Structure and Functioning of the “Egg Bank” of a Fairy Shrimp in a Temporary Pool: Chirocephalus ruffoi from Pollino National Park (Southern Italy) as a Case Study

Abstract

To investigate their distribution and total numbers, resting eggs of the anostracan Chirocephalus ruffoi were collected from the bed of a temporary pool in southern Italy. Samples were taken at 0.5 m intervals along six transects oriented at 30°from each other, by means of a cylindrical core sampler. The horizontal distribution of intact resting eggs was extremely patchy, with cyst number per core ranging from 191 to 1,400 (CV = 32.7%), corresponding to a mean of between 0.8 and 4.3 cysts cm -3 . Differences observed were related to core position and transect orientation, total cyst numbers being markedly higher in the leeward area of the pool compared to the windward area. Marked variation was also evident in vertical distribution, a significant, though weak correlation was recorded between egg density and sediment depth. Cyst-bank size (± 95% confidence limits) of the pool bed, estimated from the mean cyst number cm -3 obtained for the 6 transects, ranged between 1.0 × 10 8 and 1.3 × 10 8 cysts.Hatching in the laboratory was very erratic. Despite significant differences in hatching, the observed variation was unrelated to most of the variables considered (position within sections, cores and transects, pre-incubation treatment) and was explained only by initial sediment conditions (moist/dry). In none of the experimental conditions tested was synchronous hatching obtained.Possible causal factors (mixing of the bottom sediments by cattle, egg age, storage conditions, differential exposure to environmental cues as well as variability in hatching response even at clutch level) are discussed. IntroductionMany residents of temporary pools produce long-lived resting eggs enabling survival during unfavourable conditions. The changing nature of these habitats requires adaptation to very different environmental regimes. Moreover, the predictability and the frequency of the wet/dry phases vary greatly among habitats, depending on factors like climate, latitude, altitude, nature of the sediment etc. Therefore population persistence requires local adaptations in life history traits and, particularly in the hatching behaviour of the resting stages (BELK and COLE, 1975;WIGGINS et al., 1980). The more unpredictable the local habitat, the higher the risk of extinction favouring the evolution of adaptations that "spread the risk" (see review in SIMOVICH and HATHAWAY, 1997;BRENDONCK et al., 1998). Not all of the eggs deposited at one time will hatch during the same subsequent inundation and thus an egg bank (sensu DE STASIO, 1989) forms in the sediment, acting as a reserve for the future. In addition to information about quantitative reproductive characteristics, understanding the behaviour and functioning of an egg bank is of fundamental importance in identifying any local adaptations to habitat uncertainty.Internat. Rev. Hydrobiol.

How do freshwater organisms cross the “dry ocean”? A review on passive dispersal and colonization processes with a special focus on temporary ponds

“…Thiéry, 1997;Brendonck et al, 1998;Mura, 2004Mura, , 2005 and the role of the wind in the dispersal of zooplankton, large branchiopods eggs and other invertebrates (Baujard & Martiny, 1994;Cáceres & Soluk, 2002;Graham & Wirth, 2008 and literature therein). As already reported for phytoplankton, the effectiveness of the dispersal of these animals seems to be linked to the distance among sites.…”

Cyst Distribution and Hatching Pattern ofChirocephalus ruffoi (Crustacea, Anostraca) in an Experimental Undisturbed Pool

“…In spite of this, and in contrast to the plethora of studies on the cyst banks of zooplankters from permanent waterbodies (see review in MARCUS, 1996;MURA, 2004), no information on the structure and functioning of anostracan cyst banks was available until recent years.…”

“…The horizontal and vertical distributions of cysts in the pool bed were determined. The comparison between cyst bank estimate (MURA, 2004) and the actual number of cysts counted in the pool bed revealed an estimate error of 20.9%. Resting eggs occurred only in the upper 2.5 cm thick soil sections and decreased within this section as depth increased.…”

Hatching response to temperature along a latitudinal gradient by the fairy shrimp Branchinecta lindahli (Crustacea; Branchiopoda; Anostraca) in culture conditions

“…Zarattini et al (2002) hatched Chirocephalus diaphanus Prévost, 1803 eggs, but reported that individual egg fractions hatched only after one, two or three hydrations, and each hatching group demonstrated a level of genetic distinction from the other two groups. Mura (2004) found variation in hatching response within individual pools depending upon whether the sediment was moist or dry prior to culture, and recognised that it may be the result of mixed genotypes within the egg bank.…”