Journal of Conference Abstracts

Mussel Population Structure and Recruitment on Two Hydrothermal Areas on the Mid-Atlantic Ridge: Lucky Strike (37°17'N, 1700 m) and Menez Gwen (37°50'N, 850 m)

Thierry Comtet Laboratoire d'Ecologie Abyssale, IFREMER, Centre de Brest, B.P. 70, 29280 Plouzané Cédex, France

tcomtet@ifremer.fr

Daniel Desbruyères Laboratoire d'Ecologie Abyssale, IFREMER, Centre de Brest, France

Alexis Khripounoff Laboratoire d'Ecologie Abyssale, IFREMER, Centre de Brest, France

Annick Vangriesheim Laboratoire d'Ecologie Abyssale, IFREMER, Centre de Brest, France

Introduction

The population dynamics of hydrothermal vent associated species is poorly known because of the lack of long term series of data and the difficulty of gathering the data needed in such studies, i.e. population structure, growth rate, reproduction, recruitment and larval dispersal... However, a few studies of population structure by means of size-frequency distribution analyses have been conducted on vestimentiferan worms (Siferd, 1984), and alvinellid polychaetes (McHugh, 1989; Zal et al., 1995). These give some indications on the recruitment of these species, in order to understand the processes controlling the evolution and maintenance of these populations.

This work presents results on mussel population structure from the Lucky Strike and Menez Gwen hydrothermal areas, with comparisons at both the intra and inter-site scales. The Lucky Strike fauna is dominated by these Bathymodiolus-like mussels' dense populations, with no equivalent on any other hydrothermal area. Original data on the occurence of bivalve larvae in a sediment trap moored at Lucky Strike are discussed in terms of recruitment periodicity.

Spatial Distribution of Mussels

On the Lucky Strike area, mussels cover all types of sulphide surfaces available (walls of hydrothermally active chimneys, flanges, and cracks of the seafloor), reaching densities up to 600 ind/m² (Van Dover et al., submitted), whereas the Menez Gwen area is characterised by the presence of small clumps of mussels composed of hundreds of individuals (Desbruyères et al., 1994). In situ observations at different sites of Lucky Strike suggest a spatial segregation of sizes.

Mussel Population Structure

During the DIVA 2 cruise using the French submersible Nautile, 12 samples of mussels were collected from 5 sites of the Lucky Strike area (Fig. 1) and one site of the Menez Gwen area. Sampling was made according to two criteria: (1) one single sample collected per dive, (2) mussels were collected with the sulphide block they occupied, to avoid size selection during sampling. Sample sizes range from 30 to 10000 individuals. A total of 8600 mussels were measured for length. Mussel lengths ranged from 0.5 to 113 mm.

Fig. 1: Lucky Strike area (37°17'N MAR, 1700 m). Map showing the sampling sites Eiffel Tower, Isabel, Sintra, PP5 and PP7. The Statue of Liberty site was not sampled during the DIVA 2 cruise. The arrow indicates the location of the moored sediment trap.

Length-frequency distributions show a clear polymodal structure (Fig. 2) indicating a discontinuous recruitment in space and time, and implying a triggering factor. These histograms are characterized by the presence, in every sample but one, of very small individuals (less than 1 mm in length) suggesting a recent recruitment event, as already hypothesized by Van Dover et al. (submitted) in 1993 (Lucky Strike 93 cruise).

Comparison of the size structures at the intra-site level (for the Eiffel Tower (Fig. 2), Isabel, and PP7 sites) illustrates the phenomenon of spatial segregation of cohorts mentioned above. Our hypothesis is that intraspecific competition for substrate and food resources, or competition by larviphagy prevent larvae from settlement on surfaces covered by adults. These results demonstrate a synchronization of recruitment at this level. Comparisons of the histograms at the inter-site level but intra-area (Lucky Strike) show that the mode values are very similar from one site to another, particularly all sites present new recruits, suggesting the existence of a synchronizing factor at this level.

Fig. 2: Population structure of the Lucky Strike mussels. Length-frequency distributions in two mussel samples collected on the Eiffel Tower site (see Fig. 1 for location); class interval = 3mm; (a) sample from dive #04, n=618; (b) sample from dive #08, n=321 (n=sample size).

The three samples collected in the Menez Gwen area provide less obvious population structure, probably due to the patchy distribution of mussels, or to an irregular recruitment. However, very small individuals were collected in two of the samples, raising the question of the existence of a synchronizing factor at a larger scale.

These results confirm and complement those given by Van Dover et al. (submitted) from four samples collected during the Lucky Strike 93 cruise.

Measurements of the length of the prodissoconch II (larval shell) on a subsample of 93 juvenile individuals give an estimation of the size at settlement of 527~m.

Occurrence of bivalve larval shells in a sediment trap: hypothesis of annual recruitment

The examination of the samples collected by means of a sediment trap moored for 15 months at about 300 m from the nearest active site of the Lucky Strike area (Fig. 1), 17 m above the bottom, reveals the presence of bivalve larval shells, whose abundance range from 0 to 247 individuals per sampling period of 14 days. Two peaks in larvae abundance were observed in July 1994 and May 1995, in correlation with two peaks in total particle flux (Fig. 3). The annual cycle of particle flux is related to the seasonal changes in sea surface production in this temperate area. The origin of the larvae remain unknown, but two hypotheses can be proposed: (1) shallow-water origin, (2) local origin, related to the high densities of mussels on the hydrothermal sites of Lucky Strike. According to the latter hypothesis, the long term record of larval abundances suggests an annual periodicity in larvae production, and indicates that the particle flux from the surface could constitute the synchronizing factor.

Two other observations tend to indicate a possible annual periodicity in the recruitment of the Lucky Strike mussels. (1) modal decomposition of the length-frequency distributions reveals the presence of different modes separated by an interval of 1 to a few cm, when growth rates of 1 cm/yr were estimated for Bathymodiolus thermophilus (Rhoads et al., 1981; Rhoads et al., 1982; Roux et al., 1989), suggesting that the different modes could represent annual cohorts; (2) the observation of newly recruited individuals (less than 1 mm to a few mm) at the same period (June) during two successive cruises (Lucky Strike in 1993 and Diva 2 in 1994) fits with an annual or pluriannual recruitment event.

Fig. 3: Long term record of bivalve larvae abundance (dotted line) and total particle flux (continuous line).

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