The selective advantage of host feminization: a case study of the green crab Carcinus maenas and the parasitic barnacle Sacculina carcini

Abstract

Male crabs infected by parasitic barnacles (Rhizocephala) are known to be morphologically feminized. Here, we investigate morphological chances in green crabs, Carcinus maenas, induced by the parasitic barnacle Sacculina carcini. Infected males acquire a broader, longer and segmented abdomen, fringed with marginal setae. Copulatory appendages and pereopods are reduced in length, and the chelae become smaller. The feminization show great individual variation. Males with scars from lost externae, the parasites reproductive organ situated under the abdomen, are less modified than males carrying an externa, and the feminization is more pronounced in smaller than in larger males. No super-feminization is evident in female crabs that remain morphologically unaffected by infection. The protective value of a parasitically induced enlargement of the male abdomen may constitute an adaptation that increases parasite longevity. The additional effects on male morphology are viewed as pleiotropic side effects of the main adaptive value of enlarging the abdomen.

Tidal elevation and parasitism: patterns of infection by the rhizocephalan parasite Sacculina carcini in shore crabs Carcinus maenas

“…Although parasitized males resemble females in appearance (feminisation), both sexes, even if parasitized, can be clearly distinguished from each other (for photographs see Kristensen et al 2012). In addition, the ventral abdomen was inspected to identify ovigerous females as well as specimens infected with S. carcini (clearly visible externa).…”

“…The parasite infects and castrates both sexes of C. maenas (Høeg, 1995). Moreover, male crabs become to a certain extent morphologically feminized, induced by effects of the parasite on the hormonal system of the host, involving an enlargement of the pleon and a reduction in cheliped size (Rubilianiet al 1980, Høeg 1995, Kristensen et al, 2012. The morphological feminisation occurs stepwise with every moulting event of the infected host, while S. carcini remains internal (Høeg 1995).…”

Muscular system in the interna of Polyascus polygenea and Sacculina pilosella (Cirripedia: Rhizocephala: Sacculinidae)

“…The externa is a temporarily formed structure containing the female's reproductive system (Høeg, 1995;Bresciani, Høeg, 2001). Nowadays there are numerous papers on the phylogeny, development, evolution, parasite-host interactions, and others aspects of the rhizocephalan biology (Alvarez et al, 1995(Alvarez et al, , 2002Høeg, 1995;Bresciani, Høeg, 2001;Graham, 2001;Bortolini, Alvarez, 2008;Kristensen et al, 2012;Belgrad, Griffen, 2015;Nagler et al, 2017;Pérez-Miguel, 2017;Lützen et al, 2018;Zacher, 2018). However, morphology of interna is still poorly described, especially in the terms of the muscular system.…”

Control region sequences indicate that multiple externae represent multiple infections by Sacculina carcini (Cirripedia: Rhizocephala)

“…The interna is an extensive structure that infiltrates most of the larger blood sinuses of the crab and serves as a trophic organ extracting nutrients from the crab hemolymph (Glenner ; for further details of the development of Sacculina carcini and its morphological and behavioral impacts on Carcinus maenas see, e.g., Kristensen et al. ; Høeg ; Høeg and Lutzen ).…”