The reproductive behaviors of terrestrial gastropods are often complex and vary substantially between species. Any similarities in these behaviors may indicate a degree of evolutionary relatedness between taxa and thus have the potential to be used as an auxiliary tool when attempting to classify species. 45 adult members of the species Polygyra cereolus were collected to study the reproductive behavior of this species and to compare it to P. septemvolva, a species whose reproductive behavior has already been described. During this process, a small protuberance of tissue appearing on the forehead between the optic tentacles was noted. This structure visually appears homologous to what has previously been described in other terrestrial gastropods as “head-wart”, “frontal organ”, and “cephalic accessory organ”. In this paper, it will simply be referred to as “head-wart”.

Head-warts have been seen before in terrestrial gastropods, particularly in members of the families Camaenidae and Bradybaenidae^[1], although they have also been observed in the genus Gymnarion^[2]. Additionally, head-warts have been observed in the helicid snail Arianta arbustorum within a small percentage of courting individuals^[3]. The appearance of this wart can vary greatly, with some species possessing warts covered in calciferous hooks^[4] and others possessing subtle, smooth warts that are only visible when everted^[1]. A particularly notable example is in the Japanese species Euhadra peliomphala, which possesses a rough, dark-colored head-wart^[5] that was found to secrete testosterone. In this species, the wart was found to have an important role in courtship, as testosterone secreted from the structure served to excite snails prior to copulation^[6]. However, till date, no such organ related to any snail in the family Polygyridae has ever been described. If this structure is common in this family, then this may serve as further evidence^[7][8][9] that there exists a close evolutionary relationship between the family Polygyridae and the families Camaenidae and Bradybaenidae.

We aim to describe the head-wart found in Polygyra cereolus and observe the behavior associated with its eversion.

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^[1]. 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^[2]. 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.

Associated Behavior

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^[6], it may be that the smell and consumption of the mucus serves in getting a partner initially interested in copulation.

This study does not examine the histology of the head-wart. Until that is done, it cannot be soundly confirmed that this structure is physically related to the wart described prior in other terrestrial gastropods. This study also has a small sample size. A larger sample needs to be observed to accurately determine what role the wart plays in the life of P. cereolus and if this structure is related to reproductive behavior like it is in other species.

Study Species

The southern flatcoil is native to the eastern United States^[10], but has since been exported to many other locations around the globe ranging from Hawaii^[11] to the United Arab Emirates^[12], likely via the international horticulture trade^[13]. It has also made its way to southern California^[14]. The specimens for this study were obtained in Oceanside, California in San Diego County. This species is primarily herbivorous; its diet consists of vegetation such as clover and alfalfa^[15].

Terrarium Set-up

The snails were placed in a 38 L glass-walled terrarium that contained 2 in of moistened coconut-fiber substrate. A mesh screen was placed over the top of the enclosure to allow for ventilation, and a cloth was placed over the screen to keep gnats and other pests out. The snails were given various fresh vegetables to eat, such as carrot, cucumber, and lettuce. Crushed eggshells were also provided to serve as a form of calcium. The snails were also misted with water twice daily to keep the humidity levels up so that they would remain active. The tank was kept at around 21°C. After initial collection and placement into the terrarium, the snails were not interfered with.

No funding was sought for this study.

Thanks go out to John Slapcinsky for his insight and advice.

The authors declare no conflicts of interest.

Not applicable.

No fraudulence is committed in performing these experiments or during processing of the data. We understand that in the case of fraudulence, the study can be retracted by ScienceMatters.