. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 106 H. J. BROOK ET Figure 4. Scanning electron micrograph of the paired penes, with openings of the duct at the distal ends, of a mature male Gnorimo- sphaeroma oregonense (scale bar = mm). in the immature form after three molts. In the September 1992 field census, a few smaller males were present, similar in size to the one-molt mature males, suggesting that molt number varies in the field as well as in the laboratory. This variability was not associated with female size at brooding or with the precocial developme
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 106 H. J. BROOK ET Figure 4. Scanning electron micrograph of the paired penes, with openings of the duct at the distal ends, of a mature male Gnorimo- sphaeroma oregonense (scale bar = mm). in the immature form after three molts. In the September 1992 field census, a few smaller males were present, similar in size to the one-molt mature males, suggesting that molt number varies in the field as well as in the laboratory. This variability was not associated with female size at brooding or with the precocial development of penes. Sex ratio of developing juveniles. The sex ratio of devel- oping juveniles was unaffected by the presence of mature male and mature female G. oregonense (Fig. 10). Due to the high mortality (50-51%) of juveniles over the course of the 52-day culture period in both experiments, replicates within a treatment were pooled for statistical analysis. The presence of adult males (A'0 . - 1 -207, P > ) or brood- ing females (A",42 == , P > ) had no significant effect on the sexual development of juveniles. The results of these experiments showed that not all G. oregonense are protogynous. Although a large percentage of individuals could not be sexed at the end of the culture period, ± (n = 126) of juveniles developed into immature males, with penes and no oostegites. In contrast, only ± (;; = 16) developed into receptive females. The remaining juveniles (n = 57) had not matured to a stage exhibiting external reproductive structures before the experiment was terminated. Immature males that were isolated and maintained in plastic vials after the end of the experiment continued to develop to the mature male form, and none became females. Discussion The results of this study provide direct evidence of pro- togynous sex change within the isopod Gnorimosphae- roma oregonense. Field surveys showed sexual size di-. Please note
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