Search

Abstract. Patterns of searching behaviour were investigated in a local population of stout-body chromis, Chromis chrysurus , feeding on zooplankton in mobile foraging aggregations. Although individuals tended to wander independently of each other, their foraging ranges overlapped considerably. The local population had a more or less common home range (17 993 m 2 ) within which there were three foraging regions (ca 1200-3000 m 2 ). Within these regions, each identified individual usually stayed for more than 1 h, irrespective of the presence or absence of prey, and searched in a tortuous pattern with reduced velocity. In contrast, movements between regions tended to be rapid with almost no feeding. Thus the species employed a local search strategy involving spatial memory and expectation, despite using a rapidly changing and dynamic zooplankton resource. This searching pattern probably enables foraging to be concentrated at the front of zooplankton-supply routes associated with tidal currents, resulting in relatively high capture rates per unit effort.

Spawning sites and spawning migration paths of tagged females of the protogynous wrasse,Halichoeres marginatus, were studied on the shallow reefs at Kuchierabu-jima Island, Japan. Males set up mating territories above prominent rocks on the offshore reef slope in the late afternoon, and pair-spawned with females, which had migrated there from their home ranges located in inshore areas. Small females migrated to the spawning sites near their home ranges, whereas large females migrated to various spawning sites located within a wide area, including downcurrent sites. Spawning at the downcurrent sites favors transport of eggs offshore, thereby increasing the female’s fitness. The spawning sites where an individual had spawned as a female were subsequently used for mating after it had changed sex. It is suggested that the wide migration of females to various spawning sites, enables the storing up of information on those sites, which later helps in the acquisition of mating territories after changing sex.

The age and growth of the labroid fish Halichoeres poecilopterus from the Seto Naikai were estimated by examining the scales based on the specimens collected from the Seto Naikai from 1981 to 1984. With respect to the results of our study, one annulus of each scale is formed once a year in the hibernating period from January to April. This species spawns in early summer from late June to July, based on the temporal fluctuation in gonad index of the female. At the age of 1, 2, 3, 4 and 5 years old, the fishes are 61, 86, 106, 124 and 140mm in standard length, respectively, as estimated by the marginal growth indices. The general growth of the fish is presented by the following formula described by BERTALANFFY: Lt(mm)=234.8{1-e-0.151(t-0.986)}. The length(mm)-weight (g) relationship of this species is given by the formula, W=4.886×10-6L3.280. Consequently the asymptotic weight is estimated as 291.6g. The mean standard length of the immature yearlings collected in December, 1983 and 1984, were 42.4 and 42.3mm, respectively. The back-calculated standard lengths of yearling fishes in early summer are expected to be 44.7mm in December. The back-calculated standard lengths of the yearling are almost equal to their actual standard length.

In oredr to clarify the age and growth of Nibea albiflora in the Seto Inland Sea, Samples were collected during one year from the landings picked up from the Sea. Each scale removed from the sample specimen was examined and measured. The results of analysis of the annual ring on the scales show that one annulus is formed once a year, in the month of June and July. As the spawning season runs from May to August and since most specimens of fish spawn in June and July, the time of the formation of the new annulus is approximately in accordance with the spawing season. The standard lengh of this species found in the Seto Inland Sea is 151, 238, 305 and 357mm on an average at the ages of full 1, 2, 3 and 4 years, respectively, The general growth formula for this species is adequately expressed by the following equation ascribed to BERTALANFFY: Lt=529 (1-e-0.262t-0.736). The growth of the yearling immature fish was studied on samples collected in August, September and November. This growth, on the basis of an analysis of the progression of mean and mode in succesive length-frequency distributions of the immature fishes, was discussed as compared with the growth estimated from the monthly changes of the marginal growth index of the scale.

This report presents the results of a study of oogenesis, gonadial development and spawning in N. albiflora. The study is based on particular examinations of the ovaries and the eggs within the ovaries of the fish collected in the Seto Inland Sea. By histological study, the oogenesis of this species is fundamentally similar to that of other marine teleosts reported until now. Frequency distribution of the ova diameter of mature ovary has three modes: one is the evident mode of the ripe eggs group segregated, and the other two are the mode of maturing eggs group. Spawning occurs from late May to early August, and the height of the spawning season is in June and early July. It may be considered in all aspects that each fish spawns more than once during the spawning period. Since on females less than 198mm in standard length are found to be fully mature, the biological minimum size is about 20cm. The relationship between the fish size (Lcm) in standard length and the number of ovarian eggs (N) more than 0.2mm in diameter is indicated by the exponential equation: N=18.02 L3.216. Futhermore, the number of eggs (N') which is shed at a single spawning by each fish may be estimated by the equation: N'=0.9090 L3.581.

The spermatozoa obtained from the mature Kuruma prawn Penaeus japonicus BATE wereobserved with the scanning electron microscope. Seminal fluid was taken out, respectively, of the spermatophore in the male and of the spermatophore inside the seminal receptacle in the female. The spermatozoa from male and female are identical in shape and size. The external mor-phology of the spermatozoa of P. japonicus consists of a hemispherical head (main body)of about 4.2 to 5.2μm in maximal diameter and an extremely slender tail (or spike) of about 5.5 to 6.6μm long, which projects from the convex side of the head. The basal side of the head is intensively hollowed at the central region. The whole surface of the head is rough whereas that of the tailis smooth. The spermatozoa of Decapoda are so far separated into the unistellate sperm of natantians and the multistellate sperm of reptantians. This separation is applicable to the present species since the spermatozoa of P. japonicus, which belongs to natantia, are typically unistellated.