Pseudogobius masago, an endangered mud-dwelling goby inhabiting mudflats in Japan, is threatened by habitat loss due to human activities, such as land-fills and coastal development. However, basic information for this species is scarce, especially on Okinawajima Island, the southernmost known population. Life history traits of the species, including longevity and maturity, were investigated by a monthly sampling program conducted at Manko Wetland, Okinawa-jima Island, from March 2010 to November 2012. The spawning season of P. masago was determined from histological observations of ovaries and appearance of recruits over a year long period. The estimated batch fecundity was 202–542 eggs, fewer than that determined for individuals on the Japanese mainland (264–961). Daily increments of sagittal otoliths were counted for age and growth determinations, validation being attempted using alizarin complexone (ALC) fluorescent stain. The maximum life span of P. masago was found to be ca. four months, individual maturation and survival differing seasonally, being dependent upon birth month. P. masago was smaller and shorter-lived on Okinawa Island compared with populations in Mie or Fukuoka Prefectures, the overall life history strategy of the species apparently differing with latitude. Populations inhabiting temperate mainland Japan grew larger and produced more eggs over a shorter spawning season, whereas those inhabiting subtropical Okinawa Island grew to a smaller maximum standard length and produced fewer eggs, but had an extended spawning season with overlapping generations. Future conservation of the species is dependent upon reducing human activity on soft mudflats as much as possible.

The species of Mugilogobius sp. (Japanese name: Izumi-haze) and M. chulae are both common species, sympatrically inhabiting the estuaries and tidal flats of Okinawajima Island. Some species of Mugilogobius has a urea cycle, and their processing ability is higher, especially in northern distributed species, such as M. abei and Mugilogobius sp. It is possible to differentiate their life history traits and habitats on the basis of urea processing ability. However, because little is known in detail of their life history and habitat, Mugilogobius sp. and M. chulae were investigated through monthly sampling conducted on the southern part of the Sashiki Tidal flat and Manko Wetland on Okinawa-jima Island, from January 2012 to January 2013. The spawning seasons of Mugilogobius sp. and M. chulae are from January to October and April to October, respectively, with peaks in the summer period (April to October), indicating that maturity in both species is closely related to changes in water temperature. A comparison with the spawning season of M. abei, a temperate region species, revealed the subtropical species to spawn over a longer period. Estimates of daily age from otolith analyses indicated the possibility of recruitment from other areas, since histological observations revealed mismatches of gonads and birthmonths between different spawning seasons, as estimated by back-calculations of daily age. The longevity of Mugilogobius species was about six months. The study also revealed the growth rate of Mugilogobius sp. to be similar to that of M. chulae. Mugilogobius chulae locally inhabited mangrove habitats, whereas Mugilogobius sp. showed wide habitat use and was less susceptible to decreasing habitat than the former. Accordingly, future conservation of M. chulae is dependent upon suitable mud habitat associated with mangroves.