Hydrobiologia 486: 175–183, 2002. A.M. Maeda-Mart´ınez, B.V. Timms, D.C. Rogers, F.A. Abreu-Grobois & G. Murugan (eds), 175 Studies on Large Branchiopod Biology 4. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. Population dynamics of Triops cancriformis (Crustacea: Branchiopoda: Notostraca) of the Espolla temporary pond in the northeastern Iberian peninsula Dani Boix, Jordi Sala & Ramon Moreno-Amich Institute of Aquatic Ecology and Department of Environmental Sciences, University of Girona Campus Montilivi. E-17071 Girona, Catalunya, Spain E-mail: [email protected] Key words: Triops cancriformis, sex ratio, fecundity, recruitment, hydroperiod duration Abstract The population dynamics of Triops cancriformis in Espolla temporary pond (NE Iberian peninsula) were studied during 1996 and 1997, which encompassed six flooded periods. Data were collected on each individual’s size, sex, and, if female, on number of eggs in the oostegopodes. Male-biased sex ratios were found only in the drying- out phase and variations in fecundity were strongly related to hydroperiod duration. Sex ratio variation during the drying-out phase can be attributed to female mortality because the very low recruitment observed does not support the hypothesis of an increase of males. Two hypotheses are advanced to account for female mortality: (1) differential reproductive effort, and (2) size selective predation by herons. This population is characterised by low values of maximum densities compared with other notostracan populations, and by higher densities in the spring–summer hydroperiods than in the winter ones. Introduction criformis is now a protected species. In spite of this protection and T. cancriformis’ local popularity (fest- In recent times, many notostracan and anostracan pop- ival, street, rock group and shops all bear the name of ulations in the Iberian peninsula have disappeared (Ar- this notostracan species), its population dynamics is mengol et al., 1975), because of the loss of many tem- poorly known. porary aquatic environments (Seminario sobre Bases In the northern half of the Iberian peninsula T. Científicas para la Protección de los Humedales en cancriformis populations are fewer than in the south. España, 1986), similar to that observed worldwide Northern populations, as the studied one, correspond (Löffler, 1993; Franzén, 1996; Kuller & Gasith, 1996; to simplex form, while southern ones correspond to Brown, 1998; Giudicelli & Thiéry, 1998). In addi- mauritanicus form. The simplex form is found in tion, in the case of Triops cancriformis (Lamarck, clayey or transparent waters, while the mauritanicus 1801), rice-growing led to its elimination with pesti- form is more frequent in clayey waters (Alonso, 1985). cides (Font de Mora, 1923). Such treatment still occurs In contrast to the populations inhabiting clayey waters in rice cultivation, albeit with controlled application, (Alonso pers. com.), the study population (Espolla to eliminate the exotic red swamp crayfish Procam- temporary pond) is characterised by the overlapping barus clarkii (Girard). In the rice fields of València, of cohorts (Margalef, 1951; Vila & Abellà, 1990) and where the presence of T. cancriformis is documented by an abundance of T. cancriformis throughout the historically (Arévalo, 1915), reduction has been ob- hydroperiod, so that many die with drying out. served coinciding with a switch in treatment from The aim of this work was to study the population organochlorates to organophosphates. Reduction has dynamics of the tadpole shrimp T. cancriformis from been such that in some areas, like the autonomous the Espolla temporary pond in NE Iberian peninsula region of Catalunya (NE Iberian peninsula), T. can- through the analysis of the fecundity, recruitment pat- 176 Table 1. Water characteristics of the Espolla temporary pond during 1996–1997. The median is included due to the high val- ues of chlorophyll a and dissolved oxygen reached in the last days of the hydroperiod, when the pond was drying Range Mean Median ◦ Temperature ( C) 5.6 − 33.0 17.3 16.2 − Conductivity (µScm 1) 212 −1046 895.0 892.0 pH 6.5 − 8.7 7.7 7.7 − Dissolved oxygen (mg l 1)4.1− 14.6 7.4 6.8 − Chlorophyll a (µgl 1)0.14− 350.42 13.09 2.93 the pond at high temperature (19 ◦C) and low oxygen concentration (3 mg l−1 approx.), but mean values of pond water are 17.3 ◦C and 7.4 mg l−1 respect- ively (Table 1). The flooding dynamics of this pond are irregular. Some years there are two complete hy- droperiods (autumn and spring) separated by a period of total desiccation. It is not, however, uncommon to have years with only one complete hydroperiod, in late autumn or winter, and occasionally no flooding at all. Figure 1. Bathymetric map of the Espolla temporary pond showing The animal community richness is dominated by in- the location (in grey) of the seven transects. sects, with 82 out of 118 taxa, followed by crustaceans (14 taxa) and amphibians (11 taxa). The remainder is made up of turbellarians, nematodes, oligochaetes, bryozoans and gastropods (Boix et al., 2001). The ecological value of this ecosystem has been officially recognised since 1992 (Government of Catalunya, De- cree 328/1992 of 14 December, by means of which approval is given to the Areas of Natural Interest Plan). Despite this, measures for its protection have still not been introduced. Sampling was carried out during six hydroperiods between 1996 and 1997 (Table 2). Complete flood- ing events (hydroperiods 1, 2, and 3, in which almost Figure 2. The modified Elster beam trawl (redrawn from Schwoer- the whole area of the pond was flooded) were distin- bel, 1966). guished from those in which only the lower parts of the basin were flooded (hydroperiods a, b and c). With the tern, sex ratio, and abundance of this crustacean in six exception of hydroperiod 1, all flooding events began consecutive hydroperiods. with the pond completely dry. Sampling was conducted weekly except at the end of each hydroperiod when frequency was increased. Materials and methods The pond was divided into seven areas by bathymetry, hydrology, and vegetation. A modified Elster beam Espolla temporary pond (42◦0906N, 02◦4601E; trawl (Schwoerbel, 1966) was pulled along a 20-m surface = 3.13 ha, maximum depth = 4 m) is located transect in each area (Figs 1 and 2). This sampling in the Banyoles karstic area (NE Iberian peninsula). device had an opening of 50 × 30 cm and a mesh size It has the same groundwater supply as Lake Banyoles of 250 µm. Samples were preserved ‘in situ’ with 4% and the other ponds in the area. Groundwater reaches formalin. 177 Table 2. Details of the hydroperiods sampled at the Espolla temporary pond Hydroperiod Start End Duration Number of Maximum of inundation sampling flooded area (days) (days) (m2) H1 11-01-1996 19-04-1996 100 17 28 131 H2 02-05-1996 16-06-1996 46 7 23 825 Ha 21-11-1996 30-11-1996 10 2 719 H3 09-12-1996 14-03-1997 96 15 26 175 Hb 06-06-1997 08-06-1997 3 1 125 Hc 04-07-1997 21-07-1997 18 5 2969 Figure 3. Proportion of females with eggs and eggs per female of Triops cancriformis throughout the hydroperiods. Table 3. Length-dry weight regressions for the Triops cancri- fit. These lengths were total length (furca excluded), formis population in Espolla temporary pond. Central carapace length was used to calculate biomass. Dry weight (DW) in mg and maximum carapace length and central carapace length length (L)inmm (from the anterior edge to the median carinal spine on the posterior sulcus). The three equations (Table 3) b DW+aL gave the same fit, so central carapace length (CCL) Lengths measured nr2 a±SE ±SE was chosen for its ease of measurement. Two dif- Central carapace length 170 0.98 0.0111±0.0010 2.8960±0.0346 ferent equations were used, one for T. cancriformis Maximum carapace length 170 0.98 0.0054±0.0005 3.0242±0.0341 Total length (furca excluded) 170 0.98 0.0034±0.0003 2.9678±0.0349 males and another for females, because of sexual di- morphism (Table 4). A side caliper (± 0.1 mm) was used to measure each specimen, except for the smaller ones (CCL < 10 mm) which were measured with a stereomicroscope (×10) with an ocular micrometer. The total number of individuals on a given day Females were identified primarily by the presence was estimated as the sum of abundances in each area. of oostegopod (egg sac) on the XIth thoracic apendage Area abundance was calculated as density (individu- −2 (Alonso, 1996), but also by the number of apodous als m ) multiplied by flooded area on the day. Three body rings (Margalef, 1951) and the shape of the morphological lengths were measured in order to de- carapace (Cottarelli & Mura, 1983). Immatures were termine which weight-to-length equation gave the best 178 Figure 4b. Figure 4a. Size structure of Triops cancriformis during all sampling days. Legend: Black columns are females, white columns are males, and grey columns are immature. the population by means of two variables: percentage of females with eggs and number of eggs per female. considered to be individuals with CCL less than 9 mm, because in small individuals the presence of oost- Results egopod is more difficult to observe and mistakes are easier. Individuals with less than 4 mm CCL were Fecundity, as indicated by the proportion of egg- considered recruits. The total number of eggs (from bearing females and the number of eggs per female, both oostegopods) for each female was counted with showed a bell-shaped pattern (Fig. 3) in the longer the aim to identify the temporal pattern of fecundity of hydroperiods H1 and H3, while H2 showed a re- 179 droperiod, but not H1, which unlike the others began when the pond was not completely dry.