Jun Shoji

Seagrass habitat reduces vulnerability of red sea bream Pagrus major juveniles to piscivorous fish predator

Predation experiments were conducted in mesocosms to test the hypothesis that habitat complexity affects vulnerability of red sea bream Pagrus major juveniles to piscivorous fish predators. Juvenile behavior was video-recorded for 6 h in two structurally different habitats: vegetated with sea grass Zostera marina, and unvegetated 0.5-t tanks. Association behavior with seagrass was observed in the vegetated tank throughout the trials. Predation experiments were conducted with 30 red sea bream juveniles (29.9 mm body length) exposed to two individuals of the piscivorous fish predator Chinese sea bass Lateolabrax sp. (261.6 mm), for 6 h in 1.0-t tanks. Predation rate (no. of fish predated on per predator per h) was significantly lower in the vegetated tank (0.02/predator per h) than in the unvegetated tank (0.27/predator per h). Present experiments indicate that habitat complexity reduces vulnerability of juvenile red sea bream to predation by piscivorous fish by serving as physical and/or visual barriers and limiting the predator’s ability to pursue and capture prey. Seagrass beds in the shallow coastal waters around Japan are suggested to be an important nursery for red sea bream since they provide the juveniles with habitat complexity as well as serve as a feeding ground.

Indomethacin induction of metamorphosis from the asexual stage to sexual stage in the moon jellyfish, Aurelia aurita.

We found while screening a chemical library that indomethacin, an inhibitor of prostaglandin biosynthesis, induced strobilation (metamorphosis from the asexual to sexual stage) in the moon jellyfish, Aurelia aurita. Indomethacin initiated strobilation in a dose-dependent manner, but was not involved in the progression of strobilation. Pharmacological experiments suggested that indomethacin could induce strobilation independently of prostaglandin biosynthesis.

Within species support for the expensive tissue hypothesis: a negative association between brain size and visceral fat storage in females of the Pacific seaweed pipefish

Abstract The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the high cost of brain development and maintenance is predicted to constrain adaptive brain size evolution (the expensive tissue hypothesis, ETH). Here, we test the ETH in a teleost fish with predominant female mating competition (reversed sex roles) and male pregnancy, the pacific seaweed pipefish Syngnathus schlegeli. The relative size of the brain and other energetically expensive organs (kidney, liver, heart, gut, visceral fat, and ovary/testis) was compared among three groups: pregnant males, nonpregnant males and egg producing females. Brood size in pregnant males was unrelated to brain size or the size of any other organ, whereas positive relationships were found between ovary size, kidney size, and liver size in females. Moreover, we found that the size of energetically expensive organs (brain, heart, gut, kidney, and liver) as well as the amount of visceral fat did not differ between pregnant and nonpregnant males. However, we found marked differences in relative size of the expensive organs between sexes. Females had larger liver and kidney than males, whereas males stored more visceral fat than females. Furthermore, in females we found a negative correlation between brain size and the amount of visceral fat, whereas in males, a positive trend between brain size and both liver and heart size was found. These results suggest that, while the majority of variation in the size of various expensive organs in this species likely reflects that individuals in good condition can afford to allocate resources to several organs, the cost of the expensive brain was visible in the visceral fat content of females, possibly due to the high costs associated with female egg production.

Day–night change in fish community structure in a seagrass bed in subarctic waters

The day–night change in fish community structure over a year was examined in a seagrass bed in Lake Notoro, Hokkaido, northern Japan, to examine nocturnal increases in predation risk. This seagrass bed has previously been considered a predation refuge for juvenile and small-sized fishes. Species richness, abundance and biomass of piscivorous fishes during nighttime were higher than those during daytime on all sample dates surveyed (May, August and November 2013), indicating an increase in predation risk for juvenile and small-sized fishes during nighttime. The mean biomass-weighted trophic level of fish communities in the seagrass bed was also higher at night than in the day. The piscivorous fishes collected in the seagrass bed during the night included important fishery species. These may obtain energy through nocturnal feeding in the seagrass bed. Therefore, the function of seagrass beds as fish habitats should be re-evaluated by considering two possible characteristics contributing to fishery production: as a daytime predation refuge for juvenile and small-sized fishes, and as a nighttime foraging ground for piscivorous fishes.

Increase in predation risk and trophic level induced by nocturnal visits of piscivorous fishes in a temperate seagrass bed

The majority of surveys on food webs of aquatic ecosystems have been conducted during the day owning to difficulties in sampling animals at night. In this study, to examine diurnal changes in predator-prey interactions in a temperate seagrass Zostera marina bed, a quantitative day/night survey of fish, the dominant animal community, coupled with acoustic telemetry of their predators, was conducted. The number of species, abundance, and biomass of piscivorous predators and mean trophic level during the night were significantly higher than those in the day in all seasons. Analysis of the stomach contents of 182 piscivorous predators showed that no fish predation occurred during the day whereas predation occurred during the night in winter, spring, and summer. Acoustic telemetry demonstrated nocturnal visits by dominant piscivorous fish species (rockfishes and conger eel) to the seagrass bed. We conclude that the nocturnal visits by piscivorous fishes increased the predation risk and trophic level in the fish nursery. The ecological functions of seagrass beds should be reevaluated accounting for day/night changes in food webs; these areas serve as nurseries for juvenile and small-sized fishes during the day and as foraging grounds for predators during the night.

High-resolution mapping and time-series measurements of 222Rn concentrations and biogeochemical properties related to submarine groundwater discharge along the coast of Obama Bay, a semi-enclosed sea in Japan

High-resolution mapping along the coast and time-series measurements of the radon-222 (222Rn) concentrations in the shallow zone in a semi-enclosed sea, Obama Bay, Japan, were undertaken in 2013. The temporal and spatial variations in the 222Rn concentrations were analyzed in parallel with meteorological conditions, physical–biogeochemical characteristics, and the submarine groundwater discharge (SGD) flux measured with a seepage meter. These data indicate that the groundwater influences the water properties of the bay and that the groundwater supply pathways are not limited to the local SGD. The concentrations of 222Rn flowing into the bay from rivers was known to be relatively high because groundwater seeps from the river bed. High-222Rn water was almost always present around the river mouth, and northward advection of the water affected the distribution of 222Rn concentrations in the bay. The southward wind suppressed the advection of the high-222Rn water and largely controlled the temporal variations in 222Rn concentrations at a station located on the north side of the river mouth, whereas the local SGD affected the short-term changes in the 222Rn concentrations. The concentrations of 222Rn and chlorophyll-a, an indicator of phytoplankton biomass, show a significant positive correlation in the surface layer along the coastline in seasons when the nutrient supply was the main factor limiting primary productivity.

Spatial–temporal variations in the composition of two Zostera species in a seagrass bed: implications for population management of a commercially exploited grass shrimp

We examined spatial and temporal variations in the species composition of two seagrass species, Zostera marina and Z. caespitosa, in a lagoon facing the Sea of Okhotsk. We also considered how those variations affected habitat quality for motile epifauna, especially for a commercial shrimp. A long-interval comparison between 1996 and 2013 showed that seagrass species composition in the lagoon did not vary, while their relative abundances did. A survey in 2012 revealed that the abundance of Z. caespitosa was affected by the abundance of Z. marina, water depth, and location in the lagoon. Although these seagrass species have similar aboveground morphology, differences in their fine structures were detected. Diversities of motile animals inhabiting the seagrass species were the same. Differences in seagrass utilization were observed when we focused on a commercial shrimp, Pandalus latirostris. This shrimp always preferred higher densities of shoots irrespective of the species and seasonally changed their preference for the number of leaves per shoot. They were therefore more abundant in Z. caespitosa, which had those structural characteristics. The results suggest that the management of shrimp resources could be improved by flexibly changing protected areas in accordance with the dynamics of seagrass distribution in the lagoon.

Age and growth of three rockfish species, Sebastes inermis, S. ventricosus and S. cheni, in the central Seto Inland Sea, Japan

Inter-specific comparison of growth was conducted among three Sebastes species (Sebastes inermis, Sebastes ventricosus and Sebastes cheni) which have previously been treated as a single species before the 2008 taxonomic review. Fish were collected in April 2010 for DNA analysis and each month from June 2011 to May 2012 for growth analysis in the central Seto Inland Sea, southwestern Japan. AFLP analysis of each of the three Sebastes species confirmed significant frequency differences in fragments. The result supported the identification of the three Sebastes species based on the morphological characters. Analysis of otoliths from a total of 94 S. inermis, 125 S. ventricosus and 71 S. cheni showed that an opaque zone at the outer margin of the sagittal otolith increased from April to October and the zone was validated as an annual ring. Growth of the three Sebastes species was estimated using the von Bertalanffy growth formula. There was no significant difference in the growth formulas between males and females within each species. The growth patterns among the three Sebastes species were significantly different and S. inermis grows slower than S. ventricosus and S. cheni in the central Seto Inland Sea.

Simultaneous observation of intermittent locomotion of multiple fish by fine-scale spatiotemporal three-dimensional positioning

Recent advances in biotelemetry techniques, especially positioning methods, have revealed the detailed behaviour and movement of aquatic organisms. Behavioural intermittence in animal locomotion, such as the Lévy walk, is a popular topic in the field of movement ecology. Previous attempts to describe intermittent locomotion quantitatively have been constrained by the spatial and temporal resolution possible with conventional biotelemetry systems. This study developed a fine-scale spatiotemporal three-dimensional positioning method using a new biotelemetry system with a positional precision of <10 cm and positioning interval of <10 s. Using this proposed positioning method, the intermittent stop-and-go locomotion of Siebold’s wrasses (Pseudolabrus sieboldi) was observed during travel from an unsuitable to a suitable location following displacement. The fish displayed behavioural intermittence in relocating to a suitable location. Initially, their movement halted for reorientation, after which they moved intermittently yet in a straight line to the suitable location. To test the positioning ability of the proposed method, data sets were resampled at intervals of 5, 10, 30, 60, and 300 s. Longer sampling intervals failed to identify reorientations and underestimated the number of stops, distance travelled, and speed. Overall, the results highlighted the adequacy and ability of the proposed positioning method to observe the intermittent locomotion of fish, such as stop-and-go behaviour, in a natural environment.

Relationships Between Submarine Groundwater Discharge and Coastal Fisheries as a Water-Food Nexus

We introduced the results of investigations conducted in Japan (Yamagata, Fukui, Hiroshima and Oita Prefectures) and discussed the contribution of SGD to the production of fishery resources and their prey (meiobenthic communities). Our recent surveys provided evidence of the contribution of SGD to the production of fishes and their prey. These results strongly support the hypothesis that the contribution of nutrients from SGD to the coastal fishery resources production is much higher than previously suggested. The negative ecological impacts of SGD on meiofaunal abundance were confirmed at the high seepage areas of Kamaiso where the small spatial scale presented strong heterogeneity in SGD environmental conditions. However, the meiofaunal abundance was not low at the low seepage area located slightly apart from the high seepage area. Therefore, the effect of SGD on meiofaunal communities could be limited. However, the environmental heterogeneity significantly enhanced the conservation of the meiofaunal diversity. Thus, SGD determined the property of benthic communities.