Infestation of the spider crab Menaethius monoceros (Epialtidae: Brachyura) from the Egyptian Red Sea coasts by Parasacculina pilosella (Polyascidae: Rhizocephala: Cirripedia

The results of this study showed that out of 287 individuals (135 males and 152 females) of the spider crab Menaethius monoceros collected from the Red Sea and the Gulfs of Suez and Aqaba, 15 individuals (6 males and 9 females) were infected with Parasacculina pilosella (Van Kampen & Boschma, 1925) (Polyascidae: Rhizocephala: Cirripedia: Crustacea). A total of 20 externae were recorded, including 11 singles, three doubles, and only one triple on the infected crab abdomens. The overall infection rate was 5.23% for the combined sexes at all sites, declined to 4.44% in males, but increased to 5.92% in females, reaching a higher rate of 16.07% in the populations of Marsa Alam and Shalateen sectors during summer. The examined externae have variable colors, sizes, and shapes based on their sexual maturity, which has been classified into four stages: immature or virgin, early mature, maturing, and late (spent externae). The largest number of externae was eight extruded on the 3rd abdominal segment, while it decreased to six, two, and one on the 4th, 2nd and 5th segments, respectively. The size of externae varied from 1.3 × 1.0 × 1.0 to 6.3 × 4.5 × 2.5 mm (Breadth × Height × Thickness). These results showed changes in the external secondary sexual characters of the infected males, represented by the remarkable broadness of the abdomen fringed with marginal setae and invagination of the sternum, but were limited in infected females.