Journal of Conchology 44/4

a M art Í nez -o rt Í eT al . 362

Figure 20 Variation of the length of the shell (in mm) of Bulinus truncatus with respect to the time in days, from El Ejido (Almería, Spain). Scales=2mm.

9.6–11.0mm in diameter. The maximum height can reach exceptionally 20mm (Giusti et al ., 1995; Malek & Cheng, 1974). The features of the habitat (e.g. marshy bottom or not, abundance of food or scarcity, limestone or not, etc.) can influence both the size and shape of the shell (Schwetz, 1954). According to the data obtained in Table 1 of 37 specimens the shell length (L) varies between 8.62±0.57mm in specimens from Almería and 16.12±0.65mm from Barcelona (for the 4 bigger specimens), and the height of aperture (HA)/ Length (L) relation of 33 specimens measured varies between 0.62 and 0.74. The height aperture (HA) of 33 specimens varies between 4.66±0.55 of Almería and 6.68±0.29mm of Sardinia. The data obtained of Fraga de Azebedo et al . (1969) was 0.57, although we do not know the number of specimens measured. These data indicate con chological variation among the different popula tions of this species, meaning one is unable to differentiate any of them by the shell. Although the population of Almería corresponds to the one with the smallest shell and the largest one to that of Barcelona populations. The protoconch, as in the genus Bulinus and the majority of planorbids, presents a punctiform microsculpture, whose

spots appear arranged in spiral rows (Figs 14–15, 17–19) (Germain, 1931; Walter, 1962; Martínez– Ortí et al ., 2015). Shell growth in captivity (Fig. 20) The growth in length of the shell has been followed in captive B. truncatus specimens from El Ejido (Almería, Spain). It has been shown that this growth is logistic type, since it increases rapidly fromhatch ing and as the population approaches the carry ing capacity of the environment, the growth rate becomes slower until it finally stabilizes (Molles, 2006). The shell size grows rapidly during the first ninety days, and during the following ninety days, the growth decreases and then from 150 to 180 days it increases scarcely. From September, the length of the shell continues to increase, although more slowly than at the beginning. After one year since hatching the growth is fully stabilized, giving rise to a linear growth during the following ninety days. Growth stabilization may be due, in part, to environmental resistance (Molles, 2006), which in our case includes abiotic factors such as temperature (20ºC), light (photo period 12 h light / 12 h dark), pH (close to 7), humidity (50%), properties of the mineral water