Pierre Chevaldonné

Time-series of temperature from three deep-sea hydrothermal vent sites

Abstract Temperature time-series have been recorded with THYDRO, a new multi-probe temperature recorder, in three different hydrothermal vent sites: 13°N on the East Pacific Rise (EPR), the Lau Back-Arc Basin, and the North-Fiji Basin. The probes were placed on clumps of living animals (molluscs and polychaetes) for periods of up to 47 h, in zones where hydrothermal fluids mix with seawater. Spatial variability of temperature was often very important, thus demonstrating the necessity of a thorough study of the hydrothermal microenvironment. Temporal variability was analysed through standard spectral analysis methods. Several periodicities are superimposed, ranging from tens of seconds to ten of hours. The origin of these periods is discussed, and although the tidal cycle seems to be of greater importance in the driving forces of these variations, other periods exist that may be generated by the turbulent mixing occurring in this environment, and/or by variations in the hydrothermal fluid discharge. Aperiodic phenomena were also noted and, therefore, random processes should not be neglected. Consequences of the encountered variabilities on the vent fauna are also discussed, especially for Alvinella species that live in the studied zone and must have developed adaptive responses.

HYDROTHERMAL-VENT ALVINELLID POLYCHAETE DISPERSAL IN THE EASTERN PACIFIC. 2. A METAPOPULATION MODEL BASED ON HABITAT SHIFTS

Marine organisms typically fall into two main categories: those with a high level of population structuring and those with a low one. The first are often found to be poor dispersers, following isolation by distance or stepping‐stone theoretical predictions. The second are commonly associated with high‐dispersal taxa and are best described by the island model. Deep‐sea hydrothermal vent systems represent a good model for studying one‐dimensional metapopulations. Whereas isolation by distance might be expected to be the rule in such a system for species with limited dispersal capabilities, a biological paradox can be observed: an apparent genetic homogeneity in some vent species with short‐scale dispersal potential, in a one‐dimensional fragmented habitat. This can be explained if one key assumption of the existing models is not met: gene flow between populations and genetic drift may not have the time to equilibrate. Geophysical models revealed that hydrothermal convection is intrinsically unstable, inducing processes of coalescence or splitting of venting areas in a chaotic manner. This is likely to generate frequent extinctions and recolonizations. Theoretical genetic predictions derived from extinctions/recolonizations cannot satisfactorily model a situation where habitat shifts are frequent and constantly affect the metapopulation equilibrium. Because neither the island and the stepping‐stone models nor the classical metapopulation models resemble the hydrothermal vent reality, we present here a realistic model developed to provide a compromise between existing conceptual models and what is currently known of the biology and ecology of one of the most peculiar and best‐studied vent species, the polychaete Alvinella pompejana. This model allows us to define the boundaries between which the metapopulation is evolutionary stable in an unstable context. Simulations show different patterns in which metapopulation size and recolonization vary but reach an equilibrium despite chaotic vent extinctions. In contrast, the model also shows that displacing habitat continuously affects the equilibrium between gene flow and drift. As a consequence, the time required to balance these evolutionary forces can never be attained, leading to chaotic fluctuations in F‐statistics. Those fluctuations are mainly due to stochastic changes of the interpatch distance which affect migration rates. The shifting of active zones of venting can episodically counterbalance differentiation and allow a long‐term genetic homogenization at the ridge scale. These findings lead to a new concept in which the exchanges between populations would mainly depend on the habitats movements along the ridge axis rather than the organims dispersal. We therefore propose a new model based on patch‐network displacements in which transient contact zones allow low levels of gene flow throughout the metapopulation.

Reproductive biology and population structure of the deep-sea hydrothermal vent worm Paralvinella grasslei (Polychaeta: Alvinellidae) at 13°N on the East Pacific Rise

Paralvinella grasslei is a polychaetous annelid living in the harsh, unstable and heterogeneous environmental conditions found at deep-sea hydrothermal vent sites in the eastern Pacific. The aim of this work was to examine the possible influence of the reproductive biology of P. grasslei on the structure of its populations. Maximum observed oocyte size inside the oviduct is 275 μm, and fecundity is relatively low. Examination of gametes and young specimens suggested a direct benthic development for this species. The population structure of P. grasslei at 13°N/EPR (EPR=East Pacific Rise) revealed a discontinuous recruitment which seems to be synchronized within vent sites and fields. The data also suggested the occurrence of discrete breeding periods. P. grasslei probably reproduces several times a years, with an apparent periodicity. Tidal signals could be a possible cue for the coordination of the reproductive cycle. The life-history of P. grasslei is discussed in light of the reproductive biology of other terebellomorph polychaetes, and seems to be well adapted for colonizing the unstable environment of hot vents. Two main hypotheses can explain the dissemination processes of this species along axial oceanic ridges. The influence of nearbottom currents occurring along the central “graben” of the East Pacific Rise can be cosidered to account for part of the transport of larvae and juveniles, but the observations of polychaete erpochaetes on the test of hydrothermal bythograeid crabs and evidence that crab migrations occur between vents also support the possibility of zoochory for the dissemination of alvinellid polychaetes.