In many species individuals can employ alternative reproductive phenotypes, with profound consequences for individual fitness and population dynamics. This is particularly relevant for self-compatible hermaphrodites, which have exceptionally many reproductive options. Here we investigated the occurrence of reproductive phenotypes in the simultaneously hermaphroditic freshwater snail Radix balthica under experimentally simulated conditions of low vs. moderate population density. We captured all mating behavior on camera and measured individual female lifetime reproductive success. We found every possible reproductive phenotype: (1) both male and female (i.e. truly hermaphroditic) reproduction, (2) purely female and (3) purely male reproduction, (4) male reproduction combined with self-fertilization and (5) female mating activity, (6) pure self‑fertilization without mating and (7-8) two types of reproductive failure. Variation in alternative reproductive phenotypes was explained by mate availability (10.8%) and individual condition, approximated by a snail’s mean daily growth rate (17.5%). Increased mate availability resulted in a lower diversity of reproductive phenotypes, in particular increasing the frequency of true hermaphrodites. However, it lowered phenotype-specific fecundities and hence reduced the population growth rate. Snails in better condition were more likely to reproduce as true hermaphrodites or pure females, while low-condition snails tended to suffer reproductive failure. Overall, we show substantial variation in alternative reproductive phenotypes in a hermaphrodite, which is possibly in part maintained by fluctuations in population density and thus mate availability, and by variation in individual condition. We also provide evidence of an almost two-fold increase in clutch size that can be ascribed specifically to mating as a female.

Please find this information in the manuscript and supporting information.

General note: Please also read the ReadMe files that accompany each dataset. They explain the column headers in each dataset.

Focal individuals

This file provides information about each of 268 first-generation, laboratory-reared offspring of field-caught mothers, e.g., the experimental treatment snails experienced, the temporal block they were assigned to, the identity of their mother, their mean daily growth rate, lifetime reproductive phenotype, lifetime female reproductive output, and lifetime male and female mating success. Note that six of the 274 snails that were subjected to laboratory mating trials were excluded from this file. Five snails were excluded because they were solely paired with a related snail, and one snail was excluded due to missing data on female reproductive output. The data in this file were used to produce Figs. 2 and 4 in the main manuscript, and Figs. S2, S4, S5 and S6 in the supporting information, as well as to write the Supporting Results 2 (Randomness of reproductive patterns) and 4 (Relationship between female infertility and mating activity).

Mating behavior

This file provides information about the mating behavior of each of 215 snails, separately for each of the mating opportunities they were given. For each snail and mating opportunity, the file states the identity of the snail's mating partner, and whether the snail did not mate, mated just as a male, just as a female, or in both roles (i.e. sequentially reciprocally). Note that 53 of the 268 snails included in the file "Focal individuals" were excluded from this file, because their mating behavior could not be assessed with very high accuracy. The data in this file were used to produce Fig. 3 in the main manuscript, and Fig. S3 in the supporting information.

Clutch sizes

This file provides information about 2117 egg clutches laid by 147 snails. For each clutch, the file includes, e.g., the identity of the clutch-laying snail, the date on which the clutch was collected from the snail, the number of eggs and developed embryos, and two classifications of clutches, one regarding the experimental treatment of the mother and one regarding the maternal mating behavior. Note that 10 of the 147 snails whose clutches are included in this file did not produce any eggs; these snails laid clutches that only consisted of jelly. One egg-laying snail (snail identity: 45_1.8) was excluded from this file because of an unknown growth rate. The data in this file were used to produce Fig. 5 in the main manuscript, and Fig. S7 in the supporting information.

Mating partner shell length

This file provides information about 441 pairs of snails that were allowed to mate with one another in the course of the laboratory mating trial. For each pair, the file states, e.g., the identity of both mating partners, their shell lengths, and their mating activity. Note that 22 pairs of snails were excluded from this file, because the mating partners had the same mother and were thus related. The data in this file were used to write the Supporting Results 3 (Effects of size-matching first mating partners).

Snail marking trial

This file provides the results of a smaller laboratory experiment done to assess effects of marking snails individually on juvenile growth and survival after a two-week period. The snails used for this experiment were separate from those used in the mating trial. The file includes information about 30 marked and 20 unmarked control snails. These data were used to write the Supporting Results 1 (Effects of marking on survival and growth), where more information about the experiment can be found. Note that 19 of the 50 snails included in this trial originated from the study population (Uerikon, Lake Zurich, Switzerland), while 31 snails were from another population (Kilchberg) also located in Lake Zurich.