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2 -275- THE ECOLOGY OF EITHYNIA LEACHI (SHEPPARr., 1823) (GASTROPODA: PROSOBRANCHIA) IN THE LIT'!'ORAL ZONE OF ~EE LAKES MAARSSEVEEN W.J.R. de Wijs and E. van de~ 3roek Observations in previous years (see Keulen, internal report 1979) indicated that the prosobranch snail Bithynia Leachi is c~~it.û~ in Lake 11 and occurs fairly regularly in Lake I. Very little is known of ~he biology of this species in Western Europe. In the period from April 12th to November 2Jth 1979, samples were taken from both lakes, generally every fortnight, at permanent collecting stations: two in Lake I, four in Lake 11. If possible, at each s~ation the following substrates were examined: stones, polythene bags, wood, and large emergent macrophytes, both ali ve and decaying. The most sui table macrophytes were: Phl'agrm: tes, Typha, CaY'ex, Spal'ganium; these will in the following be inè.icated as "reed". Sampling was done at a dep th between 0 and 50 cm, and four species of prosobranchs were collected. The results of the study are summarized very briefly. 1. Relative abundance of B. leachi. See tables land Population dynamics: growth. Growth is indicated by the change in relative abundance of size classes (length of shell in mm) throughout the observatiop period. In Fig. 1 this pattern is represented for Lake 11 where the population of B. leachi is apparently undisturbed. This is concluded from the general trend in the pattern which can be described briefly a~ follows. From April through June: general increase in size. End of June, first weeks of July: size class distribution becomes bimodal; diminished frequency of large-sized animals; appearance of a new generation. From second half of July through September: the old generation disappears and is no longer present af ter the middle of August. The new generation increases quickly in size. From October through November: growth has stopped, general population structure as in April. The most striking deviations from this pattern found in Lake I are: a. Reduction of size in the older generation (see below). b. Prolonged appearance of the older generation in the samples.

3 -n6- c. Appearance of new generation about four weeks retarded. 3. Population dynamics: numbers. The numbers given are the total numbers found in Lake 11. These are calculated as the numbers of snails found per minute of searching time (i,e. corrected collecting time) and therefore can be considered proportional with density. Presentation is given in Fig Preference for type of substrate. Since the various types of substrate will have a different alg al flora, a preference for one of the categories of substrate by the prosobranchs will be influenced by a preference for a certain type of food. Table 111 shows the mean numbers of B. leachi per minute searching time, averaged during a) the whole investigation period, b) the oviposition period (samples of 22-V, 6-VI, 19-VI and 3-VII). "Reed" seems to be the less preferred substrate. Stone as a substrate was of ten much less available than the other substrate types which is the probable cause of the great standard deviation. Nevertheless the densities per substrate type varied almost parallel throughout the research period with one exception: plastics were clearly preferred during the oviposition period (last week of May - last week of June), see Table Preference for food. Examination of faeces of animals that were fixed immediately af ter collection, showed that in spring (May 7th) algal substances, especially diatoms, dominated and that in autumn (October 23rd) vegetable detritus was abundant besides pennate diatoms and remains of fungi. During choice experiments in petri dishes it appeared that the diatoms Asterionella formosa and Fragilaria crotonensis were preferred as food to Chlorella vulgare and, to some extent, to detritus from the littoral of Lake I. 6. Oviposition. The rate of oviposition was determined by counting the number of eggs per egg mass as well as the number of eggs per female. Table IV shows that the values for both parameters decreased strongly af ter the end of May, probably caused by increasing senility of the parent. Apart from the season, the temperature was important: the higher the temperature, the higher the number of eggs laid during a certain period. No significant differences in oviposition were found between

4 animals taken from Lake land Lake 11, or between animals kept in water from Lake land Lake Influence of temperature on egg development rate. It is illustrated by Fig. 3a that eggs laid at room temperature and kept in tanks at different temperatures, showed a maximal survival at 22 0 C. For the surviving eggs, the perioa between laying and hatching was clearly dependent on temperature (Fig. 3b). It would seem that the high mortality of theeggs placed at temperatures other than room temperature, is caused by a temperature shock, and not by the damaging influence of an unsuitable temperature in i~self. y ng 8. Interactions between B. leachi and the other prosobranchs (see ~ble I). Among the gastropods of the littoral zone in Lake 11, B. leachi is the most numerous species (according to Keulen (1979 l.c.) about 50% of the collected gastropods are B. leachi). In Lake I, however, its numbers are very small in comparison with those of other snails. In this lake, Potamopyrgus jenkinsi is very abundant, especially from July through September. Since B. leachi and P. jenkinsi occur in the same habitat and are about equal in size, the hypothesis is put forward that the two species compete, and that in Lake I B. leachi is partly suppressed by P. jenkinsi. Since the population pattern of B. leachi in Lake 11 can be considered as normal and undisturbed, deviations from this pattern as found in Lake I are probably caused by the presence of P. jenkinsi. These are: a. In Lake I the density (number per minute searching time) of B. leachi falls steeply from June 19th onward, i.e. when P. jenkinsi becomes abundant. b. In Lake I the parent generation disappears much later from the samples and oviposition occurs much (= ab out four weeks) later as weil. c. Specimens from Lake I are significantly smaller (see Table V), as was also observed in 1978 by Keulen (1979, l.c.). This apparent success of P. j enk-l-r,s,: when compared Vii th B. leachi in Lake I can partly be explained by certain properties of the former species that place it in a more favourable position. On several of these properties, cbservations were carried out. The average speed of movement of P. jenkinsi (more than 3 m per hour) was about twice that of B. Ieachi (and of B. tentacu Iata). This enables P. j enk1:nsi to find favourable feeding sites before other prosobranchs do. P. jenkinsi was nearly always observed on the upper side of the substrate, which is more exposed to light and will probably have a more dense algal growth

5 -278- than the underside, which is occupied by Bithynia spp. Predation by cyprinid fish was studied in the laboratory. Medium-sized fishes (about 12 cm in length) rejected far more P. jenkinsi than B. leachi of a certain size. It seems that the shells of P. jenkinsi are much harder and more difficult to crush by means of the pharyngeal teeth of the predators than those of Bithynia spp. This means th at P. jenkinsi is better protected against the smaller fish that occur in the littoral zone than B. leachi and can expose itself at a lesser risk. Moreover, the species is ovoviviparous and its eggs are therefore well-protected also. The problem why P. jenkinsi does not occur in large numbers in Lake 11 falls beyond the scope of the present investigation. Summarizing it can be stated that B. leachi shows a normal, undis~urbed population pattern in Lake 11; that the population of B. leachi is suppressed in Lake I by P. jenkinsi; and that this suppression is possible because P. jenkinsi has a number of properties th at favour this species when in competition with Bithynia leachi.

6 Table I. Numbers of counted specimens of the four prosobranch species collected in the Lakes Maarsseveen in the period April 12th - November 20th Lake I Lake II Bithynia Zeachi (Sheppard, 1823) B. tentacuzata (L., 1758) Potamopyrgus jenkinsi (Smith, 1889) thousands 1 ) 34 Marstoniopsis schoztzi (Schmidt, 1856) ) During the early summer P. jenkinsl: increased tremendously in Lake I which made counting impossible. Table 11. Proportion of B. Zeachi of all collected Bithynia specimens (Keulen) 1979 (De Wijs) Lake I 35.9% 59.9'% Lake II, 93.0% 97.2% - Table 111. Choice of substrate of B. Zeachi in Lake 11: mean, (sd). Further explanation: see text. period n polythene bags stone wood "reed" a (3.25) 4.88 (4.72) 4.01 (2.25) 2.65 (1.66 ) b (3.67) ) 5.04 (1. 96) ,32 )

7 Table IV. Oviposition of B. Zeachi in 7 days periods in glass jars. --- number of eggs period n range mean sd t -- per mass April-May June-July p«o per female April-May June-July P <0.001 Table V. Shell height in mm of specimens of B. Zeachi. Both lakes compared. \.; n range mean sd t Lake I Lake II P < 0.001

8 7 6 5 ~ N=,., P ~ ~ z 4 3 w N 2 (/) 0 l- APRIL MAY JUNE JULY AUG SEPT OCT NOV I tv 00 fj I FIG.1 SIZE DISTRIBUTION OF 81THYNIA LEACHI IN PERCENTAGES OF TOTAL; LAKE Il

9 -282- FIG.2. NUMBERS OF BLEACHI PER MINUTE OF SEARCHING TIME IN LAKE Il, SUMMARIZED FROM ALL CATEGORIES OF SUBSTRATE APR MAY JU NE JULY AUG SEPT OCT NOV " >- f- :::; f! a: 0 ::. 50 (IJ?i L 12 N_ c c FIG 3A RELATION BETWEEN TEMPERATURE ANO MORTALITY OF EGGS OF B LEACHf FIG 3B RELATION BETWEEN TEMPERATURE ANO TIME OF HATCHING OF EGGS OF B.LEACHf SHOWN ARE MEAN. RANGE ANO STANOARO ÖEVIATION LlNE FITTEO BY EYE EGGS LAIO AT ROOM. TEMPERATURE ON SLiOES ANO KEPT AT CONSTANT TEMPERATURES