Deroceras cf. laeve ( Mueller , 1774)

Rowson, Ben, Paustian, Megan & Goethem, Jackie Van, 2017, New species and records of terrestrial slugs from East Africa (Gastropoda, Urocyclidae, Veronicellidae, Agriolimacidae), ZooKeys 723, pp. 11-42 : 17

Deroceras cf. laeve ( Mueller, 1774)

Material.

TANZANIA: NMW.Z.2001.040.00001: 2 ads., central Mbeya (8.91°S, 33.46°E), Mbeya District, in a cabbage from town market, approx. 1600-1800 m alt., leg. MBS, PT & AR, 25 Jun. 2001. The cabbage, probably locally grown, also harboured a juvenile Elisolimax or Bukobia sp. (NMW.Z.2001.040.00002). Comparative material of non-African Deroceras spp.: specimens cited in Rowson et al. 2014a; b).

Remarks.

The genus Deroceras is primarily Palaearctic, but nonetheless is represented by a few species in Ethiopia. It includes several species spread widely by humans. These include the pest D. reticulatum ( Müller, 1774) and the "tramp slug" D. invadens Reise, Hutchinson, Schunack & Schlitt, 2011 (see Reise et al. 2011 for synonymy). Although both species and D. laeve ( Müller, 1774) are well-established in South Africa ( Herbert 2010), records of Deroceras in tropical Africa are few. Verdcourt (1960a) recorded D. laeve andecolum ( D’Orbigny, 1837) from a Nairobi garden. He later listed D. laeve from Muguga and Ruiru, both near Nairobi, and from Thika where it was damaging orchids, later listing it from the "Nairobi area" generally ( Verdcourt 1965, 2006). Nairobi and Mbeya have relatively similar, cool climates when compared to “Zanzibar”, from which two Deroceras have been reported: D. laeve ( Simroth 1895) and D. reticulatum ( Ellis 1969). The D. laeve record seems plausible, given that species’ apparently very broad ecological tolerance ( Wiktor 2000), although there is some evidence D. laeve may comprise more than one species ( Rowson et al. 2014a). Rowson (2007) considered the D. reticulatum record doubtful and to require confirmation. Deroceras invadens has now been intercepted on Kenyan flowers arriving in the USA ( Hutchinson et al. 2014). These authors reported D. laeve from São Tomé, where it had been identified as D. invadens , but also suggest that some records of " D. laeve " outside Europe may refer to D. invadens . Deroceras laeve was also reported from Ethiopia by Simroth (1895) along with Agriolimax jickelii “Heynemann”, a species Wiktor (2000) considers a nomen dubium. Simroth later (1904) described 14 other Deroceras species from Ethiopia. Some could potentially be confused with introduced species, but others are highly distinctive and doubtless endemic. Wiktor (2000) maintained ten of them in his revision. According to Wiktor (2000) the most southerly native occurrence of Agriolimacidae is D. uataderensis (Simroth, 1904), described from Lake Gandjule in the southern Ethiopian Rift (Lake Abaya or Lake Chamo, approx. 6°N, approx. 1200 m alt.). Terrestrial molluscs with apparently Ethiopian or Palaeartic links are known from the archipelago-like Afroalpine and Afromontane regions isolated on the highest East African mountains (e.g. Tattersfield et al. 2001, Verdcourt 2006). However, on a broader scale, and at more moderate altitudes, the Ethiopian biota is biogeographically very distinct from that of southern Tanzania (e.g. Linder et al. 2012).

The Mbeya slugs are 17.5 and 16.8 mm long, larger than most of the preserved D. laeve examined but smaller than most D. invadens , so in fact within the range of overlap. They resemble D. reticulatum (and some Ethiopian species) in being pale cream with black-brown tentacle retractors, and a dusting of light brown pigment along the centre of the mantle and forming a network between the tubercles at the top of the tail. The skin is thin, with the part of the mantle underlain by the shell relatively obvious. The pneumostome is surrounded by a contrastingly pale ring. The tail tip is steeply truncate. The skin, pneumostome and tail features are often considered diagnostic of D. invadens or D. laeve as compared to D. reticulatum , as is the length of the tail, although none may be infallible (e.g. Rowson et al. 2014b and references therein). Indeed, Herbert (2010) notes that some D. invadens in South Africa closely resemble D. reticulatum externally. Internally, no rectal caecum was found in the Mbeya slugs, ruling out D. reticulatum which has a large one ( Wiktor 2000). This also would seem to rule out a group of taxa including D. invadens , which Wiktor treats as having a shallow, pocket-like caecum. However, it is clear that the caecum can be so shallow as to be undetectable in D. invadens ( Quick 1960, Reise et al. 2011). The ovotestis lies relatively far forward, anterior of the rectum, and is scarcely exposed. Quick (1960) showed an anterior ovotestis for D. invadens and described a "less exposed" ovotestis for D. laeve . The female genitalia are well developed but the penis is reduced to a tiny nub without a retractor muscle (an aphallic condition). The combination of aphally and no rectal caecum makes the Mbeya slugs key to D. laeve in Wiktor (2000); indeed, aphally has often been used to attribute putative D. invadens specimens to D. laeve (e.g. Quick 1960, Wiktor 2000, Reise et al. 2006, Hutchinson et al. 2014). Although it has been suggested that D. invadens could potentially be aphallic ( de Winter 1988), there is as yet no substantiated report of aphally in any Deroceras species other than D. laeve (J.M.C. Hutchinson pers. comm. 2017). Genetic data also suggest that worldwide D. laeve might consist of more than one species (e.g. Rowson et al. 2014a). We therefore attribute the Mbeya slugs to D. cf. laeve , a matter that could be settled with molecular data from this population. Until then any evidence of the spreading of non-native slugs in tropical Africa seems worth reporting, given the potential economic and conservation implications.