Naoto Okumoto

Behavioral and electrophysiological evidences that the lateral line is involved in the inter-sexual vibrational communication of the himé salmon (landlocked red salmon, Oncorhynchus nerka)

Characteristic vibrational signals are suggested to be exchanged between the sexes during the spawning behavior in the himé salmon (landlocked red salmon, Oncorhynchus nerka). To check whether the lateral line is used to detect and process these vibrational signals, we examined how Co2+, which is known to block the mechano-electrical transduction in the lateral line detector, affects both the spawning behavior and lateral line response of the male himé salmon. The results showed that Co2+ blocked both the spawning behavior towards the vibrating model (Fig. 2) and the lateral line response to the vibrational stimuli (Figs. 5, 6), if the fish were forced to swim in the water containing 1.0 mM Co2+ for 1 to 1.5 h or longer in the presence of 0.25 mM Ca2+. 0.1 mM Co2+ had similar but weaker effects. These results indicate that the vibrational signals from the vibrating model are detected and processed by the lateral line system to elicit the spawning behavior. These are the first experimental evidences that the lateral line sense is involved in the communicational behavior of the fish.

Effects of sex steroids on the smoltification of masu salmon, Oncorhynchus masou

Testosterone (T; 25-micrograms/g diet), estradiol-17 beta (E2; 2.5-, 25-micrograms/g diet), and 5 alpha-dihydrotestosterone (DHT; 2.5-, 25-micrograms/g diet) were administered to yearling masu salmon from February 3 through May 8. T and E2 inhibited the natural smoltification in spring and stimulated gonadotropin (GtH) accumulation in the pituitary, but DHT had no effect. T (25-micrograms/g diet) was administered to underyearling masu salmon from September 30 through December 14. At the end of the T treatment, fish body color darkened and pituitary GtH content increased in the T-treated group. On May 8 in the next spring, however, plasma T levels and pituitary GtH contents already decreased and smoltification occurred in both control and T-treated groups. On October 29, underyearling masu salmon reared under the short daylength (8 L:16 D) for 4 months were switched to a long daylength (16 L:8 D) and simultaneously administered with T (25-micrograms/g diet) or 11-ketotestosterone (11-KT; 25-micrograms/g diet). On December 17, the proportion of smolts was 52.8% in the control, whereas T and 11-KT administration inhibited smoltification under the artificially increased daylength. Pituitary GtH content significantly increased both in the T-treated and 11-KT-treated groups, though it was much lower in the latter. These results indicate that the springtime smoltification which is induced by increasing daylength is inhibited by biologically active sex steroids such as E2, T, and 11-KT.

Vibrational communication during spawning behavior in the himé salmon (landlocked red salmon,Oncorhynchus nerka)

SummarySexual behaviors of the salmon are composed of a stimulus-reaction chain, which ensures synchronous spawning between the sexes and successful fertilization. To characterize the signals involved in such a stimulusreaction chain, the body vibration and electromyographic activity of the trunk muscles during spawning were simultaneously recorded from freely behaving male and female pairs of himé salmon (landlocked red salmon,Oncorhynchus nerkd) and were analyzed in combination with a videographic analysis of behavior sequences. The results showed that the himé salmon have an elaborate communication system in which characteristic vibrational signals are exchanged. These are produced by body vibration due to trunk muscle activity related to spawning and are transmitted between the sexes with an accurate timing through the stimulus-reaction chain. They act as timing cues to synchronize gamete release and are thought to be shared among a wide variety of fishes. It was hypothesized that the lateral line sense is involved in the detection of these vibrational signals. Furthermore, based on the sequence matrix analysis as well as on information theory, intersexual behavioral sequences during spawning were analyzed statistically. The results showed that statistically significant interactions occur between the sexes and statistically significant amounts of information are transmitted through the interactions, supporting the results from recording experiments mentioned above. Characters of the signalling system and possible origins of the vibrational signals are also discussed.

Effects of thyroxine and methyltestosterone on smoltification of masu salmon (Oncorhynchus masou)

Abstract Administration of thyroxine (T 4 ; 250 μg/g diet) from 16 July to 24 December induced silvery skin in underyearling masu salmon, but failed to induce seawater adaptability, darkening of dorsal and caudal fin margins and slimness of the body. There was no difference in the percentage of precocious males in autumn between T 4 -treated and untreated groups. Administration of methyltestosterone (MT; 2.5 μg or 25 μg/g diet) from 10 February to 27 April inhibited natural smoltification both in T 4 -pre-treated and untreated yearling masu salmon. Fish treated with MT resembled parr or dark parr, and seawater tolerance did not develop. In contrast, approximately 90% of the fish in the control group became smolts. Increases in body length and weight were greatest in the control group and MT 2.5 μg group, respectively. Condition factor decreased in control fish but remained unchanged in the groups treated with MT. The MT 25 μg group showed the slowest growth. A dose-dependent decrease in plasma T 4 level was observed in the MT-treated groups. On the other hand, a dose-dependent increase was found in plasma and pituitary GtH content.

Characterization of vibrational and visual signals which elicit spawning behavior in the male himé salmon (landlocked red salmon, Oncorhynchus nerka)

Himé salmon (landlocked red salmon, Oncorhynchus nerka) have an elaborate communication system, which ensures synchronous spawning by both sexes and successful fertilization. Both the vibrational and visual signals have been suggested to be involved in this communication system. To characterize these signals, vibrating or stationary three-dimensional models of various types were presented to the male, and male behavior elicited was examined. The results showed that the vibrating model mimicking certain aspects of the female behavior (‘prespawning act’) elicits the spawning in the male (Figs. 2, 3, 5). Furthermore, it was found that (1) the presence of both the vibrational and visual cues (Figs. 6, 7), and (2) spatial coincidence of these two cues (Fig. 8) are necessary for eliciting this behavior. Based on these results it was supposed that the vibrational and visual informations converge at the central nervous system where they are integrated to elicit the spawning.

An HRP study of afferent connections of the supracommissural ventral telencephalon and the medial preoptic area in himé salmon (landlocked red salmon, Oncorhynchus nerka).

The supracommissural ventral telencephalon and the medial preoptic area have been shown to play important roles in the sexual behavior of himé salmon (landlocked red salmon, Oncorhynchus nerka). In the present study, the sites of neurons projecting to these regions were examined by means of the retrograde horseradish peroxidase (HRP) tracing method. The morphology of neurons in these sites of origin was also studied by means of the Golgi method. The nucleus preopticus periventricularis and the rostral part of nucleus preopticus (NPP-rNPO) received bilateral projections from the middle parts of the area ventralis telencephali pars ventralis (Vv) and the area ventralis telencephali pars dorsalis (Vd), NPP and lateral part of the preoptic area (LPOA), ipsilateral projections from the caudal part of Vv, nucleus anterioris periventricularis (NAPv), nucleus ventromedialis thalami (NVM) at the level of the posterior commissure, nucleus lateralis tuberis pars medialis (NLTm), nucleus anterior tuberis (NAT), nucleus saccus vasculosus (NSV), nucleus recessus posterioris (NRP) and midbrain tegmentum (TG), and a projection from the nucleus posterior tuberis (NPT), which is situated on the midline of the brain. The area ventralis telencephali pars supracommissuralis and neighboring caudal ventral telencephalon (Vs-cV) received ipsilateral projections from almost all parts of the Vv, the middle and caudal parts of Vd, almost all parts of the NNP, the NPO at the level between the habenula and the posterior commissure, and the rostral part of the nucleus dorsomedialis thalami (NDM). The Vs-cV also received a projection from NPT. These findings seem to give anatomical bases for understanding the neural mechanisms involved in sexual behavior as well as neuroendocrine functions.

Effects of photoperiod on smolting and hormonal secretion in masu salmon, Oncorhynchus masou

Abstract Underyearling masu salmon were reared under a 8L:16D photoperiod for approximately 3 months, and subsequently transferred to 8L, 10L, 11L, 12L, 13L, 14L or 16L photoperiod in October, in order to determine the suitable photoperiod for induction of smolting (Experiment 1). Smolt was judged by the external appearance of fish, i.e., silvery body color, black margin on the dorsal and caudal fins, and slim body shape. As early as 1 month following this transfer, smolts appeared in the 16L group. After 2 months, the 13L group had the highest frequency of smolts. After 3 months, most smolts were found in the 11L group and presmolts appeared even under constant short daylength (8L:16D). Overall, the largest number of smolts appeared in the group transferred from 8L:16D to 13L:11D. Underyearling fish reared under natural photoperiod were also transferred to 11L, 12L, 13L, 14L, 15L or 16L photoperiod in October (Experiment 2). Only a few smolts appeared during this experiment. In order to determine a suitable short daylength for pretreatment prior to the transfer to long daylength, underyearling masu salmon were reared under 6L, 7L, 8L, 9L, 10L, 11L, 12L and 13L photoperiods for approximately 5 months, and subsequently transferred to a 13L:11D photoperiod in October (Experiment 3). One month after the transfer, a small number of smolts appeared in only the group transferred from 6L. After 2 months, a large number of smolts appeared in the groups transferred from 10L and downward. After 3 months, a small number of smolts newly appeared in the groups transferred from 12L and 11L. A few smolts appeared in the group under constant 13L after 6 months. Finally, effects of increasing daylength on the secretion of growth hormone (GH), thyroxine (T 4 ) and triidothyronine (T 3 ) were investigated (Experiment 4). Seven days after transfer from short (8L:16D) to long (13L:11D) daylength, plasma GH levels were significantly higher than those in the control group under constant 8L. Plasma GH levels tended to increase gradually in both groups throughout the experiment. Plasma T 4 and T 3 levels were not affected by increasing daylength. These results indicate that it is necessary to rear fish under short daylength for a sufficient period of time before daylength can be changed to long-day for purposes of artificial induction of smolting, and the larger the difference in daylength after changing photoperiod to long daylength, the earlier smolts appear. Furthermore, an increase in daylength may stimulate the transient secretion of GH, which may participate in the process of smolting.

Efferents from the supracommissural ventral telencephalon in the hime salmon (landlocked red salmon, Oncorhynchus nerka): An anterograde degeneration study

Efferents from the area ventralis telencephali pars supracommissuralis (Vs) of hime salmon (landlocked red salmon, Oncorhynchus nerka) were studied using anterograde degeneration methods. It was found that Vs sends intratelencephalic fibers bilaterally to the internal cell layer of the olfactory bulb and wide areas of telencephalon, while it sends extratelencephalic fibers bilaterally to habenulae, inferior lobes and midbrain tegmenti through medial and lateral forebrain bundles and lateral preoptic area. A possibility was pointed out that some of these extratelencephalic projections are involved in the pathway controlling the sexual behavior which are suggested from behavioral experiments.

Rhythmic electromyographic activities of trunk muscles characterize the sexual behavior in the Himé salmon (landlocked sockeye salmon,Oncorhynchus nerka)

SummaryIn order to describe precisely the fixed action patterns of salmon sexual behavior, we recorded the electromyographic (EMG) activities of trunk and jaw muscles from freely behaving male and female Himé salmon (landlocked sockeye salmon,Oncorhynchus nerka). A series of action patterns (quivering and spawning act in males, digging, covering, prespawning act and spawning act in females, and the swimming and turning movements in both sexes) were characterized by rhythmic activities of the trunk muscles. Each of these activity patterns is quantitatively distinct from the others in such parameters as frequency, bout duration, duty value, intersegmental phase delay, and spatial distribution of rhythmic activities. However, all of these rhythms share a qualitatively homologous pattern with the forward swimming movement: rhythmic activities alternate on both sides of the body (bilateral coupling) and are posteriorly propagated (intersegmental coupling). In addition, a 3∶1 intersegmental phase coupling occurs in the most anterior trunk muscles during the spawning act in some males. Based on these observations, we discussed the biomechanics for these motor patterns (oviposition, ejaculation, body vibration, and mouth opening), and the neural mechanisms for the pattern generation. A possibility was pointed out that the locomotor pattern generator in the spinal cord may be modulated by descending supraspinal signals and recruited to generate such diverse forms of action patterns in sexual behavior.