A Note on Banana Slugs


In December 2019 I noticed that there were significant inconsistencies in the identification of the banana slugs (genus Ariolimax) on iNaturalist. Since then, I have gathered a collection of literature and reviewed each of the 12,500 or so observations present as of May 2020, correcting and confirming where I could improve the Community Taxon. I will continue to to do this into the foreseeable future. My identifications have yielded some questions, and I hope to explain these below. The first thing I should say is that I am by no means an expert on banana slugs. I have taxonomic and field experience with Californian terrestrial gastropods and other mollusks in general, but my field experience with banana slugs is limited to four short encounters, all but one of which occurred before I discovered iNaturalist in May 2019. I have neither dissected a banana slug nor done any genetic work with one, although I'm sure both of those would be quite interesting. In preparing this, though, I have read a considerable amount of the literature on banana slugs.

Natural History

Banana slugs are some of the largest terrestrial slugs on earth, reaching up to 260 mm when fully extended (Pilsbry, 1948) and weighing up to 150 g (Leonard et al., 2002). They are common in moist habitats along the West Coast of North America (Leonard et al., 2007), ranging from Juneau to San Diego (Leonard et al., 2002). The population on Palomar Mountain near San Diego represents a new species, as does the population on Fremont Peak near Salinas (Leonard et al., 2009*, 2013*; Pearse et al., 2010*; Pearse & Leonard, 2010, unpublished data; JL Leonard, personal communication, 2020). On iNaturalist we have records along this entire range, including individuals from both purportedly undescribed species. Like many slugs, banana slugs are herbivores, eating a wide variety of fruits and vegetables, both wild and cultivated, possibly playing a significant role in seed dispersal (Pearson et al., 2005; Gervais et al., 1997). They also eat mammal feces (Gervais et al., 1997) and mushrooms (Leonard et al., 2002). In turn, they have a variety of vertebrate and gastropod predators (Pearse & Leonard, 2010, unpublished data). In the lab, animals can live for over 2 years but sexual maturity can be reached in less than 1 (Leonard et al., 2002, 2007). Mating seasons have not been well studied, but it appears that mating is most common in the spring and summer and egg-laying occurs in the fall and winter (Cooper, 1872; Leonard et al., 2002, 2007). Egg clutches range from 1 to well over 100 and eggs can take from 3 to 11 weeks to hatch; sometimes more than one individual can hatch from the same egg (Leonard et al., 2002, 2007).

Perhaps the most studied and most bizarre aspect of banana slugs is their mating habits. Like other land snails and slugs, banana slugs are simultaneous hermaphrodites—meaning they have male and female sexual organs at the same time. That being said, sexually mature individuals can have no male reproductive organs (aphally), reduced male reproductive organs (hemiphally), or normal male reproductive organs (euphally) (Roth, 2004; Leonard et al., 2007). Euphallic banana slugs can mate either as a male or a female (unilateral reproduction) or they can perform both roles at the same time (simultaneously reciprocal reproduction), the latter only occurring in pairs (Leonard et al., 2002, 2007). Mating behavior begins with biting and head-swinging as the individuals twist together to align their genital openings, forming a yin and yang shape (Heath, 1916; Leonard et al., 2002). Copulation can then last almost 24 hours before separation, although precise behaviors appear to be dependent on species (Leonard et al., 2004a*, 2007, 2010*, 2011*). The bizarre part is this: during copulation, sometimes one or both individuals will proceed to chew through the penis or penes that connect them; this is called apophallation (Heath, 1916). The precise reason for this behavior is unknown, but a variety of explanations have been hypothesized. Perhaps the penis recipient benefits by forcing the penis donor to reproduce only as a female and thus focus on laying eggs (Reise & Hutchinson, 2002). Maybe the penis donor stands to benefit by stoppering its sperm with a "plug" (Heath, 1916) and, I might add, preventing competition with other slugs' sperm. However, this seems unlikely as the penis severer usually eats the penis after it is dismembered (Leonard et al., 2002). Apophallation, then, would be a nutritional benefit to one or possibly both slugs. Maybe the slugs are unable of withdrawing an everted penis back into their body (Pilsbry, 1948). It may be important to note that apophallation has only been observed in A. californicus and A. dolichophallus (Leonard et al., 2002) and that regeneration has never been observed, but it seems unlikely in such short-lived animals. In addition to sexual reproduction, banana slugs can also successfully self-fertilize (Miller & Sinervo, 2007; Leonard et al., 2007).


The genus Ariolimax was described in 1859 with A. columbianus as the type species (Mörch, 1859). The genus is a combination of the generic names Arion and Limax, Arion being the closer relative but Limax being the original genus under which A. columbianus ("of Columbia") had been described (Mead, 1943). The type locality was Puget Sound (Gould, 1851). The next species described was A. californicus ("Californian") from the "coast range" of the San Francisco Peninsula (Cooper, 1872; cf. Mead, 1943). Ariolimax stramineus was described next from Santa Cruz Island as a variety of A. columbianus (Hemphill, 1891). Pilsbry & Vanatta (1896) described A. buttoni ("of Button") from Oakland in a new genus, Aphallarion ("Arion without a penis"). This work was the first thorough treatment of the genus, giving detailed descriptions of the genitalia, which have proven to be the only reliable way to distinguish species morphologically (Mead, 1943). Aphallarion was described on the basis of the complete lack of a penis—an organ that would be expected in every individual because they are simultaneous hermaphrodites. Ariolimax buttoni was the type species of this monotypic genus. However, Heath (1916) discovered that banana slugs will chew through penes while mating (apophallation), suggesting that the individuals described as A. buttoni did not have penes because they were apophallated, not because they never had one to begin with (cf. Pilsbry, 1948). Mead (1943) and Pilsbry (1948) agreed, the latter following this treatment of the genus in his monumental work on the terrestrial gastropods of North America. Mead (1943) also described 2 new taxa: A. dolichophallus ("long penis") from Saratoga and A. californicus brachyphallus ("short penis") from Mt. Davidson, San Francisco. Mead (1943) stated multiple times that a "later paper" would discuss the "genital physiology of the genus" in more detail. This could be a reference to his PhD dissertation, which I have not been able to find; otherwise it was never published (cf. van Bruggen & Mead, 2011).

This taxonomy of 3 species and 2 additional subspecies remained unchallenged for over 60 years until Roth & Sadeghian (2006) resurrected A. buttoni from synonymy with A. columbianus and elevated A. stramineus to species level following the evidence presented by Roth (2004), Leonard et al. (2004a*, 2004b*), Pearse et al. (2010*), and additional unpublished work. Roth (2004) had shown that the original description of A. buttoni did indeed show aphally as opposed to apophally. He also found aphallic and hemiphallic individuals of this species that were sexually mature. Moreover, these were only found in the southern population of A. columbianus—the population that included the type locality of A. buttoni—and not in the population of the Pacific Northwest (Roth, 2004). When it was suggested that this southern population was genetically distinct (Pearse et al., 2010*), Roth & Sadeghian (2006) resurrected A. buttoni. Given that the genetic data remains to be formally published, this is up for questioning. Even so, Leonard et al. (2007) showed that A. buttoni exhibited aphallic, hemiphallic, and euphallic (normal penis) individuals, confirming the phally polymorphism proposed by Roth (2004).

Ariolimax stramineus was also elevated in Roth & Sadeghian (2006) based on the genetic data proposed by Pearse et al. (2010*) and personal correspondence with JS Pearse. This is corroborated by the phylogenetic trees in Pearse & Leonard (2010, unpublished data): if A. stramineus is treated as a subspecies of A. columbianus, A. buttoni must also be a subspecies of A. columbianus and A. columbianus would be polyphyletic. Both A. stramineus and A. buttoni must be elevated to species level to make A. columbianus monophyletic. Note that the use of the genus Aphallarion for A. buttoni would make Ariolimax paraphyletic and so it remains in synonymy with Ariolimax (most appropriately the subgenus Ariolimax), although it could potentially be used as a subgenus. The subgenus Ariolimax also appears to be paraphyletic (cf. Pearse & Leonard, 2010, unpublished data), and resurrecting Aphallarion as a subgenus could be part of a potential solution. Personally, I find subgenera unnecessarily excessive and they are not used on MolluscaBase.

The status of Ariolimax brachyphallus is a rather peculiar situation. It was described as a subspecies of A. californicus (Mead, 1943) and this is the status in Roth & Sadeghian (2006). However, some had already been referring to it as a full species as early as 2005 (Leonard et al., 2010*). This continued in Leonard et al. (2007) and all subsequent presentations. The phylogenetic trees in Pearse & Leonard (2010, unpublished data) are somewhat contradictory. The one on p. 27 indicates A. californicus is closer to A. dolichophallus than A. brachyphallus, meaning that if A. dolichophallus is a full species, A. brachyphallus should be too. However, this is not the case on p. 25, and that tree could support A. brachyphallus as a subspecies of A. californicus. MolluscaBase lists A. brachyphallus as a subspecies of A. californicus, which I think is appropriate given that there was no explanation for its elevation—published or otherwise. However, A. brachyphallus was added as a full species here on iNaturalist and I think I prefer this given the nasty complications inherent in subspecific identifications (see the discussion here). Ariolimax brachyphallus would not present a major problem as there are relatively few observations, but demoting it to subspecies would require demoting A. californicus to the nominate subspecies. This would affect over 1500 observations and the sort of upheaval that would cause seems unnecessary when precise relationships are unknown and research is currently being conducted that so far suggests A. brachyphallus is a valid species.

As of 2008, John S Pearse of University of California, Santa Cruz was working on describing the new species present on Fremont Peak (Leonard et al., 2009*). Currently, this description is still in progress, as is the description of the Palomar Mountain species. There have been some hiccups in publishing the genetic data for the whole genus, but work on that has recently resumed (JL Leonard, personal communication, 2020). Some of the more interesting findings indicate that euphallic individuals of A. buttoni are identical to A. columbianus, but they are still genetically distinct. The opposite is true of A. californicus and A. dolichophallus, which are not molecularly distinct but show markedly different sexual behavior and genitalia, suggesting there has been rapid evolution (Mead, 1943; Leonard et al., 2002, 2004a*, 2009*, 2010*, 2011*, 2013*; Pearse et al., 2010*; Pearse & Leonard, 2010, unpublished data).

My Identification Philosophy

My top priority in identification is consistency. The only thing consistent about the state of the genus on iNaturalist as of December 2019 was AI suggestions, which is synonymous with chaos when species are only differentiable by internal morphology and perhaps molecular sequencing. I generally do not identify observations by others that I cannot independently verify in the literature. I do not disagree with identifications that are potentially but unverifiably incorrect. I generally do not add identifications that will not improve the Community Taxon, unless the Community Taxon is above species and, at this point, cannot be refined any further. However, I do not mark the Community Taxon as "No, it's as good as it can be" unless it has been proven that there is no way (including range) to distinguish one or more species. This is not yet the case for banana slugs, but it could be in certain regions upon publishing of further research. I do not prefer to rely solely on range to identify species, but when no other traits are available and ranges are clearly defined (as with banana slugs), I do not find it inappropriate. This is how Leonard et al. (2002, 2007) selected specimens for study and it is supported by Mead (1943). Moreover, if range was not important in identification, atlases would not be used for splitting taxa. Although unpublished, almost all of the map data in Pearse & Leonard (2010, unpublished data) are supported by other references; they just provide the most accessible, inclusive, and precise ranges. The ranges for each region, north to south, are listed below. Please let me know if you find any of my identifications that do not align with these parameters, or if you would like to debate the boundaries provided. In no way do I have a monopoly on identifications.

The Pacific Northwest only contains the type species: Ariolimax columbianus. It ranges from southeastern Alaska through British Columbia, Washington, Oregon, and Del Norte and Humboldt Counties in Northern California (Roth & Sadeghian, 2006). I have also included Siskiyou County because the banana slugs found there are very close to these other 2 counties. This species also ranges east into Idaho (Frest & Johannes, 2000). I identify all observations within this range as A. columbianus, except observations in southeastern Humboldt County as these are close to the range of A. buttoni; I refrain from identifying such ambiguous observations.

The majority of Northern California contains A. buttoni. The counties listed by Roth & Sadeghian (2006) include Trinity, Mendocino, Nevada, Sonoma, Napa, Yuba, Sacramento, Marin, Contra Costa, and Alameda. They state that populations in Shasta, Plumas, Butte, Lake, Placer, El Dorado, Yolo, Solano, Calaveras, and Tuolumne Counties also represent A. buttoni. Mead (1943) confirms much of this in his documented range of A. columbianus, which we now know as A. buttoni in these areas (Roth, 2004; Leonard et al., 2007). I do not think it unreasonable to also add Tehama, Glenn, Sierra, Colusa, and Amador Counties to fill in these gaps, which is consistent with Pearse & Leonard (2010, unpublished data). As of right now, all of these counties contain banana slug observations here on iNaturalist except Sacramento. Perhaps that is an old museum record collected before significant development, or perhaps there is a relict population still alive somewhere in the Central Valley. This would somewhat validate the San Joaquin County observation. The Fresno County observation is also somewhat unusual. If an accurate location, it is presumably most closely related to A. buttoni given that this is the only species in the Sierra Nevadas. We may also expect to find A. buttoni in Stanislaus, Merced, Mariposa, and Madera Counties; Pearse & Leonard (2010, unpublished data) would support this. I identify all observations within this whole range as A. buttoni, except observations in northwestern Mendocino County and western Trinity County as these are close to the range of A. columbianus; I refrain from identifying such ambiguous observations.

San Francisco County contains A. brachyphallus, A. buttoni, and possibly A. californicus. Ariolimax brachyphallus and A. buttoni have been found on the mainland while the individuals on Alcatraz Island appear to exhibit a new genital morphology that is closest to A. brachyphallus. Genetically, however, these island individuals cluster with A. californicus. An individual with that morphology has also been found on the San Francisco mainland (Leonard et al., 2009*). I refrain from identifying any San Francisco County observations to species, except the spotted individuals, which I identify as A. buttoni, although this species can also be spotless (Mead, 1943; cf. Roth, 2004; cf. Leonard et al., 2007).

San Mateo County contains A. californicus and probably A. brachyphallus. Mead (1943) stated that A. brachyphallus is restricted to the San Francisco Peninsula, which is not extremely precise. The only exact localities he listed are Mt. Davidson and Mt. Sutro, although these would likely be the only localities in which he had actually found it. The range he provided is broader and presumably reflects his acknowledgement that the species was likely found outside of just these two peaks. It is no surprise, then, that Pearse & Leonard (2010, unpublished data) reported A. brachyphallus from northern San Mateo County. Roth & Sadeghian (2006) reported A. stramineus from San Mateo County, but this would seem to be either a typographical error or labelling error copied from a museum lot as no other records (published or otherwise) have documented A. stramineus anywhere near here. Mead (1943) listed a questionable record of A. columbianus from San Mateo County. This has not been verified so I do not believe it is accurate, but any records of A. columbianus in this area we now know to be A. buttoni (Roth, 2004; Leonard et al., 2007). I identify all observations in San Mateo County north of Pacifica as A. brachyphallus and all observations between Redwood City and Pescadero as A. californicus.

Santa Clara County contains A. californicus, A. dolichophallus, and A. buttoni. The only precise record of A. californicus is from Stanford University (Mead, 1943). Mead (1943) described A. dolichophallus from Saratoga and he also listed it from Congress Springs and Los Gatos. He listed A. columbianus from Alum Rock Park, and we now know all records from this area to be A. buttoni (Roth, 2004; Leonard, et al., 2007). Pearse & Leonard (2010, unpublished data) also indicated A. buttoni is found farther east and south in the county. It would seem, then, that A. californicus and A. dolichophallus are found on the borders with San Mateo and Santa Cruz Counties, respectively, and A. buttoni is found in the eastern half of the county. However, it appears that A. buttoni extends farther west into Santa Clara County than has been reported (published or otherwise). Only A. columbianus and A. buttoni have spots, although these species can also be spotless (Mead, 1943; cf. Roth, 2004; cf. Leonard et al., 2007), so spotted individuals in Santa Clara County are A. buttoni but spotless individuals are ambiguous. There are observations of spotted individuals west to Hwy 17 and a few in the vicinity of Saratoga. I refrain from identifying observations in Santa Clara County to species, except spotted individuals and observations north and/or east of Hwy 101, which I identify as A. buttoni.

Santa Cruz County contains A. dolichophallus and probably A. californicus. Mead (1943) only listed A. dolichophallus from Santa Cruz County but Roth & Sadeghian (2006) also listed A. californicus. Pearse & Leonard (2010, unpublished data) showed 1 individual of A. californicus from the extreme western edge of the county. It would not be surprising that A. californicus is found here because it is found in the neighboring San Mateo County, although Mead (1943) stated that the boundary is quite distinct. It is possible that A. buttoni is found along the northeast border with Santa Clara County, but no one has reported this. I identify all observations in Santa Cruz County as A. dolichophallus, except for those both north and west of Boulder Creek as this is close to the range of A. californicus; I refrain from identifying such ambiguous observations.

San Benito County contains A. buttoni and a new species. Roth & Sadeghian (2006) only listed A. buttoni from San Benito County, but a new species has been recently discovered on Fremont Peak. Ariolimax buttoni has also been found on Fremont Peak. This new species is currently being described, and it may also be found in Pinnacles National Park (Leonard et al., 2009*, 2013*; Pearse et al., 2010*; Pearse & Leonard, 2010, unpublished data; JL Leonard, personal communication, 2020). Roth & Sadeghian (2006) could not have overlooked this new species because it is mentioned in Pearse et al. (2010*), which they cite, so it is somewhat surprising that they did not list it. I identify all observations in San Benito County as A. buttoni, except those from the Fremont Peak area, which I refrain from identifying.

Monterey County contains A. buttoni, A. dolichophallus, A. stramineus, and possibly A. brachyphallus. Mead (1943) reported A. columbianus and A. dolichophallus reaching south to Salinas Valley, and we now know records of A. columbianus in this area to represent A. buttoni (Roth, 2004; Leonard et al., 2007). Roth & Sadeghian (2006) listed A. buttoni from Monterey but they did not list A. dolichophallus. It is likely that the species reaches a little farther south into Monterey County. Both Mead (1943) and Roth & Sadeghian (2006) listed A. stramineus, the former stating it ranges north to Salinas Valley, indicating Salinas Valley to be a zone of sympatry for these 3 species. The findings of Pearse & Leonard (2010, unpublished data) corroborate all of this, but indicate Monterey Peninsula is the zone of sympatry. They also state that a disjunct population of A. brachyphallus lives on the Peninsula, which is rather odd. The record of A. californicus californicus in Monterey County (Roth & Sadeghian, 2006) is likely a typographical error, or a museum lot collected before the description of A. dolichophallus and A. brachyphallus that has not been reidentified. It could potentially be a misidentified record of A. brachyphallus, which they rightly call a subspecies of A. californicus (see Taxonomy). I identify all observations south of Point Lobos as A. stramineus and refrain from identifying those north of Point Lobos to species.

Banana slugs in San Luis Obispo County have not been well-documented. Roth & Sadeghian (2006) did not list San Luis Obispo for any of the species, suggesting there are no lots from this county in museum collections. Mead (1943) listed one record of A. stramineus from Little Pico Creek, likely collected by WH Lange. If his collection survives, it should be searched for such specimens. Pearse & Leonard (2010, unpublished data) also indicated very scant information on this county. Curiously, they listed A. brachyphallus from here as an isolated population in the vicinity of Cambria, which would include Lange's Little Pico Creek record. They say nothing about populations in the other part of the county, so it would appear that such observations on iNaturalist represent previously undiscovered populations. If we only looked at published records, the entire county would presumably just contain A. stramineus. However, Pearse & Leonard (2010, unpublished data) are the only ones who have studied this region in any detail and they only document A. brachyphallus. I refrain from identifying observations in San Luis Obispo to species unless they occur in the extreme northwest corner, in which case they are probably A. stramineus.

Santa Barbara and Ventura Counties both only contain A. stramineus (Mead, 1943; Roth & Sadeghian, 2006). This species has also been found on Santa Cruz (from where it was described) and Santa Rosa Islands, but it is present on none of the smaller islands. This makes sense given the ecology of the Channel Islands (Mead, 1943). I identify all observations in these counties as A. stramineus.

San Diego County contains a new species on Palomar Mountain (Pearse et al., 2010*; Pearse & Leonard, 2010, unpublished data; Leonard et al., 2013*; JL Leonard, personal communication, 2020). Roth & Sadeghian (2006) could not have overlooked this new species because it is mentioned in Pearse et al. (2010*), which they cite, so it is somewhat surprising that they did not list it. This population is very far removed from the others so its presence is somewhat surprising. However, Mead (1943) speculated that there could be a population in San Diego County that "migrated along the coast ranges" when he found a record of Orcutt having discovered banana slugs there. I have not been able to locate a publication of this record and it may simply represent a museum lot. However, Mead (1943) suspected it was inaccurate. Mead's coast range hypothesis would not be consistent with the genetic data in Pearse & Leonard (2010, unpublished data), which indicates this new species is closer to A. buttoni (the nearest population being from the Sierra Nevada Mountains) rather than A. stramineus (the coast range species). The Fresno County observation could represent a link between A. buttoni and this new species. Naturally, I refrain from identifying any San Diego County observations to species.

Summary of Ariolimax Species by Region

Region Species Remarks
Pacific Northwest: Alaska, British Columbia, Washington, Idaho, Oregon
California Counties: Del Norte, Siskiyou, and Humboldt (map)
A. columbianus Individuals can be spotted or spotless.
California Counties: Trinity, Shasta, Tehama, Plumas, Butte, Mendocino, Glenn, Sierra, Yuba, Lake, Nevada, Colusa, Placer, El Dorado, Yolo, Sonoma, Napa, Sacramento, Amador, Solano, Calaveras, Tuolumne, Marin, San Joaquin, Contra Costa, Alameda, and Fresno (?) (map) A. buttoni Individuals can be spotted or spotless.
San Francisco County, CA (map) A. brachyphallus
A. buttoni
A. californicus?
A. brachyphallus and A. buttoni (some of which are spotted) are found on the mainland. The individuals on Alcatraz Island may be A. californicus, or possibly A. brachyphallus or something new.
San Mateo County, CA (map) A. brachyphallus
A. californicus
San Mateo proper divides A. brachyphallus (to the north) from A. californicus (to the south).
Santa Clara County, CA (map) A. buttoni
A. californicus
A. dolichophallus
A. dolichophallus and A. californicus are found next to San Mateo and Santa Cruz Counties, respectively. The western extent of A. buttoni (some of which are spotted) is unclear, but it is the only species northeast of Hwy 101.
Santa Cruz County, CA (map) A. dolichophallus
A. californicus?
A. californicus would only be found near the northwest border with San Mateo County.
San Benito County, CA (map) A. buttoni
New species
The new species (which is in the process of description) is restricted to Fremont Peak, where A. buttoni is also present.
Monterey County, CA (map) A. buttoni
A. dolichophallus
A. stramineus
A. brachyphallus?
A. stramineus is the only species south of the Monterey Peninsula and the other 3 are all present on and north of the Peninsula.
San Luis Obispo County, CA (map) A. brachyphallus?
A. stramineus?
Ranges are poorly documented here.
California Counties: Santa Barbara and Ventura (map) A. stramineus This includes Santa Rosa and Santa Cruz Islands.
San Diego County, CA (map) New species Restricted to Palomar Mountain; in the process of description.


At the end of the day, this genus is a work in progress. Every genus is a work in progress. Ariolimax is still one of the better known genera of native terrestrial gastropods on the West Coast—both scientifically speaking and as far as the general public is concerned. I hope to condense the literature into one place and provide an easy reference point for identification. At the very least, I got to read some cool papers, learn some HTML, and add every applicable observation to the Molluscan Mycology project. Additionally, if and when more research is published, it will be much easier to curate the taxa. If you're ever in Santa Barbara, let me know and perhaps we can go looking for some banana slugs. Who knows what we might find!

Literature Cited

*Some of these references are presentation and poster abstracts. The date listed is the date in which they were published, not presented, because I was not present at the conferences (unlike some authors who cite them with the presentation date). While they technically have been published, at least some were not peer-reviewed.

Cooper JG 1872. On new Californian Pulmonata, etc. Proceedings of the Academy of Natural Sciences of Philadelphia 24: 143-154.
Frest TJ & Johannes EJ 2000. An Annotated Checklist of Idaho Land and Freshwater Mollusks. Journal of the Idaho Academy of Science 36(2): 1–51.
Gervais JA, Traveset A, & Willson MF 1997. The Potential for Seed Dispersal by the Banana Slug (Ariolimax columbianus). The American Midland Naturalist 140: 103–110.
Gould J 1851. Pneumobranchiata: Limacidae. The Terrestrial Air-Breathing Mollusks of the United States, and the Adjacent Territories of North America, ed. A Binney, Boston, Mass. 2: 1–44.
Heath H 1916. The conjugation of Ariolimax californicus. The Nautilus 30(1): 22–24.
Hemphill H 1891. General Notes. The Nautilus 4(10): 120.
Leonard JL, Pearse JS, & Harper AB 2002. Comparative Reproductive Biology of Ariolimax californicus and A. dolichophallus (Gastropoda: Stylommatophora). Invertebrate Reproduction & Development 41: 83–93.
*Leonard JL, Pearse JS, Breughelmans [sic] K, & Backeljau T 2004a. Rapid Evolution in Banana Slugs (Ariolimax spp.) (Gastropoda; Stylommatophora: Arionidae): Sexual Behavior Indicates the Direction Of Evolution. Integrative and Comparative Biology 44(6): 591.
*Leonard JL, Pearse JS, Breughelmans [sic] K, Backeljau T, Diep PJ, Robles M, & Townsend RC 2004b. Rapid Evolution in Banana Slugs (Ariolimax spp.) (Gastropoda; Stylommatophora: Arionidae): Life History Parameters. Integrative and Comparative Biology 44(6): 718.
Leonard JL, Westfall JA, & Pearse JS 2007. Phally polymorphism and reproductive biology in Ariolimax (Ariolimax) buttoni (Pilsbry and Vanatta, 1896) (Stylommatophora: Arionidae). American Malacological Bulletin 23: 121–135.
*Leonard JL, Pearse JS, Breugelmans K, Backeljau T, & Long JM 2009. Banana Slugs in the Bay Area National Parks: Distribution of Ariolimax (Stylommatophora: Arionidae) Species. The Western Society of Malacologists: Annual Report for 2008 41: 29–30.
*Leonard JL, Westfall JA, & Pearse JS 2010. Sexual Selection and Mating Systems in the Genus Ariolimax (Stylommatophora: Gastropoda). The Western Society of Malacologists: Annual Report for 2005 38: 41.
*Leonard JL, Pearse JS, Helsen S, Van Houtte N, Breugelmans K, Jordaens K, & Backeljau T 2011. Evidence for rapid evolution with diversification of sexual behavior in the genus Ariolimax (Gastropoda: Stylommatophora). The Western Society of Malacologists: Annual Report for 2010 43: 55.
*Leonard JL, Pearse JS, Elejalde A, Helsen S, Van Houtte N, Breugelmans, K, Jordaens K, & Backeljau T 2013. Phylogeography and Rapid Evolution in Banana Slugs (Arionidae: Ariolimax spp.). The Western Society of Malacologists: Annual Report for 2011 44: 218–220.
Mead AR 1943. Revision of the Giant West Coast Land Slugs of the Genus Ariolimax Moerch (Pulmonata: Arionidae). The American Midland Naturalist 30: 675.
Miller BLW & Sinervo B 2007. Heritable body size mediates apparent life-history trade-offs in a simultaneous hermaphrodite. Journal of Evolution Biology 20: 1554–1562.
Mörch OAL 1859. Beiträge zur Molluskenfauna Central-Amerika's. Malakozoologische Blätter 6: 102–126.
*Pearse JS, Breugelmans K, Backeljau T, & Leonard JL 2010. Phylogeography of banana slugs (Ariolimax spp.) (Gastropoda; Stylommatophora: Arionidae). The Western Society of Malacologists: Annual Report for 2005 38: 56.
Pearse J & Leonard J 2010. All About Banana Slugs. Unpublished 56 pp.
Pearson AK, Pearson OP, & Ralph PL 2005. Growth and Activity Patterns in a Backyard Population of the Banana Slug. The Veliger 48(3): 143–150.
Pilsbry HA 1948. Land Mollusca of North America (North of Mexico) Volume 2 Part 2. Academy of Natural Sciences of Philadelphia, Philadelphia, Penn. 1113 pp.
Pilsbry HA & Vanatta EG 1896. Revision of the North American Slugs: Ariolimax and Aphallarion. Proceedings of the Academy of Natural Sciences of Philadelphia 48(2): 339–350.
Reise H & Hutchinson JMC 2002. Penis-biting slugs: wild claims and confusions. TRENDS in Ecology & Evolution 17: 163.
Roth B 2004. Observations on the Taxonomy and Range of Hesperarion Simroth, 1891 and the Evidence for Genital Polymorphism in Ariolimax Mörch, 1860 (Gastropoda: Pulmonata: Arionidae: Ariolimacinae). The Veliger 47(1): 38–46.
Roth B & Sadeghian PS 2006. Checklist of the Land Snails and Slugs of California. Santa Barbara Museum of Natural History, Santa Barbara, Calif. 82 pp.
(also in spreadsheet version)
Van Bruggen AC & Mead JI 2011. Albert R. Mead, 1915–2009, noted American malacologist: An obituary. The Nautilus 125(4): 228–233.

Potentially helpful resources that I do not have:

Harper AB 1988. The Banana Slug: A Close Look at a Giant Forest Slug of Western North America. Bay Leaves Press, Aptos, Calif. 32 pp.
Mead AR 1942. The Taxonomy, Biology, and Genital Physiology of the Giant West Coast Land Slugs of the Genus Ariolimax Mörch (Gastropoda: Pulmonata). Abstracts of Theses, Cornell Univ., New York 312–314.
Waste RJ 1937 (or is it 1940?). The land slugs of California. MA Thesis, Univ. Calif. Berkeley., Calif. Unpublished 111 pp.

See also these pages:


Posted on 16 May, 2020 21:32 by thomaseverest thomaseverest


Photos / Sounds


Southern Pacific Banana Slug (Ariolimax stramineus)




March 28, 2020 03:00 PM PDT


A little surprised to see one so high up above all the creeks, but it was quite damp and overcast


Wowie! Such great work to put this together! Thank you so much Thomas!!!

Posted by susanhewitt almost 4 years ago

Thanks Susan! It was a lot of fun!

Posted by thomaseverest almost 4 years ago

Thanks for this! I am sharing it with the San Mateo County BioBlitz project forthwith! Also @merav should see this, given her level of fandom for Ariolimax...

Posted by gyrrlfalcon almost 4 years ago

Thanks for summarizing what we currently know and for applying that knowledge (such as it is) to our observations. I'll keep an eye open for spotted individuals now that I know they have extra diagnostic power.

Posted by chris_nelson almost 4 years ago

@chris_nelson Yes I recently found this observation. Immature, so they're rather faint. If A. buttoni is present in San Mateo, that complicates things a bit, but it might explain why the species is present in San Francisco. Or maybe other species can be spotted too…

Posted by thomaseverest almost 4 years ago

This definitely lines up with my admittedly nebulous understanding of the state of Ariolimax taxonomy, so I thank you for stating it so comprehensively and with extensive citations and with less nebulosity, even if I don't always agree with your identification approach (I'm still waiting for the publication of Pearse's genetic data in a peer-reviewed journal to have absolute faith in geographic IDs; if what you say about the renewal of that work is true, maybe I won't have long to wait).

Posted by kueda almost 4 years ago

@thomaseverest, thank you so much for collating the current knowledge about Ariolimax into this concise, understandable treatise. It answers several questions I've had over the years. Very well done and much appreciated!

Posted by truthseqr almost 4 years ago

Wow! Thank you for putting together such a comprehensive summary!

Posted by rjadams55 almost 4 years ago

Incredibly helpful – thanks!

Posted by davidarrivee almost 4 years ago

Awesome work! Thanks for this useful summary.

Posted by ameet almost 4 years ago

Excellent contribution here...clarifies many things...and I especially appreciate all the links to literature given in the bibliography, to facilitate further study for us readers.

Posted by arbonius over 3 years ago

Great post!
Just a bit of a correction to the taxonomy section: the ICZN Code deals strictly with nomenclature, not taxonomy; it's irrelevant in any given author's decision whether to treat two taxa as subspecies of the same species or as separate species in their own right (or even to recognize them as separate taxa in the first place) - that is to say, none of the Code articles cited here are relevant to what they're being cited for. There's no "official" process as far as taxonomy goes - you can base taxonomic changes on any data (published or not), or even no data or explanation at all (not that anyone else needs to follow any given suggested taxonomic change)! The only part of the process that's under the Code's purview is the names applied to the taxa being split/lumped/etc. (what names have priority, new names having to be introduced in specific Code-compliant ways, and all that).

Posted by maxkirsch over 3 years ago

@maxkirsch Interesting. So the Code deals with the availability of names, not their validity. At the end of the day, A. buttoni is a validly proposed name, but there are no rules on it needing to be "validly" established as a species concept (i.e. distinct from A. columbianus)—that's essentially all up to personal interpretation, which the current scientific community prefers to have established through peer-reviewed published data. And we can just start calling A. stramineus and A. brachyphallus full species without needing to publish an explanation or declaration of such an elevation. That would make sense according to Principles 1 and 2. So what counts as a nomenclatural act other than species description, as mentioned in Article 7?

Posted by thomaseverest over 3 years ago

It's the availability of names and the validity of names applied to taxa that the ICZN Code governs, rather than how taxonomists circumscribe the taxonomic concepts denoted by those names. (e.g., whether a scientist treats A. buttoni and A. columbianus as conspecific or as separate species, the Code doesn't care how either conclusion was reached - all it cares about is that, in the event that they're treated as one broader species, that species is called A. columbianus rather than A. buttoni, since the former name has priority and is thus the valid name for that broader species. Like you said, other members of the scientific community certainly care how either conclusion was reached, but that's outside the Code's purview.) Or in other words, it's just the names of taxonomic concepts that the Code cares about, rather than the concepts themselves. i.e., the Code is completely irrelevant to the question of whether a given population should be treated as its own species or a subspecies of a broader species, and how either conclusion was reached.
(Could you clarify what you're referring to as Principles 1 and 2, since the Code doesn't have numbered principles, only chapters and articles [a few of which are statements of principles - but again, those principles aren't numbered like chapters and articles]?)

A nomenclatural act is "a published [in the sense of the Code] act which affects the nomenclatural status of a scientific name or the typification of a nominal taxon." That includes things like First Reviser actions, fixation of the type species of a nominal genus, designating a holotype or syntypes when creating a species-group name, designating a lectotype or a neotype for an already-existing species-group name, etc., as well as publishing a new name.

Posted by maxkirsch over 3 years ago

@maxkirsch Thanks for the help; I've updated the relevant portions here. Principles 1 and 2 are in the Introduction.

Posted by thomaseverest over 3 years ago

This is so rad. Nice work, and a big thank you, @thomaseverest

Posted by siznax over 3 years ago

Thank you for posting this

Posted by dsacer over 3 years ago

Really an incredible effort! An amazing resource.

Posted by dylangomes about 3 years ago

Great information!

Posted by sedgequeen about 3 years ago

Excellent work - thank you for this!

Posted by jdctgp almost 3 years ago

Thank you! This is exactly what I need to get better at identifying these slugs!

Posted by katiemharding over 2 years ago

All I wanted to do is enter a banana slug and new species.. ") haha jk. Thank you for the break down fascinating, somewhat confusing and frustaing, yet fascinating.

Posted by brianhindsakafundad almost 2 years ago

Incredible thank you!

Posted by syy7 almost 2 years ago

Thank you so much for this work! Can you please check my observation? I currently doing the naturalist course and plan on a presentation on the banana slug in Point Reyes. Thanks, Zoj

Posted by spourgitaki 7 months ago

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