CYCLOMORPHOSIS AS A FACTOR EXPLAINING SUCCESS OF A DAPHNIA HYBRID IN TJEUKEMEER

Recent studies have shown that Daphnia hybrids are intermediate to the ir parental species for several life history traits, and that they com bine advantageous traits of the parental species, thereby decreasing t heir risk of being preyed upon by planktivorous fish. In this study D. galeata, D. cucullata and their interspecific hybrid are compared wit h respect to cyclomorphosis and juvenile growth as a potential mechani sm to avoid invertebrate predation. Helmets and tail spines are known to reduce predation risk, whereas fast juvenile growth reduces the per iod that Daphnia are vulnerable to predation. In laboratory cultures, I tested the hypothesis that, with regard to cyclomorphosis, juvenile hybrids are more comparable to the juveniles of D. cucullata, and with respect to growth rate, they are more comparable to D. galeata. The h ybrid did not differ significantly from D, galeata in the time needed to reach 0.9 mm body length. In contrast, the relative spine and helme t length of juvenile hybrids were more comparable with those of D. cuc ullata, whereas in adult instars these characters were more similar to D. galeata. Thus during the first juvenile instars, the hybrids combi ne the relative long helmet and spine of D. cucullata, with the faster growth of D. galeata. This combination of characteristics might reduc e hybrid vulnerability to invertebrate predation in field situations.