Growth and developmental time in the parthenogenetic scorpion Tityus stigmurus (Thorell, 1876) (Scorpiones: Buthidae)

Crescimento e tempo de desenvolvimento do escorpião partenogenético Tityus stigmurus (Thorell, 1876) (Scorpiones: Buthidae)

To understand the dynamics of arthropod populations, it is important to determine life-history characteristics that contribute to reproductive success. Juvenile growth rate and development time are key determinates of organism survival, size, age of maturity and adult reproduction. These factors are also important for understanding the dynamics of arthropod populations (Kleinteich & Schneider, 2011; Roff, 1992; Stearns, 1992). For most animals, variations in growth rates and developmental time have mainly been related to differences in the interaction between the individual genome and in sensitivity to environmental changes (Dmitriew, 2011; Fusco & Minelli, 2010).

Amongst the 1900 scorpion species (Stockmann & Ythier, 2010), parthenogenesis has been reported in approximately eight species (Francke, 2007; Ross, 2010). Seven of these species belong to the genus Tityus C. L. Koch, 1836 (Ross, 2010; Schneider & Cella, 2010), which is the largest genus of the Buthidae family, widely distributed across the Americas, from Costa Rica (Francke & Stocwell, 1987) to Central Argentina (Ojanguren Affilastro, 2005) and the Dominican Republic (Armas & Abud Antun, 2004).

Despite medical interest in the genus Tityus, which includes scorpions responsible for many deaths and injuries in the tropics each year (Albuquerque, Santa Neto, Amorim, & Pires, 2013; Chippaux & Goyffon, 2008; Stockmann & Ythier, 2010), little information is available about the life history of many species (Lourenço, 1979a, b, 2002, 2007, 2008; Lourenço & Eickstedt, 1988; Lourenço & Cloudsley-Thompson, 1998, 2010; Matthiesen, 1971; Rouaud, Huber, & Lourenço, 2002), especially with regard to growth pattern. Scorpions grow at different rates, which means that adults are capable of producing offspring at highly variable times. Post-birth development (PBD) has been studied in only 22 species, and a large variation in PBD duration (6 to 85 months), with four to eight moltings prior to adulthood was observed (Francke, 1984; Lourenço, Andrzejewski & Cloudsley-Thompson, 2003; Lourenço, Huber & Cloudsley-Thompson, 1999; Lourenço, Ythier & Cloudsley-Thompson, 2008; Polis & Sissom, 1990; Quijano-Ravell, Ponce-Saavedra, & Francke, 2011; Rouaud et al., 2002; Seiter, 2012).

Tityus stigmurus (Thorell 1876) is a parthenogenetic scorpion (Ross, 2010) widely spread across urban areas of northeastern Brazil (Lira-da-Silva, Amorim, & Brazil, 2000; Ministério da saúde [Brasil], 2009), a region inhabited by over 53 million people (Instituto Brasileiro de Geografia e Estatística. Brasília [IBGE], 2015). It is the scorpion that possess the most significant health threat in this region, being responsible for most scorpion stings, including fatal cases (Albuquerque et al., 2013). The aim of this study was to determine the life cycle and intermolt period of T. stigmurus. Knowledge of the natural history of this scorpion is fundamental to gaining a better understanding of the ecoepidemiology of human accidents.

The only T. stigmurus specimen that reached the adult stage underwent six moults and became an adult 871 days after birth. The average durations of each postembryonic instar (first to sixth) are presented in Table 1.

Table 1.
Intermolt period in days and cumulative age (mean ± SD) of the parthenogenetic scorpion T. stigmurus (Thorell, 1876) reared in controlled conditions.

The first molt took place on the dorsum of the mother three to four days after birth, and this juveniles starting dispersion in the next one or two days. Mortality was high in the second instar for juveniles, totalling 44.4% of the sample. Unlike for the first instar, there was a noticeable variation in the duration of the second instar (36 to 89 days). Variation in the total duration of the third instar (Coefficient of variation = 18.5%) was lower than in the second instar (Coefficient of variation = 23.3%), ranging from 87 to 138 days. The mortality rate in the third instar was approximately 38%.

Only five of the 10 specimens that reached the fourth instar survived to the fifth instar. The time spent in the fourth instar for three of the five specimens averaged 148 days, and the remaining two moulted approximately 180 days later. Four of the five surviving specimens reached the sixth instar; one matured during the seventh instar, after spending 186 days in the sixth instar. Morphological measurements and the growth factors (Dyar’s ratio) for all the instars are presented in Table 2

Table 2.
Morphological measurements in millimetres (mean ± SD) and growth factors (mean ± SD) for the three corporeal body structures during in different instars of T. stigmurus (Thorell, 1876) during in different instars.

Overlapping in measurement extremes between instars was registered for all structures analysed from both populations (Figure 1).

Analyses of covariance for the three structures measured (Met V (F = 1.5098, d.f. = 1.75, p = 0.2209), MF (F = 1.4911, d.f. = 1.74, p= 0.2239 and CL (F = 0.0028, d.f.= 1.75 p = 0.9571)) showed that scorpions from the colony and the experimental group did not differ, therefore permitting their use as complementary data.

Figure 1.
Overlapping size (in mm) in on carapace length, movable finger, V metasomal segment, and instars among individuals of T. stigmurus specimens (Thorell, 1876).

Studies on scorpion life cycles have shown that their post-embryonic development is usually long (Lourenço, 2002; Polis & Sissom, 1990) with a relatively high juvenile mortality rate (Brown, 1997; Francke & Jones, 1982; Sissom & Francke, 1983). Specimens of the genus Tityus reached adulthood within six (Polis & Sissom, 1990) to 25 months (Lourenço, 2002). In the current study, T. stigmurus reached adulthood a mean of 29 months after birth, contrasting with previous research which presented developmental duration as between 10 (Ross, 2010) and 17 months (Matthiesen, 1971). This is probably the longest variation in PBD described for the genus Tityus. However, because of the limited information available on scorpions’ rearing conditions, it is difficult to make a more detailed comparison. In addition, in this study, adulthood was reached after six moults, one moult more than found by other authors.

In this study, the duration of instars 2-5 was considerably shorter than reported by Matthiesen (1971) (148 days, n = 5; 281 days, n = 4; 362 days, n= 2 and 520 days, n = 1) for the same species. However, the author did not provide data on the experimental conditions in which animals were kept, such as their food supply and temperature. These factors may well affect developmental parameters. In addition, the scorpions used by Matthiesen were from Pesqueira, a city with a semiarid climate, 215 km west of the coastal city of Recife from which our sample was taken. Therefore, population differences may also partly account for the differences between the two studies.

To distinguish between different juvenile instars, morphometric studies of different parts of the scorpion body have been made on several species, forming an index (Francke, 1979; Francke & Sissom, 1984; Lourenço, 1979a, b; Lourenço & Goodman, 2006; Lourenço, Ythier & Cloudsley-Thompson, 2007; Lourenço & Cloudsley-Thompson, 1999; Lourenço et al., 2003; Lourenço et al., 2008; Rouaud et al., 2002; Sissom & Francke, 1983). In the genus Tityus, Dyar’s rule has been applied to only five species: T. mattogrossensis (Borelli, 1901), T. fasciolatus (Pessôa, 1935), T. neblina (Lourenço, 2008), T. ocelote (Francke & Stockwell 1987), and T. confluens (Borelli, 1899), and ranged from 1.13-1.57, depending on the species and parameters analysed (CL, MF, and MetV) (Lourenço, 1979a, b; Lourenço & Cloudsley-Thompson, 2010; Seiter, 2012). Variation (1.14-1.34) was also found in a study of T. stigmurus, with the size distributions overlapping to some extent between the different instars. However, despite the observed variation, the different instars could be distinguished using the CL, Met V, and MF lengths. Although an overlap in body structures is not common in arthropods, it has also been found in coleopterans P. darlingensis (Carter, 1919), O. discicolle (Brullé 1840) and in lepidopteran Helicoverpa armigera (Hubner 1805) (Allsopp & Adam, 1979; Mohammadi, Abad, Rashidi, & Mohammadi, 2010; Velásquez & Viloria, 2010).

In summary, by monitoring individual juveniles for more than two years, we have observed variation in individual size at each instar and in growth rate for the parthenogenetic scorpion T. stigmurus. Our findings indicate that this species may have the longest post-embryonic development period in the genus Tityus, reaching adulthood after an average of 29 months and six moults.

Our most sincere thanks to the environmental health agents at the Health District for collecting scorpions. We would also like to thank PIBIC/CNPq for the scholarship they provided to the third author and CAPES for the doctoral scholarship they provided to the first and third authors.