Abstract
Geographic variations in reproductive traits are important for evolutionary biology, but often difficult to investigate because of the need for a large-scale survey and the ephemeral nature of secondary sexual characteristics. Here, using web image searches (Google Images and Twitter), we revealed large-scale geographic variations (>1500 km) in breeding timing and nuptial coloration in the mutually ornamented fish, Japanese dace (Tribolodon hakonensis). As this fish is easily caught and many anglers upload their photographs on the web, we were able to find a total of 401 high-resolution photographs from all over Japan. Breeding periods were determined from dates of photographs with/without nuptial coloration, which matched with previous studies. We also found that breeding periods might have advanced three weeks over the last 80 years in the Chitose River, potentially because of climate change. Additionally, the pattern of latitudinal cline for breeding timing revealed delayed timing in higher latitudes, although regional variations were also high. Finally, we quantified the patterns of nuptial coloration for this mutually ornamented fish, and confirmed that over 80% of individuals showed typical colorations, but others showed rare phenotypes that were never previously described, including an intermediate coloration between T. hakonensis and a potentially hybridizing species, T. brandtii. Our web-based method for estimating breeding phenology could be adopted for organisms with temporal sexual characteristics, such as plants (e.g., flowering time) and some fishes. Web image analyses are still preliminarily with many limitations, but could be promising for investigating variations in visible traits.
Highlights
Massive amounts of information have been made available on the web by people worldwide, and the quantity of information is growing at an overwhelming rate
We found a total of 401 photographs of Japanese dace from most areas of Japan (Table 1); of the 42 prefectures in which Japanese dace were found during the National River Census and the 46 prefectures that make up their known range, we acquired Japanese dace photographs from a total of 39 prefectures
We demonstrated the usefulness of web-based citizen data to investigate breeding season and the associated latitudinal cline, and nuptial color variations of a mutually ornamented fish, Japanese dace
Summary
Massive amounts of information have been made available on the web by people worldwide, and the quantity of information is growing at an overwhelming rate. Japanese dace is distributed in Far East Asia, and ranges from Kyushu to Sakhalin and Sea of Japan (Sakai 1995) In this fish, males and females exhibit remarkable nuptial coloration during breeding season (e.g., Nakamura 1969). Understanding inter- or intra-population variation in nuptial coloration could be important for revealing the function of this ornamentation Because this species ranges widely in latitude, breeding period varies geographically: February to March in Kyushu, southwestern Japan (Tabeta and Tsukahara 1964), and July in Hokkaido, northeastern Japan (Okada 1935). (4) we assessed inter- and intrapopulation variations in nuptial coloration, and considered allopatric and sympatric distributions with the potentially hybridizing species T. brandtii To our knowledge, this is the first study to estimate breeding season from web image searches, which might be a promising approach for phenology studies
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