Nozaki H

Morphology and evolution of sexual reproduction in the Volvocaceae (Chlorophyta)
J PLANT RES 109 (1095): 353-361 SEP 1996

Abstract:
Morphological features of sexual reproduction in the Volvocaceae are reviewed, focusing particularly on gametic union and zygote germination. Both of the two conjugating gametes of the isogamous genera Pandorina, Volvulina and Yamagishiella bear a tubular mating structure (mating papilla), and plasmogamy is initiated by union of the papillae tips. On zygote germination, a single biflagellate gone cell is released from the zygote wall. Although all the anisogamous and oogamous genera of the Volvocaceae produce ''sperm packets'' during gametogenesis and a single gone cell at zygote germination, some difference can be recognized in the male gametes. The male gametes of Eudorina bear a tubular cytoplasmic protuberance (putative mating papilla) near the base of the flagella, whereas such a structure recognized at the light microscopic level is not evident in Pleodorina and Volvox, Evolution of the sexual reproduction characteristics of volvocacean algae is discussed on the basis of recent cladistic analysis of morphological data as well as of the ribosomal (r) RNA phylogeny and large subunit of the ribulose-1,5-biphosphate carboxylase/oxygenase(rbcL) gene trees.

Holomuzki JR, Hemphill N

Snail-tadpole interactions in streamside pools
AM MIDL NAT 136 (2): 315-327 OCT 1996

Abstract:
We studied competitive interactions and grazing effects of the snail Physella integra and the American toad Bufo americanus in ephemeral streamside pools in W-central Kentucky. Snails and tadpoles reared alone and combined at ambient densities in artificial pools revealed that costs of coexisting differed between taxa. Snails invested resources into growth and survival at the expense of reproduction when with anuran competitors, whereas tadpoles exhibited slowed development and decreased biomass per pool in combined-species treatments. These negative effects were likely a result of food limitation, considering that herbivory by both taxa significantly reduced algal abundance relative to ungrazed controls. Herbivory by both species also affected the assemblage of benthic algae. Diatoms, particularly Nitzschia spp. were the predominant algae in both single-species treatments, whereas the green algae Eudorina and Oedogonium comprised most of the community biovolume in combined-species treatments. The filamentous green algae Cladophora was dominant in ungrazed controls.

A survey of 16 isolated pools along three third-ol der streams revealed patterns of snail egg production and benthic algal assemblage similar to treatment effects in the artificial pool experiment. The survey also indicated that snails and tadpoles seldom co-occur in these pools; thus competition between them may be infrequent. Linkages between phosphate availability predation and habitat persistence seem particularly important in mediating competitive interactions between these taxa.