Geosesarma De Man 1892

Ng, Peter K. L. & Liu, H. - C., 2004, Geosesarma Hednon, A New Species Of Terrestrial Crab (Crustacea: Decapoda: Brachyura: Sesarmidae) From Taiwan And The Philippines, Raffles Bulletin of Zoology 52 (1), pp. 239-249 : 239-240

Geosesarma De Man 1892

Remarks. – The genus Geosesarma De Man 1892 (sensu Serène & Soh, 1970) is clearly not homogeneous, and Ng (1988: 119) recognised three separate groups for convenience. As has been discussed by some authors (see Ng, 1988: 118), the genus Geosesarma is not well defined, and is characterised mainly by their wholly freshwater habits, swollen basal antennular segment and large eggs (ca. 1.0 mm in diameter or larger). Many other sesarmid genera, however, also have representatives living in fresh water habitats, sometimes great distances from the sea. The swollen antennular segment is a useful character, but is not always reliable as it is also evident in members of other genera (e.g. Labuanium ) (unpublished data). The egg sizes of many species currently referred to Geosesarma are still not known, and it is doubtful if this character alone is sufficient to diagnose a genus. The development of only two species is known for certain, G. peraccae and G. notophorum . Geosesarma peraccae has an abbreviated development, with the advanced zoeae possessing their own food reserves and developing rapidly in the burrows ( Soh, 1969), while G. notophorum has direct development with the young crabs staying on the mother’s back for a short time (Ng & Tan, 1995). Large eggs and abbreviated development have also been reported for other inland and coastal sesarmid taxa ( Hartnoll, 1964; Schubart & Cuesta, 1998; Cuesta et al., 1999; Schubart & Ng, in prep.).

The present description of a new species, G. hednon , from the central Philippines and Taiwan is noteworthy as the two sites are some distance apart. However, many ovigerous females of G. hednon had been obtained and the eggs are small, much smaller than those known for other Geosesarma species (see Ng 1988: 118). The eggs from some of these ovigerous females hatched into normal planktonic zoeae and they will be described at a later date. As such, this species has a rather wide distribution compared to more typical Geosesarma species. Geosesarma hednon thus lacks one of the key attributes of the genus, i.e. the possession of large eggs with abbreviated or direct development. The authors have on hand, ovigerous females of G. nemesis Ng, 1986 , G. cataracta Ng, 1986 , G. tiomanicum Ng, 1986 , G. penangense Tweedie, 1940 , G. malayanum Ng & Lim , in Ng, 1986, G. aurantium Ng, 1995 , and G. noduliferum (De Man, 1892) , all of which have large eggs. We expect some of the other Geosesarma species (e.g. G. maculatum ) to also have small eggs like G. hednon . While we have seriously considered establishing a new genus for G. hednon and its close allies (the type species of Geosesarma is G. noduliferum ), we have deferred from doing so as the affinities of too many of the species in the genus are still poorly understood. Most of the species have not even been reported or redescribed since their original discovery.