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We surveyed microsite conditions in the oviposition location used by female saw-combed fishflies (Nigronia serricornis) to investigate the relationship between egg hatching success and egg mass features and oviposition site selection. Egg-laying females showed a significant tree species preference at two of three sites sampled. Leaves used as ovipositon sites differed in length, width, area, and height among sites, but differences in oviposition site characteristics were attributable to stream habitat conditions, not preference. Egg hatchmg rates were very high (ca. 95%) and did not differ between streams. Egg mass size was sipficantly different among sites, but average egg size was not. Egg size was not correlated with egg mass size, but hatching rate was significantly correlated with egg mass size. Recruitment estimates ranged from <25 to >95 hatchlings per m2. Although leaf height, leaf length, and stream site were significant in predicting egg mass size, females showed great plasticity in selection of oviposition sites and no correlations were found between hatching success and microsite conditions.
We surveyed microsite conditions in the oviposition location used by female saw-combed fishflies (Nigronia serricornis) to investigate the relationship between egg hatching success and egg mass features and oviposition site selection. Egg-laying females showed a significant tree species preference at two of three sites sampled. Leaves used as ovipositon sites differed in length, width, area, and height among sites, but differences in oviposition site characteristics were attributable to stream habitat conditions, not preference. Egg hatchmg rates were very high (ca. 95%) and did not differ between streams. Egg mass size was sipficantly different among sites, but average egg size was not. Egg size was not correlated with egg mass size, but hatching rate was significantly correlated with egg mass size. Recruitment estimates ranged from <25 to >95 hatchlings per m2. Although leaf height, leaf length, and stream site were significant in predicting egg mass size, females showed great plasticity in selection of oviposition sites and no correlations were found between hatching success and microsite conditions.
Symbiotic species often have preferential attachment sites on their host body and thus some sites may only be occupied if the supposed best sites are already occupied. Alternatively, colonization may be similar in all sites, but removal may differ among them. Both mechanisms might generate a nested pattern. Moreover, the host size can alter the quality of the site for symbiotic occupation; for instance, by increasing the area and anchorage structures or reducing removal. We predicted that the spatial distribution of the ectosymbiotic chironomid Ichthyocladius lilianae Mendes, Andersen and Sæther, 2004 on their host, the armored catfish Pareiorhaphis hypselurus (Pereira and Reis, 2002), would present a nested pattern with regards to body-part preference and variation according to host size. We found that (i) suboptimal sites on the host body were usually only occupied by a symbiont when the optimal sites were already occupied and (ii) sites occupied by larvae on small host body surface were a subset of the sites occupied on large hosts. Our results indicate a nested distribution, yet the mechanisms generating this pattern are unclear. One possibility is that symbionts have preferential sites for attachment on the host body surface and compete for these sites. As an alternative, symbionts may present no preference and colonize equally all sites, but they remain attached to sites that offer better resources or protection from removal.
Host-symbiont theory suggests that the abundance of an obligate symbiont will correlate positively with the density and size of its host. We examined these expectations using an obligate chironomid ectosymbiont -host fishfly (Nanocladius (Plecopteracoluthus) sp. No. 5 -Nigronia serricornis) system in 13 streams of southern Maine over 2 years (1996)(1997)(1998). In addition, we investigated field conditions and macroinvertebrate-community composition, to search for correlates with symbiont prevalence and mean infestation intensity. Prevalence and infestation intensity were high in both years of the study, but neither infestation intensity nor prevalence was correlated with host population density across sites. Within sites, prevalence increased significantly when host density declined, contrary to our expectation. Symbiont-infestation intensity was not related to host size. No symbiont-population attributes correlated with physicochemical conditions in either sample year but prevalence was positively correlated with infestation intensity. Ectosymbiotic midges exhibited a significant host and attachment-site bias. Collectively, these data suggest that this symbiont-host interaction is obligate and stable, but the dynamics in our streams do not support the general expectation of increasing symbiont-population size with either increasing host-population or body size. Intraspecific interactions between symbionts are thought to have a stronger influence on population dynamics than do host populations or abiotic conditions for this ectosymbiotic midge.Résumé : La théorie sur les relations hôte-symbionte semble indiquer que l'abondance d'un symbionte obligé est en corrélation positive avec la densité et la taille de ses hôtes. Nous avons examiné ces prévisions dans un système ectosymbionte-hôte, dans 13 ruisseaux du sud du Maine, où le symbionte était un chironomidé (Nanocladius (Plecopteracoluthus) sp. No. 5) et l'hôte, un corydalidé (Nigronia serricornis). De plus, nous avons examiné les conditions du milieu et la composition de la communauté de macroinvertébrés afin d'établir des corrélations avec la prévalence des symbiontes et l'intensité moyenne des infestations. La prévalence et l'intensité des infestations ont été élevées les 2 années de l'étude, mais ni l'une ni l'autre de ces deux variables n'était reliée à la densité de la population hôte à aucun des sites. Contrairement à nos prévisions, la prévalence a augmenté significativement aux divers sites lorsque la densité des hôtes a diminué. L'intensité des infestations de symbiontes n'était pas reliée à la taille des hôtes. Aucune des caracté-ristiques démographiques du symbionte n'était en corrélation positive avec les conditions physicochimiques ni l'une ni l'autre des 2 années d'échantillonnage, mais elles étaient en corrélation positive avec l'intensité des infestations. Les chironomidés ectosymbiontes ont une préférence marquée pour des hôtes particuliers et pour certains points d'attache sur leurs hôtes. Dans leur ensemble, ces données semblent indiquer que ...
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