Yoh Yamashita

Ecophysiology of juvenile flatfish in nursery grounds

Abstract Relationships between biotic and abiotic factors and the ecological performance of late larval and juvenile flatfish in nursery grounds are examined from ecophysiological viewpoints. The first events in the nursery are metamorphosis and settlement. Development of organs, osmoregulation and behavioural changes during metamorphosis, and size at metamorphosis are regulated by environmental factors. Various hormones play critical roles in this regulation. Effects of environmental conditions on individual growth in the nursery grounds are described on the basis of Frys five environmental factors: limiting, controlling, masking, directive and lethal factors. The main limiting factors are food and dissolved oxygen; controlling factors are temperature and body size; masking factors are salinity and pollutants; lethal factors are extreme environments; and directive factors are food, predators and dissolved oxygen. In addition to temperature, it has been indicated that dissolved oxygen seems to be relatively important for flatfish of the eastern US and northern European countries, while food abundance appears to be more critical for Japanese flounder. The feasibility is discussed of ecophysiological modelling to predict individual growth and subpopulation production based on the assessment of the role of environmental variability using the above classification, which organises and integrates environmental effects.

Microsatellite-based pedigree tracing in a Japanese flounder Paralichthys olivaceus hatchery strain: implications for hatchery management related to stock enhancement program

Effective population size in captive populations is affected by several factors such as the number of contributing broodstock to the next generation, sex ratio of parents, and variations in family size, which can be accurately evaluated by examination of the pedigree structure in the populations of interest. Here we present an example of microsatellite-based pedigree tracing in a hatchery strain of Japanese flounder Paralichthys olivaceus to be stocked into natural sea areas. We also detail the potential effect of selective operations (size selection) on the pedigree structure. The hatchery strain we screened was founded by using 18 wild captives (6 males and 12 females) through the use of the mesocosm spawning method, and the pedigree of the offspring including 113 individual larvae collected within 24 h after hatching, 216 individuals of 1 month old, and 407 individuals of 4 month old was unambiguously identified. The contribution of candidate broodstock to the next generation was highly skewed as the contribution to almost all of the offspring was monopolized by a single male, and a half of the females did not produce any offspring. The contribution of one family to the released fish selected for larger size (total length) was significantly high, while those of other two families were low (P<0.008).

Dietary value of benthic diatoms for the growth of post-larval abalone Haliotis discus hannai

The feeding behaviour and the growth of post-larval abalone (1–2 mm in shell length) Haliotis discus hannai fed on nine species of benthic diatom were examined in the laboratory. Digestion efficiencies of abalone fed on the nine diatom species were also measured. All abalone showed active feeding behaviour and diatom cells were observed in their stomachs. However, the growth rates of abalone fed on four diatom species, which were readily digested, were higher than those of abalone fed on the other five species of diatom. The cell walls of the former four diatom species were easily broken open when they were grazed by abalone because of their high adhesive strength (three species), or due to the weakly silicified cell walls (one species with a low adhesive strength). The other five species, whose adhesive strengths are low, were easily ingested by abalone without any deformation of the cell walls and the majority of them were excreted whole and still alive. It is considered that post-larval abalone cannot digest diatom cell contents in their alimentary canal without first rupturing the cell wall with the radula. It is thought that the post-larvae which ingested the latter five diatom species could not readily absorb the diatom cell contents as the cell wall was intact and thus their growth rates were low.

Identification of consumed stone flounder, Kareius bicoloratus (Basilewsky), from the stomach contents of sand shrimp, Crangon affinis (De Haan) using mitochondrial DNA analysis

Abstract We have developed a method for the identification of consumed stone flounder, Kareius bicoloratus (Basilewsky) larvae and juveniles from the stomach contents of sand shrimp, Crangon affinis (De Haan). We succeeded in isolating stone flounder DNA from the stomach contents of sand shrimp, using a cell lysis buffer containing 8 M urea. DNA for PCR analysis was obtained from stomach contents even when it was isolated 5 hours after the termination of predation. However, the total DNA yield decreased as the time after predation progressed. Stone flounder was distinguished from other prey in the stomach contents of sand shrimp using PCR with universal primers for fish and restriction analysis of mtDNA. Successful amplification of stone flounder mtDNA was possible up until 4 h after the termination of predation, but no amplification was possible 5 h after the termination of predation. In addition, a part of the D-loop region of stone flounder mtDNA was cloned, sequenced, and used to design species-specific PCR primers that allow amplification for stone flounder. PCR products were not obtained from the total DNA of other possible prey of the sand shrimp: 11 fish species; 2 mysid species; one amphipod species; and 2 polychaete species. In this paper, we discuss the advantages and disadvantages of using the PCR method coupled with restriction analysis, and the species-specific PCR method to identify consumed larval and juvenile fish species in the guts of predators.

Survival and growth rates of post-larval abalone Haliotis discus hannai fed conspecific trail mucus and/or benthic diatom Cocconeis scutellum var. parva

Abstract The settlement rate and growth rate of post-larval Haliotis discus hannai were compared in the laboratory when raised on the following three experimental substrata. (1) Mucus: trail mucus of juvenile H. discus hannai (3 cm in shell length); (2) diatom: monoculture of the benthic diatom Cocconeis scutellum var. parva; (3) diatom and mucus: C. scutellum var. parva coated with trail mucus of juvenile abalone. The mean percentage of larvae that settled were 48.7 on mucus, 98.0 on diatom, and 97.3% on diatom and mucus substrata. All post-larval abalone that settled on the mucus substratum died by the end of the third week, with a mean shell length during the third week of 711.9 μm. Individuals on the diatom substratum died by the end of the second week, with a mean shell length during the second week of 481.6 μm. Post-larvae on the diatom substratum and on the mucus substratum did not reach 800 μm in shell length; the size at which they are able to utilize the cell contents of C. scutellum var. parva. In contrast, on the diatom and mucus substratum, 70.0% of the individuals survived to the end of the experimental period (4 weeks). Individuals on the diatom and mucus substratum grew well over the entire experimental period and reached approximately 1.4 mm in shell length 4 weeks after settlement. Post-larvae reared on the diatom and mucus substratum grew to more than 800 μm by feeding initially on the trail mucus of conspecific juveniles, which was probably supplemented with extracellular diatom mucus and decomposed cells of the diatom. Once they reached about 800 μm, they were able to access diatom cell contents.

Effects of delayed metamorphosis on larval competence, and postlarval survival and growth of abalone Haliotis discus hannai

The effects of delayed metamorphosis on larval competence, and the postlarval survival and growth of Haliotis discus hannai (H. discus hannai) were examined. Competent larvae were induced to metamorphose at 5, 10, 15, and 19 days after fertilization by the addition of 1 AM g-aminobutyric acid (GABA). Larvae in another group were maintained until individuals metamorphosed spontaneously. Metamorphosed individuals (postlarvae) were reared in the laboratory at 20 jC fed on a benthic diatom Cylindrotheca closterium or without any food in equivalent conditions. Metamorphosis, survival, and growth (determined from shell length) rates were measured. The percentage of individuals that had metamorphosed 2 days after the addition of GABA increased with the length of larval swimming period; larvae that were 5 and 19 days old when induced to metamorphose showed 19% (F5.6; S.E.) and 96% (F3.6) metamorphosis, respectively. The percentage of postlarvae that metamorphosed spontaneously increased after 17 days from fertilization and reached 96% (F3.0) at 24 days postfertilization. Survival rates of fed postlarvae depended on larval swimming period. For larvae that swam for V15 days, more than 80% of postlarvae survived throughout the 20-day experimental period. In contrast, the survival rate was only 57% (F2.9) for 19-day-old larvae. Postlarval growth rates did not differ significantly between larval swimming periods V15 days. However, the growth rate of postlarvae from 19-day-old larvae was significantly lower than that of V15-day-old larvae. In the starved treatments, survival rate was lower, and the final shell length of the dead animals was less, as larval period became longer. D 2002 Elsevier Science B.V. All rights reserved.

Jack mackerel Trachurus japonicus juveniles use jellyfish for predator avoidance and as a prey collector

Juveniles of carangid fishes including jack mackerel Trachurus japonicus are known to associate with jellyfishes. The function of this association behavior was studied through rearing experiments and underwater visual observations. Association behavior of jack mackerel with moon jellyfish in experimental tanks was more frequent in the presence compared to the absence of predators (chub mackerel Scomber japonicus). In the experimental tanks, the presence of jellyfish, however, did not mitigate predation by these predators. Although jack mackerel did not feed on the jellyfish itself, they frequently fed on the captured prey (Artemia nauplii) whilst in the gut cavity of the jellyfish. Underwater observations of giant jellyfish Nemopilema nomurai off Kyoto and Fukui prefectures revealed that approximately 30% of these jellyfish were accompanied by jack mackerel juveniles with body sizes ranging 10–45 mm standard length (SL). Considering that jack mackerel juveniles found in subtidal rocky reefs ranged 40–120 mm SL, we considered that jack mackerel from 10 to 45 mm SL associate with jellyfish as a hiding place as well as a food collector, until they find a suitable reef habitat when they attain approximately 40 mm SL.

Predation on fish larvae by moon jellyfish Aurelia aurita under low dissolved oxygen concentrations

Laboratory experiments were conducted to test the hypothesis that low dissolved oxygen concentrations have the potential to enhance the predation rate on fish larvae by moon jellyfish Aurelia aurita which is increasing in abundance in the coastal waters of Japan. Larvae of the red sea bream Pagrus major in four size classes (2.9, 4.1, 6.2 and 8.6 mm in standard length) were used as prey in a short-term predation experiment. No change in the bell contraction rate of the jellyfish (mean bell diameter, 100.4 ± 10.2 mm) was observed at the oxygen concentrations tested (1,2 and 4 mg/L, and air-saturated, 5.5–6.0 mg/L), suggesting a strong tolerance to a decline in the oxygen concentration. More than 80% of the 2.5 and 4.1-mm size-class larvae were predated on by the jellyfish at all oxygen concentrations during the 15-min trials in 10-L tanks. The 6.2 and 8.6-mm size-class larvae were able to escape from the jellyfish apparently due to their developed swimming ability at the two higher dissolved oxygen concentrations. There was an increase in predation on larvae of these two size classes at the two lower dissolved oxygen concentrations. Predation by moon jellyfish on fish larvae can be more intense at low dissolved oxygen concentrations that commonly occur in the coastal waters of Japan.

Contribution of diatoms as food sources for post-larval abalone Haliotis discus hannai on a crustose coralline alga

Abstract Survival and growth rates of post-larval abalone Haliotis discus hannai reared on a crustose (non-geniculate) coralline alga (CCA) Lithophyllum yessoense with diatoms (CCA + diatoms) and without diatoms (CCA − diatoms) were compared in the laboratory in order to determine the contribution of diatoms on CCA as food sources for post-larvae and the dietary value of CCA themselves. Experiments were performed for 5 weeks with two groups of different developmental stage, newly metamorphosed stage (younger postlarvae) and over 1 mm stage (older post-larvae). Both stages of post-larvae reared on CCA + diatoms grew well (53.4 ± 2.4 μm/day; mean ± SE and 85.0 ± 4.0 μm/day; mean ± SE, respectively). Younger post-larvae grew to over 2 mm in 5 weeks, and older ones reached over 4 mm in 5 weeks. There was no difference in shell length of younger post-larvae reared on CCA + diatom and CCA − diatom at the week 1. However, the mean growth rate of abalone on CCA − diatoms from the second to fifth week (24.4 ± 1.9 ...

Migration ecology of juvenile temperate seabass Lateolabrax japonicus: a carbon stable-isotope approach

The migration pattern of juvenile temperate seabass Lateolabrax japonicus in the stratified estuary of the Yura River was examined using carbon stable-isotope ratios (δ(13)C). δ(13)C values of mysids, which are the most important prey items for this species, were consistently enriched in the lower estuary and surf zone (LES), while depleted in the freshwater zone (FW). δ(13)C values of juveniles in LES were enriched, while those in FW were depleted, consistent with the δ(13)C difference in prey items. The results of δ(13)C showed that many juveniles migrated upstream from April to June and most of them stayed in FW until at least July. Juveniles that stayed in FW and LES for a relatively longer period (> c. 20 days) showed higher condition factors than those that stayed in FW for a short period (several days). This indicates that residence in FW enabled juveniles to achieve as good a body condition as residence in LES.