Takanobu Mizuta

Gonadal state of wild amphioxus populations and spawning success in captive conditions during the breeding period in Japan

Abstract Adult individuals of amphioxus (Branchiostoma belcheri) were collected by dredging from a research vessel at selected stations in two areas off the coast of Japan in July 2000: Deyama and Takamatsu so named by local fishermen in the Enshu Nada Sea. The number of males collected exceeded that of females at all the stations in Takamatsu and at four of five stations in Deyama. The over all sex ratio (males : females) of the collected animals was 1.2 : 1. The animals showed various maturational stages of the gonad, and approximately 70% had mature gonads. However, post-spawning animals were identified only at two stations in Takamatsu. Mature animals were placed in laboratory tanks. These animals remained in good conditions for about two months, and many animals spontaneously spawned in the tanks. This is the first report of spontaneous spawning of B. belcheri in Japan.

In vitro conversion of sex steroids and expression of sex steroidogenic enzyme genes in amphioxus ovary

Sex steroids are essential hormones for reproduction in vertebrates. The existence of a sex steroidogenic pathway in invertebrates is controversial because cytochrome P450 (CYP) genes have not been detected in the genomes of an echinoderm and a urochordate. However, cloning and gene expressions of sex steroid-metabolizing enzymes have been reported in a cephalochordate. In this study, in vitro conversion of sex steroids from pregnenolone (P5) to progesterone (P4), from 17alpha-hydroxyprogesterone (17alpha-P4) to 17alpha, 20beta-dihydroxy-4-pregnen-3-one (17alpha, 20beta-P) and 17alpha, 20alpha-P, and from androstenedione (AD) to estrone (E1), estradiol-17beta (E2), and testosterone (T) were confirmed by an incubation experiment performed using (14)C-labeled precursors and mature ovarian homogenates of the amphioxus Branchiostoma belcheri. In amphioxus, the ability of immature ovaries to synthesize sex steroids was much lower than that of mature ovaries. Post-spawning, the mRNA of CYP11A significantly decreased in females. Transcripts of the CYP19 gene also declined in one-fourth of the females after spawning, although this trend was not supported statistically. The mRNAs of CYP17 and 17beta-HSD did not change before and after spawning. Our results indicate the presence of Delta(4) and another derivative pathway in the amphioxus ovary and suggest that the synthesis of sex steroids, particularly estrogen synthesis, may be low in females after spawning behavior.

Preferential acceleration of heavy ions in multi‐component plasmas

We discuss preferential, perpendicular acceleration of α particles in large amplitude magnetohydrodynamic (MHD) waves by using a test particle simulation. In a 3 component plasma that consists of electrons, protons and α particles, one of the wave branches has phase velocity larger than the Alfven velocity in a long wave length regime. We find that the wave-particle interaction between this wave and the other Alfven wave leads to strong anisotropic acceleration in perpendicular direction to the ambient magnetic field. We examine the wave-particle interaction for a 3 component plasma, and obtain good agreement with the test particle simulation.

Regulations' effectiveness for market turbulence by large erroneous orders using multi agent simulation

We built an artificial market model and investigated the impact of large erroneous orders on price formations. Comparing the case of consented large erroneous orders in the short term with that of continuous small erroneous orders in the long term, if amounts of orders are the same, we found that the orders induced almost the same price fall range. We also analyzed effects of price variation limits for erroneous orders and found that price variation limits that employ a limitation term shorter than the time erroneous orders exist effectively prevent large price fluctuations. We also investigated effects of up-tick rules adopting the trigger method that the Japan Financial Services Agency adopted on November 2013.

Acrosome Reaction in Spermatozoa from the Amphioxus Acrosome Reaction in Branchiostoma belcheri (Cephalochordata, Chordata)

Abstract The formation of an acrosomal process at acrosomal exocytosis in spermatozoa of the amphioxus was described in the present report for the first time. A non-reacted acrosome was located in front of the nucleus, where a cup-shaped acrosomal vesicle covered a conical accumulation of subacrosomal material. When naturally spawned spermatozoa were treated with a calcium ionophore, ionomycin, the acrosomal vesicle opened at the apex and an acrosomal process was projected. The process exhibited a filamentous structure. The reaction followed the mode typically seen in marine invertebrates. These observations suggest that the features and function of the acrosome of amphioxus, whose position is on the border between invertebrates and vertebrates, reflect their ecological adaptation and phylogenic position.

Acrosome reaction in spermatozoa from the amphioxus Branchiostoma belcheri (Cephalochordata, Chordata).

Abstract The formation of an acrosomal process at acrosomal exocytosis in spermatozoa of the amphioxus was described in the present report for the first time. A non-reacted acrosome was located in front of the nucleus, where a cup-shaped acrosomal vesicle covered a conical accumulation of subacrosomal material. When naturally spawned spermatozoa were treated with a calcium ionophore, ionomycin, the acrosomal vesicle opened at the apex and an acrosomal process was projected. The process exhibited a filamentous structure. The reaction followed the mode typically seen in marine invertebrates. These observations suggest that the features and function of the acrosome of amphioxus, whose position is on the border between invertebrates and vertebrates, reflect their ecological adaptation and phylogenic position.

Effects of Price Regulations and Dark Pools on Financial Market Stability: An Investigation by Multiagent Simulations

We built an artificial market model and investigated the impact of large erroneous orders on financial market price formations. Comparing the case of consented large erroneous orders in the short term with that of continuous small erroneous orders in the long term, if amounts of orders are the same, we found that the orders induced almost the same price fall range. We also analysed effects of price variation limits for erroneous orders and found that price variation limits that employ a limitation term shorter than the time erroneous orders exist effectively prevent large price fluctuations. We also investigated effects of up-tick rules, adopting the trigger method that the Japan Financial Services Agency adopted in November 2013.

A study on the reversal mechanism for large stock price declines using artificial markets

Deterioration in the fundamentals of firms due to scandals or disasters causes declines in their stock prices. We empirically know that stock prices rebound after they largely fall. In this paper, this trend is called the reversal phenomenon. There has been some preceding research on this issue; however, little has been explained about market mechanisms such as the market pricing mechanism responsible for the reversal in large declines in stock prices. We reproduced the reversal phenomenon in an artificial market with a degree of variation in expected prices, and not with the overreaction hypothesis, and found that a call market, which is a non-continuous double auction, imposes a condition where the market becomes non-efficient.

A Study on the Effectiveness of Short-Selling Regulation in View of Regulation Period Using Artificial Markets

Since the subprime mortgage crisis in the United Sates, stock markets around the world have crashed, revealing their instability. To stem the decline in stock prices, short-selling regulations have been implemented in many markets. However, their effectiveness remains unclear. In this paper, we discussed the effectiveness of short-selling regulation which was invoked temporarily after the market satisfies a regulation condition using an artificial market. First, we proposed an artificial market in which short-selling was regulated after the market satisfied the regulation condition. Next, we observed price variations in the markets in which the durations of short-selling regulation were different and discussed the bubble mechanism of them. Then, we observed the correlation between market stability and regulation periods and it was found that the longer the regulation period was, the more instable markets were. Therefore, we have found short-selling regulation had the property that it not only stemed the decline in the prices but also increased the prices excessively and market instability increased with increasing regulation period. Finally, we determined performances of each agent type in the market in which the regulation was invoked.

A Study on the Market Impact of Short-Selling Regulation Using Artificial Markets

Since the subprime mortgage crisis in the United Sates, stock markets around the world have crashed, revealing their instability. To stem the decline in stock prices, short-selling regulations have been implemented in many markets. However, their effectiveness remains unclear. In this paper, we discuss the effectiveness of short-selling regulation using artificial markets. An artificial market is an agent-based model of financial markets. We constructed an artificial market that allows short-selling and an artificial market with short-selling regulation and have observed the stock prices in both of these markets. We found that the market in which short-selling was allowed was more stable than the market with short-selling regulation, and a bubble emerged in the regulated market. We evaluated the values of assets of agents who used three trading strategies, specifically, these agents were fundamentalists, chartists, and noise traders. The fundamentalists had the best performance among the three types of agents. Finally, we observe the price variations when the market price affects the theoretical price.