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Hatching controlled by the circatidal clock, and the roleof the medulla terminalis in the optic peduncle of the eyestalk, in an estuarine crab Sesarma haematocheir

Masayuki Saigusa

Laboratory of Behavior and Evolution, Graduate School of Natural Science and Technology, Okayama University, Tsushima 3-1-1, Okayama 700-8530, Japan

View larger version (39K): [in a new window]   Fig. 1. Water-exchange method used to monitor (A) hatching of embryos attached to the female and (B) hatching of embryos detached from the female. Abbreviations: s, air stone; nt, nylon thread; ec, embryo cluster.

View larger version (27K): [in a new window]   Fig. 4. Hatching profiles of two intact females (A and B) and the embryo cluster detached from each female. Top panel: hatching of the embryos attached to the female. Middle panel: hatching of the embryos detached on the day of larval release from the female. Bottom panel: hatching of the embryos detached one day before larval release. The area depicted by broken lines shows 95% hatching, while the downward arrow shows the median time in hatching distribution. The 24 h L:D cycle is shown by the horizontal bar at the top of the figure (the filled section represents the dark phase). Vertical solid lines indicate the times of light on and light off in the experimental room. N is the total number of zoeas hatched. SI, synchrony index. HW, predicted time of high tide.

View larger version (98K): [in a new window]   Fig. 2. Optic peduncle of the eyestalk in the female of Sesarma haematocheir. (A) Schematic of the longitudinal section (lateral view). t-t' represents the position of the transverse cut in part F. Scale bar, 1 mm. (B) Right eyestalk of an intact female. Scale bar, 1 mm. (C) Eyestalk after surgery. Scale bar, 1 mm. (D) Incision on the eyestalk exoskeleton before making lesions to the optic peduncle (arrow). Scale bar, 1 mm. (E) A triangle `window' was cut in the exoskeleton. `e' shows the triangle exoskeleton removed. A cluster of neurosecretory cells (nsc) is seen from the triangle window of the eyestalk. This cluster of nsc is located at the edge of the ME (see part F). Scale bar, 1 mm. (F) Transverse section (t-t' in part A). Scale bar, 200µm. (G) The right eyestalk is removed after the experiment. The dark brown triangle (arrow) shows the piece of exoskeleton returned after surgery. Scale bar, 1 mm. (H) The optic peduncle after the transverse cut of the MT (right eyestalk). A mark of the cut (C) remains. Scale bar, 1 mm. (I) Schematic detailing the optic peduncle shown in part H. Scale bar, 1 mm. (J) Optic peduncle of the female (longitudinal section: lateral view). N1 is a cluster of neurons in the MT. The region between the white lines indicates the area where the lesions were made in experiment III-4 (Table 5). Scale bar, 100µm. (K) Ventral view of the optic peduncle (right eyestalk). Black dots (N1 and N2) indicate the cluster of neurons in the MT. t-t' represents the position of the transverse cut in part L. Scale bar, 300µm. (L) Transverse section of the MT (t-t' in part K). The ventral half of the MT (under the white line) was cut in experiment III-4 (see Fig. 3). Abbreviations: ce, compound eye; nsc, cluster of neurosecretory cells; sg, sinus gland; mt, medulla terminalis; xo, X organ; mi, medulla interna; me, medulla externa; lg, lamina ganglionalis; n, cluster of neurons on the medulla interna.

View larger version (28K): [in a new window]   Fig. 3. The optic peduncle of the right eyestalk. (A) Lateral view. This side shows the front of the crab. (B) Ventral view. The arrowhead shows the front of the crab. The dotted lines indicate the boundaries between the frontal, middle and posterior regions of the MT. Abbreviations: MT, medulla terminalis; nsc, cluster of neurosecretory cells; f, frontal region of the MT; m, middle region of the MT; p, posterior region of the MT; BA, base of the optic peduncle; TC, transverse cut of the MT; DC, dorsal-half cut of the MT; VC, ventral-half cut of the MT.

View larger version (28K): [in a new window]   Fig. 5. Timing of hatching monitored in the 24 h L:D cycles. (A) Control groups. Open circles indicate the time of hatching in intact females, and solid triangles indicate the time of hatching in the females immersed in ice water for 10 min. (B) Hatching and larval release of females from which the compound eye—retina complex was removed (experiment II-1; Fig. 2C). (C) Hatching and larval release of females from which the optic peduncles had been removed up to the medulla externa (ME) (experiment II-2; Fig. 2C). (D) Hatching and larval release of females from which the optic peduncles had been removed up to the medulla interna (MI) (experiment II-3; Fig. 2C). (E) Hatching and larval release of females from which a triangle window on the eyestalk exoskeleton had been removed and replaced (experiment III-1; Fig. 2E). The 24 h L:D cycle is shown by the horizontal bar at the top of the figure (the filled section represents the dark phase). HW connects the times of predicted high tide in the field. SS and SR indicate the times of sunset and sunrise in the field, respectively.

View larger version (32K): [in a new window]   Fig. 6. Pattern of hatching after transverse cuts were made on the medulla terminalis (MT; experiment III-2). The effects of the lesions were classified into four types (see Table 4 for details of each type). Parts (A)-(H) represent the pattern of hatching in individual females; data represent either the percentage of hatched zoeas (A-C) or the number of hatched zoeas (and that of embryos dropped without hatching) (D-H). Filled bars represent the number of mature, swimming zoeas; hatched bars represent the number of mature, but non-swimming, zoeas; stippled bars represent the number of embryos dropped from the female without hatching. The 24 h L:D cycle is shown by the horizontal bar at the top of each graph (the filled section represents the dark phase). SU indicates the date when the surgery was performed. N is the total number of zoeas hatched. The asterisks indicate the date when the optic peduncles were inspected under the stereomicroscope. In (H), the arrowhead indicates the time when the incubation was stopped.

View larger version (29K): [in a new window]   Fig. 7. Pattern of hatching after lesions were made on the ventral half of the medulla terminalis (MT; experiment III-4). The effects of the lesions were classified into four types (see Table 5 for details of each type). Parts (A)—(H) represent the pattern of hatching in individual females; data represent either the percentage of hatched zoeas (A,B) or the number of hatched zoeas (and that of embryos dropped without hatching) (C—H). Filled bars represent the number of mature, swimming zoeas; hatched bars represent the number of mature, but non-swimming, zoeas; stippled bars represent the number of embryos dropped from the female without hatching. The 24 h L:D cycle is shown by the horizontal bar at the top of each graph (the filled section represents the dark phase). SU indicates the date when the surgery was performed. N is the total number of zoeas hatched. The asterisks indicate the date when the optic peduncles were inspected under the stereomicroscope. In (E—G), the arrowhead indicates the time when the incubation was stopped.

View larger version (18K): [in a new window]   Fig. 8. Hatching profiles of four females (A—D) that had started the hatching program before surgery was performed. Filled bars represent the percentage of swimming zoeas; hatched bars represent the percentage of mature, but non-swimming, zoeas. The area surrounded by broken lines indicates 95% hatching. SU indicates the date when the surgery was performed. N is the total number of zoeas hatched.