Hikaru Endo

Inhibitory effect of 2,4‐dibromophenol and 2,4,6‐tribromophenol on larval survival and metamorphosis of the sea urchin Strongylocentrotus nudus

As a possible factor leading to the low recruitment level of sea urchins in kelp forests, the inhibitory effect of 2,4-dibromophenol (DBP) and 2,4,6-tribromophenol (TBP) released from the large perennial brown algae Ecklonia kurome and Eisenia bicyclis on survival and metamorphosis of eight-armed larvae of the sea urchin Strongylocentrotus nudus was examined. The percentage of larvae that underwent metamorphosis in filtered sea water after 1 h exposure to one-half dilution of saturated dibromomethane solution (≈60 ppm) as a chemical inducer reached approximately 100% after 1 h, while that in filtered sea water containing 1 ppm TBP was reduced to 73%. This was further reduced to less than 40% in the presence of 10 and 20 ppm TBP after 2 h. In filtered sea water containing 1 and 10 ppm DBP, the proportion of metamorphosed larvae was reduced markedly to 43 and 5% after 2 h, respectively. All larvae exposed to 50 ppm TBP and to 20 and 50 ppm DBP died after 1 h. These findings suggest that DBP is more toxic than TBP for sea urchin larvae, strongly inhibiting their metamorphosis.

Combining niche shift and population genetic analyses predicts rapid phenotypic evolution during invasion

The rapid evolution of non‐native species can facilitate invasion success, but recent reviews indicate that such microevolution rarely yields expansion of the climatic niche in the introduced habitats. However, because some invasions originate from a geographically restricted portion of the native species range and its climatic niche, it is possible that the frequency, direction, and magnitude of phenotypic evolution during invasion have been underestimated. We explored the utility of niche shift analyses in the red seaweed Gracilaria vermiculophylla, which expanded its range from the northeastern coastline of Japan to North America, Europe, and northwestern Africa within the last 100 years. A genetically informed climatic niche shift analysis indicates that native source populations occur in colder and highly seasonal habitats, while most non‐native populations typically occur in warmer, less seasonal habitats. This climatic niche expansion predicts that non‐native populations evolved greater tolerance for elevated heat conditions relative to native source populations. We assayed 935 field‐collected and 325 common‐garden thalli from 40 locations, and as predicted, non‐native populations had greater tolerance for ecologically relevant extreme heat (40°C) than did Japanese source populations. Non‐native populations also had greater tolerance for cold and low‐salinity stresses relative to source populations. The importance of local adaptation to warm temperatures during invasion was reinforced by evolution of parallel clines: Populations from warmer, lower‐latitude estuaries had greater heat tolerance than did populations from colder, higher‐latitude estuaries in both Japan and eastern North America. We conclude that rapid evolution plays an important role in facilitating the invasion success of this and perhaps other non‐native marine species. Genetically informed ecological niche analyses readily generate clear predictions of phenotypic shifts during invasions and may help to resolve debate over the frequency of niche conservatism versus rapid adaptation during invasion.

Age- and size-based morphological comparison between the brown alga Sargassum macrocarpum (Heterokonta; Fucales) from different depths at an exposed coast in northern Kyoto, Japan

Although large perennial brown algae are known to show great morphological plasticity responding to specific environmental factors such as depth and wave exposure, there are few reports showing this morphological variability taking into account age or size composition. Here, we conducted age- and size-based morphological comparison between the perennial brown alga Sargassum macrocarpum C. Agardh from their upper depth limit, middle depth, and lower depth limit at an exposed coast in northern Kyoto, Japan. Model II regression was fitted for the relationships between age or stipe diameter (SD) and morphological variables including SD, holdfast weight (HW), number of main branches (MBN), total weight of main branches (MBW), thallus weight (TW), and thallus length (TL) of 30 specimens collected seasonally from each depth zone. The differences between depths in these regression equations were analyzed using SMATR. Although SD, HW, and MBN of the specimens did not differ between depths under both age- and SD-basis, there were significant differences between depths in MBW, TW, and TL, suggesting that the differences in TW between depths are resulted from differences in MBW. Whereas TW at the lower depth limit was lower than that at the upper depth limit or middle depth in many months, TL at the upper depth limit was shorter than that at the middle depth or lower depth limit in some months. These results suggest that S. macrocarpum at shallower depths tend to have short and bushy morphology, whereas those at deeper depths have long and less bushy morphology.

Interactive effects of nutrient availability and temperature on growth and survival of different size classes of Saccharina japonica (Laminariales, Phaeophyceae)

Abstract: In northern Japan, the production of Saccharina japonica exhibits marked annual fluctuation due to changes in seawater temperature and nutrient availability during winter and spring. To better understand this phenomenon, we examined the combined effects of temperature (5°C, 10°C, 15°C and 20°C) and nutrient availability (seawater enriched with 25% Provasolis enriched seawater vs nonenriched seawater) on photosynthesis, growth and survival and nitrogen and chlorophyll a contents of juvenile sporophytes (2–3 cm) and larger size class sporophytes (100–150 cm) from southern Hokkaido. Both juvenile sporophytes and discs from 100–150-cm-class sporophytes cultured in enriched seawater showed significantly higher growth rates and photosynthetic activities than those cultured in nonenriched seawater. This difference likely was due to the significantly higher nitrogen and chlorophyll a contents of thalli grown in enriched seawater. Significant effects of temperature on growth and photosynthesis were also detected. A significant interaction between temperature and nutrient availability for relative growth rate (RGR) was detected. The positive effect of elevated nutrient availability on RGRs was magnified by an increase in temperature from 5°C to 15°C for juvenile sporophytes and from 5°C to 10°C for 100–150-cm-class sporophytes; whereas, the negative effect of elevated temperature from 10°C to 15°C on RGRs of 100–150-cm-class sporophytes was antagonised by a reduction in nutrient availability. In contrast to the 100% survival rate in enriched seawater, dead juvenile sporophytes and discs from 100–150-cm-class sporophytes were found at 15°C and 20°C in nonenriched seawater, and the survival rates decreased with increasing temperature. Therefore, the growth and survival of sporophytes of S. japonica are significantly affected by nutrient availability and temperature, which may lead to marked fluctuations in annual production.

Hidden diversity and phylogeographic history provide conservation insights for the edible seaweed Sargassum fusiforme in the Northwest Pacific

Understanding the evolutionary processes that have created diversity and the genetic potential of species to adapt to environmental change is an important premise for biodiversity conservation. Herein, we used mitochondrial trnW‐L and cox3 and plastid rbcL‐S data sets to analyze population genetic variation and phylogeographic history of the brown alga Sargassum fusiforme, whose natural resource has been largely exterminated in the Asia–Northwest Pacific in the past decades. Phylogenetic trees and network analysis consistently revealed three major haplotype groups (A, B, and C) in S. fusiforme, with A and B distributed in the Japan‐Pacific coast. Group C consisted of three subgroups (C1, C2, and C3) which were distributed in the Sea of Japan, the Yellow–Bohai Sea, and East China Sea, respectively. Isolation‐with‐migration (IMa) analysis revealed that the three groups diverged approximately during the mid‐Pleistocene (c. 756–1,224 ka). Extended Bayesian skyline plots (EBSP) showed that groups A and B underwent relatively long‐term stable population size despite a subsequent rapid demographic expansion, while subgroups C2 and C3 underwent a sudden expansion at c. 260 ka. FST and AMOVA detected low population‐level genetic variation and high degrees of divergence between groups. The cryptic diversity and phylogeographic patterns found in S. fusiforme not only are essential to understand how environmental shifts and evolutionary processes shaped diversity and distribution of coastal seaweeds but also provide additional insights for conserving and managing seaweed resources and facilitate predictions of their responses to future climate change and habitat loss.

Photosynthetic responses to photosynthetically active radiation and temperature including chilling-light stress on the heteromorphic life history stages of a brown alga, Cladosiphon okamuranus (Chordariaceae) from Ryukyu Islands, Japan: Photosynthesis of Cladosiphon okamuranus

Photosynthetic responses to temperature and photosynthetically active radiation (PAR) were investigated on the heteromorphic life history stages (macroscopic and microscopic stages) of an edible Japanese brown alga, Cladosiphon okamuranus from the Ryukyu Islands. Measurements were carried out by using optical dissolved oxygen sensors and a pulse‐amplitude modulated fluorometer. Maximum net photosynthetic rates and other parameters of the Photosynthesis – PAR curves at 28°C were somewhat similar in both life history stages, without characteristic photoinhibition at 1000 μmol photons m−2 s−1. Results of oxygenic gross photosynthesis and dark respiration experiments over a temperature range of 8–40°C revealed similar temperature optima for both stages (29.7°C, macroscopic stage; 30.3°C, microscopic stage), which support their observed occurrences in the habitat during summer. Maximum quantum yields of photosystem II (PSII) (F v/F m) were relatively stable at low temperatures with the highest at 15.1°C for the macroscopic stage and at 16.5°C for the microscopic stage; but dropped at higher temperatures especially above 28°C. Continuous exposures (6 h) to 200 and 1000 μmol photons m−2 s−1 at 8, 16, and 28°C revealed greater depressions in effective quantum yields of PSII (Φ PSII) of the microscopic stage at 8°C, as well as its F v/F m that barely increased after 6 h of dark acclimation. Whereas post‐dark acclimation F v/F m of both stages exposed to low PAR fairly recovered at 28°C, suggesting their photosynthetic tolerance to such high temperature. Under natural conditions, both heteromorphic stages of C. okamuranus may persist throughout the year in this region. Beyond its northern limit of distribution, the microscopic stage of this species may suffer from photodamage, as enhanced by low winter temperatures; hence, its restricted occurrence.

Physical Factors Involved in the Isoyake (Seaweed Forest Depletion) at Mio, Pacific Coast of Central Japan

Physical Factors Involved in the Isoyake (Seaweed Forest Depletion) at Mio, Pacific Coast of Central Japan In Japan, seaweed forest depletion is known as isoyake. We conducted a study on the marine environment to clarify the cause and the persistence of the isoyake in the sea area at Mio, on the west coast of the Kii Peninsula, central Japan. In the sea area at Mio, a layer of high-turbidity seawater formed on the sea surface (lowest Secchi disc depth: 1m) during and after precipitation. The average accumulation of sediment on the bedrock was 6.7 mg/ cm2. This value was approximately four times that recorded in the sea area at Noshima, where seaweed communities were present. These results imply that the large amounts of suspended particles in the seawater and sediment particles that settled on the bedrock were the major factors contributing to the continuation of the isoyake at Mio.

Influence of isotope fractionation on the nitrogen isotope composition of the brown macroalga Undaria pinnatifida : Isotope fractionation in macroalga

Although the stable nitrogen isotope ratio (δ15N value) of macroalgae is used to indicate sources of dissolved inorganic nitrogen in coastal marine environments, little is known about the effects of isotope fractionation on the δ15N value. We compared the δ15N values of Undaria pinnatifida cultivated at an inner bay and an exposed site (sites A and B, respectively) in Shizugawa Bay on the northeastern Pacific coast of Japan. Values for algal growth indicators, including thallus length, area, and weight, were higher at site B than at site A. The δ15N value of U. pinnatifida was significantly lower at site B (4.7 ± 1.0‰; mean ± standard deviation) than at site A (6.7 ± 0.4‰). This difference in the δ15N value of U. pinnatifida could not be explained by the δ15N value of seawater NO3 (source isotopic signature), which was similar at the two sites (7.5 ± 0.3‰). The pooled data from the two sites indicated that the δ15N value of U. pinnatifida decreased with increasing thallus size. The results suggest that the δ15N value of U. pinnatifida is influenced by isotope fractionation.

Effect of temperature and PAR on photosynthesis of an endangered freshwater red alga, Thorea okadae, from Kagoshima, Japan

Abstract The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of an endangered Japanese freshwater red alga, Thorea okadae (Thoreaceae, Thoreales), was determined by using pulse-amplitude modulation (PAM) fluorometry and dissolved oxygen sensors. The macroscopic life history stage (gametophyte) was observed on pebbles and cobbles in a shallow riverbed from early winter to late spring (December–May), in direct sunlight (∼800 μmol photons m−2 s−1) during winter noon time with a clear sky. A net oxygenic photosynthesis–PAR model revealed that the net photosynthetic rate increased linearly and saturated, with a compensation (Ec) and saturating PAR (Ek) of 13.3 μmol photon m−2 s−1 (10.3–16.3, 95% Bayesian prediction interval, BPI) and 55.2 μmol photon m−2 s−1 (42.2–72.9, 95% BPI), respectively. A temperature-dependent (i.e., 8–36°C range) model of net photosynthesis and dark respiration was fitted and showed that the gross photosynthetic rate, 17.3 μg O2 gww−1 min−1 (16...

Dietary Effect of Kelp (Saccharina japonica) on Gonad Quantity and Quality in Sea Urchins (Mesocentrotus nudus) Collected from a Barren Before the Fishing Season

ABSTRACT The sea urchin Mesocentrotus nudus is commercially harvested between June and August in northeastern Japan. A previous study showed that feeding on Saccharina japonica between April and June resulted in gonad quantity and quality improvement in this species from a barren. No information is available on the optimal culture period for harvest before the fishing season. The purpose of this study was to determine whether gonad size and quality (color, texture, and taste) can be improved before the fishing season and to verify the possibility of an early harvest. The sea urchins M. nudus were collected from a barren and placed under two types of culture conditions: suspended cages in a bay and a laboratory circulation system. They were fed S. japonica between December and May or March. Gonad indices, hardness, and color [L*, a*, and b*, and ΔEab* (a unit of acceptable commercial color difference from a fishing ground for each treatment)], and free amino acid (FAA) contents of the urchin gonads at the start of rearing experiments and at the end of cage (C) and laboratory feeding (LF) cultures were measured. These variables were also compared between LF and laboratory-starved urchins to confirm the dietary effect of this kelp. Gonad indices in treatments C and LF were significantly higher than those at the start of the experiment and those in laboratory starvation (P < 0.01), although there was no significant difference between C and LF. Gonads in treatment C were significantly softer than that at the start of the experiment (P < 0.0001). ΔEab* of gonads in treatment C was the lowest, indicating that the gonad color was close to the preferred color. There was no significant difference in the total FAA content between C and LF treatments, whereas, of the umami FAA, glutamic acid content was significantly higher in gonads from C than LF. Of the bittertasting FAA, tyrosine and valine contents were significantly higher in LF gonads than in C gonads. These results suggested that gonad size, hardness, and color of M. nudus were improved before the fishing season by initiating culture in December with S. japonica feeding.