The researcher assembled successful auction records from January 2013 through December 2023 through AucFan, an online database that aggregates listings and prices from auction sites including Yahoo! Auction, where live animals were traded in Japan. A search for the katakana term “Sawagani,” a common Japanese name for the crabs, initially returned 3,988 transactions involving 17 freshwater-crab species or unverifiable names. The analysis was narrowed to 3,431 transactions in which animals were classified as G. dehaani sensu lato, now treated as a species complex that may contain cryptic species or deeply divergent populations. Seller descriptions and photographs were used to assign colour categories, while prices, numbers of animals, collection prefectures, origin and seller characteristics were extracted from each listing.

The classification distinguished five market-facing colour groups: dark brown, red, sky blue, other colours and mixed lots. These categories differed from earlier colour classifications because they focused primarily on carapace appearance rather than combining the carapace with the colour of the walking legs. When sellers supplied no colour description, photographs were used to classify the animals; transactions without either were treated as unverifiable. Yellow, purple and other uncommon patterns were placed in the “other” category because they occurred too infrequently for detailed analysis. Origin was classified as wild-caught, captive-bred or unverifiable, and the number of wild-caught animals listed from each prefecture served as an indicator of collection pressure associated with the auctions. The study cautioned that this indicator did not represent the total collection pressure experienced by wild populations.

During the 11-year period, 46,319 crabs were traded in the analysed transactions, which generated ¥4,256,311 in total sales. More than 98% of the transactions, individual animals and sales value involved the pet category. Within this market, wild-caught animals accounted for more than half of all transactions, while captive-bred animals represented less than 1% of the totals. Every captive-bred animal was classified as sky blue, had reportedly been raised for more than three years, and was sold mainly in 2023. All transactions classified as food sales involved mixed-colour animals, principally lots combining dark-brown and red crabs. The dominance of the pet market means the results primarily describe trade motivated by collector and aquarium demand rather than food consumption.

Two distinct sales patterns emerged. Dark-brown and mixed-colour crabs followed what the study termed a low-profit, high-sales strategy: sellers offered more animals in each transaction and charged less per crab. Dark brown was the leading colour type in total transactions, total individuals and individuals per deal, while mixed lots ranked second in total transactions and individuals. Their unit prices were comparatively low. A significant contrast was recorded between dark brown and sky blue in both the difference between opening and closing prices and price per individual. At equivalent closing prices, dark-brown and mixed lots also contained more animals than sky-blue and other-coloured lots, demonstrating that the difference was not limited to the two colour groups having different overall prices.

Sky-blue and other-coloured crabs followed a high-profit, low-sales strategy. Sellers offered fewer individuals at higher prices per animal, with sky blue recording higher values across the transaction measures examined. Other-coloured animals also had high prices per individual but the lowest total number sold and lowest number per deal among the colour categories. Sky blue and other colours did not differ significantly in every comparison, including price per individual. Because the number of animals entering these transactions was lower, the greater monetary value of the unusual forms did not translate into the same auction-related collection volume as the common dark-brown and mixed-colour categories. The authors interpreted the pattern as consistent with sellers limiting supply to maintain high unit prices, while noting that the naturally limited occurrence of some other-coloured forms could also contribute to their low availability.

Annual transaction figures rose over the study period, with marked increases in the number of deals, individuals and sales value during 2018 and 2020. The number of sellers also grew, alongside an increase in transactions per seller. In 2023 alone, the dark-brown category included 12,070 animals traded in 441 transactions worth ¥594,340, a marked increase from 2015, when its transaction scale was more similar to those of the other colour types. Sky-blue activity also increased, although fewer animals were sold; by 2023, the category’s transaction value had approached that of dark brown. Generalized additive models fitted separate smooth temporal trends for each colour type and found different patterns in animals per deal and price per individual. The models explained 41.8% of the deviance in animals per deal and 47.5% in price per individual, with 95% confidence intervals displayed for the fitted curves.

Geography added another dimension to the market. Wild-caught dark-brown and mixed-colour crabs were sold across a broad area, whereas red, sky-blue and other-coloured animals were more localized. Dark brown appeared in transactions linked to 24 prefectures and mixed lots to 13, compared with three for red, eight for sky blue and seven for other colours. Wild-caught animals from 29 prefectures south of Niigata and Tochigi accounted for 2,323 transactions and 27,297 individuals. High-volume dark-brown trade was concentrated in western Japan, particularly Kyoto, Wakayama and Hyogo, while sky-blue sales occurred in a smaller set of Pacific coastal areas, including Chiba, Shizuoka, Kochi and Kagoshima. For the other-colour category, 43.6% of transactions lacked an identifiable collection locality, limiting the resolution of its geographical pattern.

Seller records suggested that much of the wild-caught trade was local. Of 139 sellers, 119 were classified as non-professionals and 96 completed only one transaction. Among 1,763 transactions with known collection sites, 93.7% matched the seller’s prefecture, while only six sellers sourced wild-caught animals from more than one location and 70 operated from a single prefecture. In several locations, including Tottori, Kyoto, Wakayama, Nagano and Chiba, one seller accounted for a dominant share of local transactions. This concentration means that the market’s geographical pattern could change if an established seller began listing larger quantities or if a new seller adopted the high-volume, low-unit-price strategy in an area with little current activity.

These findings establish patterns of auction-related collection pressure but do not show that wild populations declined. The study did not directly measure crab abundance, density, distribution or genetic structure, and not every animal offered online was necessarily collected from the wild. The colour categories were also based on auction descriptions and photographs, with some transactions lacking verifiable information. Taxonomic uncertainty further complicates interpretation because the G. dehaani grouping may include multiple species or divergent populations, and animals traded under the same historical name may not have had identical biological or conservation status. The authors therefore call for auction data to be combined with field evidence on population size, dynamics, distribution and genetic structure. Their analysis identifies where concentrated extraction is occurring and which market behaviours are associated with it, but the biological consequences remain to be measured directly.

Subject of Research: Online auction sales strategies and spatial collection pressure in the Geothelphusa dehaani species complex

Article Title: Difference in sale strategies drive spatial heterogeneity in collection pressure through online auctions: the role of body colour variation in the Japanese freshwater crabs, Geothelphusa dehaani species complex

Article References: Kunishima, T. (2025). Difference in sale strategies drive spatial heterogeneity in collection pressure through online auctions: the role of body colour variation in the Japanese freshwater crabs, Geothelphusa dehaani species complex. Nature Conservation, 59, 279-300. https://doi.org/10.3897/natureconservation.59.142085

Image Credits: AI Generated

DOI: 10.3897/natureconservation.59.142085

Keywords: wildlife trade, online auctions, freshwater crabs, collection pressure, colour polymorphism, Japan, aquarium trade, market strategies, biodiversity conservation, Geothelphusa dehaani, Difference, sale