings could be a form of sublethal predation.
The life history characteristics of the varunid crab Cyclograpsus pumilio were studied by monthly sampling at a cobble shore in Ooigawa Port (34°46′35.2″N, 138°17′59.3″E), located in the inner part of Suruga Bay, Japan, between March and December 2017. A total of 964 individuals were collected, including 410 males (2.91–5.90 mm carapace width, CW) and 554 females (3.81–10.30 mm CW). The number of individuals collected exceeded 100 each month until August and then abruptly decreased to<10 per month. The monthly frequency distribution of mode CW showed no temporal changes in males and a slight decrease in females, from 9.5 mm in March to 7.0 mm in October. Smaller females of CW<4.5 mm were obtained between April and August. The changes in the relative growth of propodus height in males and abdominal width in females versus CW were determined by model analysis. The sizes at morphological maturity were estimated to be 3.98- and 6.39-mm CW for males and females, respectively. Ovigerous females with CW 6.92–9.29 mm (n=18) were obtained between June and August and again in October. During the study period, the population of C. pumilio in Ooigawa almost went extinct due to habitat loss in September, when a layer of cobbles collapsed following a typhoon. This local population was characterized by very low proportions of ovigerous females (<11%) each month, lower and variable batch fecundity, and shorter recruitment period compared with the proportions found in other areas.
Thirty-six sediment samples were collected from Lake Hamana, on the Pacific coast of central Japan, from 2013 to 2018 for research on ostracod assemblages. The distributions of living species were determined and compared with the results of a previous study conducted in the latter half of the 1970s. The diversity of living species had decreased in the central part of the lake (from 20 species of 19 genera to 9 species of 7 genera) but increased in the Shonai branch lake (from 10 species of 10 genera to 14 species of 10 genera). The overall lake species diversity was maintained (from 24 species of 23 genera to 22 species of 13 genera). However, the number of sampling stations at which living individuals were found had decreased from 25 to 17. The systematic diversity (diversity of taxa higher than species) also decreased. Furthermore, the area over which brackish- and freshwater species were distributed had particularly reduced. In contrast, there was an increase in species diversity, due to the inward migration of marine species along with the connection with the sea.
Spartina alterniflora, a halophytic plant native to the east coast of North America and the Gulf of Mexico, grows in river estuaries and tidal flats, forming large colonies. Given that the invasion of this species often has substantial detrimental effects on local ecosystems, including the survival of benthic animals and native plants, it needs to be appropriately managed. Since 2009, S. alterniflora has been spreading into the Ohno River, which flows into in the inner region of the Yatsushiro Sea. As the Ohno River environment has obstructed access to dredging machines and thus restricted the removal of S. alterniflora, colonies of this plant have rapidly proliferated. To characterize its spreading patterns in this river, we quantified chronological changes in the area covered by S. alterniflora colonies based on a comparison of aerial photographs. The colonies were found to be distributed primarily in the intertidal zone of the Ohno River. Although the number and area of S. alterniflora colonies have increased markedly along the river estuary up to 2 km upstream, these colonies are rarely found in the upper reaches of the tidal zone, which could be attributable to the fact that large stands of Phragmites australis reed cover extensive parts of the habitat in the upper part of the intertidal zone. Nevertheless, there remain areas in the intertidal zone that are potentially suitable for further growth of S. alterniflora (i.e., bare fields), indicating that it will continue to expand its distribution along the Ohno River in the future. Continuous monitoring of the Ohno River and neighboring rivers will facilitate the early detection of this spread and thereby minimize the costs of controlling S. alterniflora growth.
Corals are known to synchronously undergo gonadal development during a period of 1 year and release gametes simultaneously during the spawning season. Synchronization of reproduction may be caused by changes in the external environment; particularly, the effects of light on reproduction have been extensively reported. In this study, we specifically focused on the type of wavelength, with the intention to elucidate whether there are wavelengths that promote reproduction in two Acropora. We exposed coral colonies to four types of light-emitting diode conditions with wavelengths suitable for keeping coral: “Coral,” “Reef,” “Fresh,” and “Sunset,” and monitored gamete maturation every 3 months, as well as the number of released gametes during the spawning season. In A. intermedia, “Sunset” conditions promoted gamete maturation; however, there was no spawning observed. In contrast, in A. muricata, the same conditions promoted a series of processes ranging from gamete maturation to gamete release. In “Fresh” and “Coral” conditions, gamete maturation was also promoted. “Sunset” condition is characterized by longer wavelengths within the red-light visible spectrum, which is relatively predominant in the shallow waters where the two target species live. Therefore, this indicates that these longer wavelengths represent a light stimulus that promotes reproduction. By contrast, gamete maturation was promoted by even shorter wavelengths in A. muricata; therefore, further experiments considering different light sensitivities among species and growth promotion through the photosynthesis of zooxanthellae are recommended. It is also suggested that, in addition to wavelength type, spawning may be affected by water temperature increase after winter season.
In Japan, the flat oyster Ostrea denselamellosa Lischke, 1869 (Mollusca: Bivalvia: Ostreidae) has been known to occur along the coasts of Mutsu Bay south of the East China Sea. This species was commonly fished, especially in the Seto Inland Sea and Mikawa Bay until the 1980s. However, this natural resource has since deteriorated, and consequently, there has been no record of O. denselamellosa in Osaka Bay for several decades. In the present study, we sampled 11 Ostrea specimens collected with the help of an anchor dredge in Osaka Bay, and identified them as O. denselamellosa based on mitochondrial DNA barcoding. To our knowledge, the present study is the first record of O. denselamellosa from Osaka Bay after the latest recorded catch in 1980. Moreover, our specimens with various shell sizes implied that O. denselamellosa successfully reproduces in Osaka Bay.
To clarify the effect of irrigation drainage from paddy fields on an estuarine tidal flat (Ushibashi Estuary, Miyagi Prefecture),we investigated the carbon and nitrogen stable isotope ratios (δ13C and δ15N) of organic matters within the paddy fields, agricultural drainage channels (rivers),and estuarine tidal flats, and those of 18 species of estuarine benthic consumers from March to December 2020. The tidal flat sediments had lower δ13C and δ15N at the riverward station (δ13C: -27.1 to -25.4‰; δ15N: 2.7 to 4.3‰) than at the seaward station (δ13C: -26.5 to -20.5‰; δ15N: 3.3 to 7.1‰).The total nitrogen to organic carbon ratio (C/N) was low in tidal flat sediments (3.3 to 7.5),and it was estimated that organic matter derived from freshwater and seawater microalgae was deposited at the estuary. However, the effect of direct supply from the paddy field to river particulate organic matter (POM) was not detected. The carbon sources of most benthic animal species residing in tidal flats were presumed to be of marine origin (marine- POM and microphytobenthos) by a δ13C-based stable isotope mixing model (simmr).The contribution of food resource for some bivalves and polychaetes were higher in terrestrial sources than in marine sources. The estuaries of the agricultural drainage channels serve as a carbon sink for storing organic matter from the rivers, providing a unique habitat for benthic animals.
Fossils of sessile organisms can be used to reconstruct relative sea-level changes in the past. The upper limit of fossil colonies of the serpulid worm Spirobranchus sp. 1 (sensu Simon et al., 2019) is an excellent indicator of palaeo-shorelines. This method was defined on the Pacific coast where the tidal range is large and Spirobranchus colonies are well developed; however, the method is difficult to apply to the coast of the Sea of Japan because the tidal range is smaller and colonies are poorly developed in this area. Therefore, we investigated the vertical zonation of intertidal living sessile organisms over almost four years using immersion panels, mainly to assess the accuracy of Spirobranchus sp. 1 as a shoreline indicator on the Sea of Japan coast. The vertical zonation was investigated by measuring the number and size of calcareous shells and tubes attached to the panels. Spirobranchus sp. 1 occurs only in a 35-cm vertical zone near mean sea level and is useful as a palaeo-shoreline indicator. In addition, we obtained information on the vertical zonation of other sessile organisms in the intertidal zone and information on changes in the status of organism attachment over time. This information is useful for recognizing palaeo-shorelines and interpreting age data on the Sea of Japan coast.
Ariake Bay, located in the western coast of Kyushu, Japan, has been praised as “fertile sea” due to its extremely high production of various sea foods. However, since the late 1990s, red tide has occurred about twice as frequently as before, although the total nutrient discharge from the lands surrounding the bay has even decreased slightly since the 1970s. The increase of organic load on the sea floor by the frequent occurrence of red tide has caused seasonal hypoxia in the bottom water, and brought mass mortality of benthic animals in a wide area of the inner parts of the bay. These phenomena have occurred commonly in the enclosed coastal seas that have been suffering from eutrophication throughout the world. In this review, I collected various information on the occurrence of red tide, nutrient discharge, water conditions when red tide occurred, characteristics of the tidal current, etc., in Ariake Bay from the previous studies, and tried to clarify the mechanism how the red tide have occurred frequently, which is the most responsible for the recent marked decline of the benthic ecosystem in the bay. I focus on the impacts of the closure of the dike in a reclamation project conducted in the inner part of Isahaya Bay (an inner bay of Ariake Bay located in its western side of the middle part of the bay) in April 1997 not only on the tidal current in Isahaya Bay, but also the anticlockwise residual current that originally dominates in Ariake Bay.