Shape of attachment structures in parasitic isopodan crustaceans: the influence of attachment site and ontogeny

Abstract

Many fields of modern systematic biology are adult-centred. This is unfortunately also the case for Cymothoidae, an ingroup of parasitic forms of Isopoda, with fishes as hosts. Different ingroups of Cymothoidae have specialised appendages that enable their fish associated lifestyles, attaching to different sites on the body of the host. The extent to which these structures vary among species and in relation different sites of attachment, and between different ontogenetic stages, is explored here. This study presents the detailed descriptions, illustrations, comparisons, and analysis of a variety of attachment structures of 13 adult and immature specimens representing three major groups Ceratothoa, Elthusa and Anilocra, along with full focus, detailed photographs of all the examined life stages. The three groups exhibit different strategies attaching to mouth, gill and externally, respectively. A statistical representation of the morphology of the dactyli, used for attaching to the host, was performed. This included a critical comparison of 10 additional species documented in literature. This is the first comprehensive description and photographs of specialised appendage morphology of immatures of Ceratothoa, as well as the first detailed micrographs of embryonic stages of Cymothoidae, and the first lateral and ventral views of immature stages of the examined species. Immature specimens possess morphological characters that can be used to distinguish between different species, but cannot be accurately identified based on diagnostic characters of adults. Quantitative analysis indicates that ontogeny plays a major role in the shape of the attachment structures (e.g. dactyli).

Description and ontogeny of a 40-million-year-old parasitic isopodan crustacean:Parvucymoides dvorakorumgen. et sp. nov.

“…23, 46; Aneesh et al, 2019 , fig. 1; Van der Wal & Haug, 2020 , figs. 1, 4, 7,11, 14, 17, 20, 26, 29).…”

“…Thus, adult male and female specimens can be well differentiated. More recently, detailed morphological descriptions and differentiating characters of different immature stages have been presented ( Bakenhaster, 2004 ; Jones et al, 2008 ; Aneesh et al, 2016 ; Van der Wal & Haug, 2020 ). A tentative restoration of the ontogenetic sequence of the examined fossils ( Fig.…”

“…The term ‘immature’ is used here to refer to all stages after hatching (post-marsupial development), but before maturation ( sensu Van der Wal & Haug, 2020 ). Immatures of extant species have a larger body length to width ratio (more elongated) that decreases over ontogenetic development ( Fig.…”

“…Immature forms of Cymothoidae have different developmental stages ( e.g. , pre-mancae, mancae and juveniles/natatory-stage individuals, sensu Van der Wal & Haug, 2020 ). These can be differentiated based on characters such as the presence or absence of developed appendages on trunk segment 7, the presence of yolk and the presence of setae on the pleopods and uropods.…”

“…Specimens P2338, P2343, P2344 and P2347 are interpreted as representing the final immature stage (immature stage 3 sensu Van der Wal & Haug, 2020 ; ‘juvenile’ sensu Brusca, 1978 ; Segal, 1987 ; Kottarathil et al, 2019 ; ‘natatory-stage’ sensu Jones et al, 2008 ) for the following reasons. Immature stages prior to immature stage 3 ( i.e.…”