n. pl.1.(Zool.) A large division of Mollusca, including all except the bivalves; - so called because the head is distinctly developed. See Illustration in Appendix.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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cephalata Verrill, 1910 but differs in the possession of alternating long (5-9) and short (1-4) dorsal cirri in mid body setigers.
Five of the species were found at one to three sites, but three other species (carinata, longicephala, and cephalata) were more broadly distributed.
Populations belonging to a specific group invariably showed close morphological similarity, and each corresponded to a different species, with the exception of two clusters, which both possessed morphological features attributed to Daphnia cephalata. Populations in these two groups were allopatric and are hereafter treated as inland/montane (A) and coastal (B) subspecies of D.
Daphnia longicephala was the sole species at six sites and was common at most others although three other species (carinata, cephalata, and projecta) were also detected.
Hetero- Heterozygote zygosity Species N (%) Total Excess Deficit carinata 74 .063 90 .19 .03 cephalata A 18 .031 15 .13 .00 cephalata B 15 .030 17 .29 .00 longicephala 98 .076 120 .15 .05 magniceps 1 .000 0 - - nivalis 10 .100 3 .00 .00 projecta 30 .102 23 .13 .13 thomsoni 57 .131 51 .19 .04 Genotypic Characteristics of Taxa
cephalata had a heterozygosity less than half those of D.
carinata and one of its three different [F.sub.1] hybrids (with cephalata, longicephala, or thomsoni), suggesting that they represented backcrosses.
Prior work (Hebert 1981) has shown some obligately parthenogenetic populations of Daphnia cephalata near Sydney, and this study confirmed the prevalence of populations showing fixed heterozygosity in this species.
Daphnia magniceps appeared to be more broadly distributed, although less so than the other three species (carinata, cephalata, and longicephala).
cephalata are known to have low fitness (Hebert 1985), suggesting that genetic divergence among taxa is now sufficient to ensure its protection from erosion by introgression.