The life-history patterns of the hydrothermal vent polychaetes, Paralvinella pandorae Desbruyères and Lubier, and Paralvinella palmiformis Desbruyères and Lubier
Date
1987
Authors
McHugh, Damhnait
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Abstract
The population structure and reproductive biology of two polychaetes of the Family Alvinellidae, Paralvinella pandorae Desbruyeres and Laubier and Paralvinella palmiformis Desbruyeres and Laubier, have been studied from eight vents at three hydrothermal sites on the Juan de Fuca Ridge and Explorer Ridge. The Alvinellidae are found exclusively in hydrothermal vent habitats; P. pandorae and P. palmiformis are known only from hot vents of the northeast Pacific. This restricted distribution appears to be due to dispersal limitations between vent fields along the Juan de Fuca and Explorer Ridges, and those situated thousands of kilometers to the south along East Pacific Rise and Galapagos Rift. Abundance of both species at different vents varies, possibly as a result of competition for space and/or food, or differences in recruitment. Absence of either species from a vent may be due to inability to disperse, unsuitable settling substratum, or inappropriate physico-chemical conditions.
Size-frequency analysis of two P. pandorae populations produces unimodal histograms, suggesting continuous or semi- continuous juvenile recruitment; in a third population two possible size classes are evident. Histograms of P. palmiformis display definite size class peaks, which indicate periodic recruitment of juveniles. Local, habitat-induced variations in environmental conditions are thought to produce the observed size range variations in both species from different vents.
Histological studies of gametogenesis show that both species are gonochoric, and gamete development is similar to that described for other polychaetes in the Order Terebellida. All populations of P. pandorae examined exhibit a full range of gametogenic stages, including spermatozoa in males. This species appears to reproduce continuously or semi-continuously. P. palmiformis, on the other hand, appears to possess a discrete breeding cycle. The control of synchronization of gametogenesis and spawning in P. palmiformis is not known.
Maximum oocyte size in P. pandorae and P. palmiformis is approximately 330l,lm and 440l,lm, respectively. Larval development, extrapolated from this parameter and based on previous terebellid studies, is thought to be lecithotrophic and demersal in both species. Such development would provide for repopulation of a vent, or colonization of new vents. Extended larval life facilitated by low ambient temperatures would allow dispersal, possibly via bottom currents along the ridge systems, to other vents. The larger maximum oocyte size of P. palmiformis suggests a longer larval developmental period than that of P. pandorae. Despite this, neither P. palmiformis nor P. pandorae appears capable of dispersal over the long distances to southeastern Pacific vents.
P. pandorae and P. palmiformis do not exhibit extremer-selected or Kselected life-history strategies. Neither species shows a high reproductive potential, rather they tend to be more K-selected in their life-history patterns. Both produce relatively small numbers of large, lecithotrophic larvae. The life-histories of P. pandorae and P. palmiformis appear to be influenced more by the limitations of their evolutionary history than the selective pressures of the environment that they inhabit. These results are in agreement with the suggestions of several authors that reproductive modes of vent fauna are mainly a function of phylogenetic constraints.