Novel type of egg-clustering in threadsnakes (Serpentes: Leptotyphlopidae)

Abstract Snakes lay their eggs in clutches of different size, which are usually attached to each other forming a cluster. Egg-clustering is a widespread phenomenon across alethinophidian snakes, mostly recorded in Pythonoidea and caenophidian clades. Here we report a new type of egg-clustering for threadsnakes (Leptotyphlopidae) that departs from the alethinophidian type. We found that females of Epictia australis and Leptotyphlops sylvicolus lay their eggs connected to each other through a filament, and we dubbed it ‘string-egg clustering’. The histomorphology of the filament linking the eggs in E. australis showed an outer calcareous layer underlain by a thick layer of collagen fibers, demonstrating that it is an integral part of the eggshell formed during its deposition process in the oviduct. String egg-clustering seems to be present only among species belonging to both subfamilies of threadsnakes, Epictinae and Leptotyphlopinae. Egg-clustering in alethinophidians has been demonstrated to have several advantages for embryo development and post-hatching survival, including fixing the position of the embryo within the egg, protection against predators, and embryo-to-embryo communication. The presence of a filament connecting the eggs in leptotyphlopid species might be relevant for maintaining the position of the embryo in the egg, to avoid the dispersion of the egg in the nesting site, and potentially for the transmission of physical cues. Thus, we hypothesize that the string-egg clustering constitutes an advantageous reproductive trait among threadsnakes.