Gametophytes of two Undaria species, U. pinnatifida and U. undarioides (Laminariales, Phaeophyceae), were studied to determine their water temperature requirements in order to understand their different distributions in Mie Prefecture, Japan. The optimal temperature for growth was 20 ° C for gametophytes of both species, and the upper critical temperature for growth was also the same for both species at 28 ° C. Therefore, the optimal and critical temperatures for growth of the gametophytes are not the main factors determining distribution. The optimal temperature for maturation of U. pinnatifida was approximately 10-15 ° C, whereas it was closer to 20-21 ° C for U. undarioides , a difference between these species of at least 5 ° C. In autumn and early winter, the seawater temperature at the mouth of Ise Bay, where U. pinnatifida is distributed, ranges from 21.6 ° C (October) to 12.7 ° C (December), and off Hamajima, where U. undarioides is found, the range is from 22.7 ° C (October) to 19.1 ° C (December). The seawater temperatures from October to December, which is the maturation season for the gametophytes, agreed well with the optimal temperature requirements for maturation of the gametophytes of both species. Thus the difference in the maturation temperature range of the gametophytes is a major factor determining distribution of these Undaria species along the Japanese coast.

SUMMARY Gametophytes of two Undaria species, U. pinnatifida and U. undarioides (Laminariales, Phaeophyceae), were studied to determine their water temperature requirements in order to understand their different distributions in Mie Prefecture, Japan. The optimal temperature for growth was 20°C for gametophytes of both species, and the upper critical temperature for growth was also the same for both species at 28°C. Therefore, the optimal and critical temperatures for growth of the gametophytes are not the main factors determining distribution. The optimal temperature for maturation of U. pinnatifida was approximately 10–15°C, whereas it was closer to 20–21°C for U. undarioides, a difference between these species of at least 5°C. In autumn and early winter, the seawater temperature at the mouth of Ise Bay, where U. pinnatifida is distributed, ranges from 21.6°C (October) to 12.7°C (December), and off Hamajima, where U. undarioides is found, the range is from 22.7°C (October) to 19.1°C (December). The seawater temperatures from October to December, which is the maturation season for the gametophytes, agreed well with the optimal temperature requirements for maturation of the gametophytes of both species. Thus the difference in the maturation temperature range of the gametophytes is a major factor determining distribution of these Undaria species along the Japanese coast.

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The relative growth rate of young sporophytes of Undaria pinnatifida (Harvey) Suringar and Undaria undarioides (Yendo) Okamura was examined in order to understand the difference in distribution of these two species around the coast of Japan. The optimal temperature for growth of both species was similar at 20 ° C and the upper critical temperature for growth was also similar, at 27 ° C for U. pinnatifida and 26 ° C for U. undarioides . Therefore, the optimal and upper critical temperatures for growth of the young sporophytes are not the main factors determining the distribution of each species. Next, the lower critical temperatures for growth were examined. For the young sporophytes of U. pinnatifida , the lower limit was less than 5 ° C while for those of U. undarioides it was 15 ° C. Thus, the difference in the lower critical temperature for growth between the two species was approximately 10 ° C. During the period of young sporophyte growth in the field, the temperature at the mouth of Ise Bay, Japan, where U. pinnatifida occurs, ranges from 12.7 ° C in December to 13.1 ° C in April, with a minimum of 7.9 ° C in February. Our experiments indicate that young sporophytes are able to grow throughout this period. The temperature off Hamajima, Japan, where U. undarioides occurs, ranges from 19.1 ° C to 14.8 ° C during the same time period. Again, young sporophytes are able to growth throughout this period, although minimum winter temperatures are only just high enough for growth. These natural temperature ranges during the growth season of the sporophytes agree well with the experimentally determined temperature requirements for growth of each species. Therefore, the difference between the two species in the critical temperature required for growth of the young sporophytes, especially in the low temperature range, is one of the major factors determining the distribution pattern of each species.

Genetic relationships among Undaria species and among populations of each species were studied based on DNA sequences of the mitochondrial cox3 gene. Although three Undaria species, U. peterseniana (Kjellman) Okamura, U. pinnatifida (Harvey) Suringar and U. undarioides (Yendo) Okamura, have been described based mostly on blade morphology, plants with intermediate morphologies have also been found. Multiple plants from several populations in Japan were collected. Morphological characters could identify most of the samples unambiguously. A few samples with intermediate morphologies were also collected. Mitochondrial haplotypes found in each population were different for each identified species, and each species had multiple haplotypes. In the cox3 haplotype network analysis, the numbers of steps between haplotypes within and between species were similar, and haplotypes of each species did not group together. The close genetic relationships among species strongly suggest that these species are conspecific. Alternatively, recent speciation could be possible with maintenance of ancestral polymorphisms within the species (i.e. incomplete lineage sorting). Haplotypes of samples with intermediate morphologies were different for each sample and the same as ones found in the local population, suggesting interspecific hybridizations among species.