Populations of the spider crab, Menaethius monoceros (Latreille,1825) (Majidae Brachyura, Crustacea) from the Gulf of
Aqaba and Egyptian coast of the Red Sea were examined. The results show that the individuals of this species usually occur in rocky substrates encrusted among algal growths and within fronds of the sea grasses at sandy areas. The carapace length is small in size ranging from 2.9 to 17.6mm, (without rostrum). Males of this species attain
their maturity at size from 8 to 12 mm, carapace length. while females become mature at 4 to 8 mm. carapace length The incubated eggs are few in number occurring during all seasons of the year. They varied from 49 to 562 with an average of 269 ± 149 eggs per female. Food consists of algae (brown, red, green as well as cyanophyta ) and animals such as amphipods, copepods, foraminiferans and eggs.There is seasonal variations in the percentage of different food items,
and a slight difference between various populations from the two localities.