Physical Description of the Head-Wart
Microscopically, the surface of the head-wart in P. cereolus appears uniform to the surface of the surrounding tissue, but it can be differentiated because of the gaps that appear between the papillae on the snail’s head when the structure becomes erect. This coincides with what was described in the snails Fruticicola fruticum and Bradybaena sieboldiana. In each of these species, the head-wart is indistinct from the rest of the epidermis, only becoming noticeable after wart eversion, which is comparable with what was observed in P. cereolus. If the wart is homologous to those present in other species, its eversion is a result of the swelling of the body wall that is brought on by internal pressure. In P. cereolus, the swelling first begins between the two optic tentacles, and it has the appearance of a small, round growth. As time passes and the growth increases in size, it eventually grows so large that the entire forehead of the snail bulges out between the area of the optic tentacles and the oral tentacles. As the protrusion of the head-wart increases its color lightens significantly in comparison to the surrounding tissue; the head-wart adopts a translucent, grayish tint, contrasting against the typical yellow or brownish integument of this species.
Over the course of 2 months, the head-wart was observed for a total of 11 times. Each time, prior to wart eversion, P. cereolus was first seen resting on the side of the terrarium motionless for several hours. Subsequently, the snail’s forehead would begin to protrude outwards, very slowly over the course of approximately 15–30 min. Once the structure was fully erect, the snails would sit very still, just barely moving their optic tentacles one at a time in small, subtle circular motions. If left uninterrupted, the snails would often remain in this posture for 1–2 h, at which time the wart would slowly retract back into the head; then the snail would move on. Of the 11 total times the structure was seen, 6 times the snail was left uninterrupted for the duration of the wart’s erection, and 5 times the snail was interrupted by another snail. In 20% of the interruptions, the intruding snail ignored the snail with the erect head-wart and continued on its way. However, in the other 80% of the interruptions, courtship behavior began. Courtship in this species consists of aggressively rasping one another followed by antiparallel circling. As these actions cycle, the genital pore gradually begins to swell. The swelling of the genital pore typically marks the withdrawal of the head-wart. However, in one instance, 2 snails went from being without everted head-warts to having fully erect ones while simultaneously having swollen genital pores. The structures continued to protrude until both snails fully everted their genitalia, at which time it appeared that the internal pressure needed to evert the genitalia was diverted away from the head-wart, thus causing the wart to shrink. Consumption of the mucus secreted by the snail with the swollen wart by the intruding snail prior to copulation was also a common sight. If this species releases testosterone or another sex pheromone via its head-wart as in E. peliomphala, it may be that the smell and consumption of the mucus serves in getting a partner initially interested in copulation.