The present research aims at broadening the range of possible Pebrilla paguri hosts, by analysing various Mediterranean hermit crab species, as well as at investigating their preferential distribution on the crustacean body. For each crab carrying P. paguri on its exoskeleton, the position and number of the ciliate's loricae were recorded and mapped. Pebrilla paguri loricae were observed only on 16.6% of the hermit crabs examined, with a greater number being observed on Calcinus tubularis and Paguristes eremita, compared to the two other crab species. A reason for this may be that the former are not very active in feeding, motion, and interaction. The highest frequency of P. paguri was observed on the abdomen, telson, and uropods, whereas only a few were found on the cephalotorax. This localisation is discussed in relation to the epibiont's preference for parts of the crab's body that are normally kept deep inside the molluscan shell and characterised by scarce water flow and illumination. These conditions will promote bacterial proliferation and accumulation of the organic residuals, which will become available for P. paguri as food. The preferential distribution of ciliates on the left side of the crab's body can be seen as a way to avoid damage by rubbing against the shell's columella.
The eight zoeal stages of the Mediterranean shrimp Periclimenes aegylios are described and illustrated in detail, using laboratory-reared specimens. This study improved the partial and unpublished descriptions of the zoeae of this species. The complete and accurate definition of the morphology of all the stages now allows comparison of the zoeae of P. aegylios with those of other Mediterranean Periclimenes species, as well as the allopatric Atlantic P. sagittifer, from which P. aegylios was separated in 1996.
Ever since its discovery, Pebrilla paguri was considered to be an exclusive epibiont of hermit crabs. In fact, up to now, it has never been recorded on substrata other than the hermit crab exoskeleton. The finding of its loricae on non-consecutive exuviae, released by the same specimen of Calcinus tubularis, suggested that the motile stage may transform into the sessile form and attach itself to the shell occupied by the crab. All the shells occupied by the C. tubularis and Clibanarius erythropus specimens showing the ciliate on the exoskeleton were identified to genus level; for each shell genus a map of the inner morphology was drawn in order to record the position and number of the loricae. The present research showed that P. paguri is not an exclusive hermit crab epibiont: it also attaches itself to shells, and particularly to the three largest whorls, corresponding to the portion occupied by the crab body. The presence of P. paguri in the shell may be determined by the preference for substrata characterized by a particular water stream or light levels. The results obtained show that the P. paguri substrata choice is more similar to that of other folliculinids, and more varied than previously reported.
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