Rediscovery of Mesotheres unguifalcula (Glassell, 1936) (Crustacea: Brachyura: Pinnotheridae) with Remarks on the Symbiotic Relationship with its New Host, the Spindle Sea Snail Leucozonia cerata (W. Wood, 1828) (Mollusa: Gastropoda: Fasciolariidae).

The symbiotic pinnotherid crab Mesotheres unguifalcula was rediscovered in Acapulco Guerrero, Mexico, and was found infesting the spindle sea snail Leucozonia cerata (Fasciolaridae), a new host record for this crab. A total of 432 snails were collected in 2020, with a prevalence of 77%, well explained by the host width frequency. Monthly prevalence varied from 54% to 90%, and the mean intensity was 1.4 +/- 0.5 crabs per host. The sex ratio of snails was 1:1, and the crab did not prefer to infest males or females. The sex ratio of the crabs was positively skewed towards females. Crabs infested both small and large snails; however, most infested snails ranged between 20 and 40 mm in width. Prevalence increased with the host size: with hosts smaller than 30 mm experiencing an average of 53% infestation, while those from 30 mm to 52 mm averaged 93% infestation. The number of crabs by host varied from 1 to 3; solitary females and males were dominant (51%), followed by heterosexual couples (24%) and other combinations that included homosexual couples and triads, which barely represented 2%. Although there are many heterosexual couples, monogamy is ruled out due to the higher number of solitary males and females and the lower number of heterosexual couples compared to those statistically expected. The available evidence about the life history of Mesotheres unguialcula, like that of other studied species of the subfamily Pinnotherinae sensu stricto, suggests a pure-search polygynandry of sedentary females as its mating system (i.e., larger, solitary, and sedentary females, and smaller males who, in reproductive season, are roaming from one host to another in search of females receptive to copulation).