# Spatial and temporal variations of recruitment in the tube worm *Riftia pachyptila* on the East Pacific Rise (9 degrees 50′ N and 13 degrees N)

E Thiebaut, X Huther, B Shillito, D Jollivet, F Gaill

► To cite this version:

E Thiebaut, X Huther, B Shillito, D Jollivet, F Gaill. Spatial and temporal variations of recruitment in the tube worm *Riftia pachyptila* on the East Pacific Rise (9 degrees 50′ N and 13 degrees N). Marine Ecology Progress Series, 2002, 234, pp.147-157. 10.3354/meps234147. hal-03483034

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# Spatial and temporal variations of recruitment in the tube worm *Riftia pachyptila* on the East Pacific Rise (9° 50′N and 13° N)

Eric Thiébaut<fnref n="1" />*, Xavier Huther², Bruce Shillito², Didier Jollivet<fnref n="3" />, Françoise Gaill²

**ABSTRACT:** The giant vestimentiferan tubeworm *Riftia pachyptila* is one of the most abundant and ecologically important species at deep-sea hydrothermal vents along the East Pacific Rise (EPR). Clumps of *R. pachyptila* could indeed be viewed as one of the most frequent habitats for the fauna typifying the eastern Pacific hydrothermal vent sites and thus deserve specific attention, as their dynamics are likely to control the spatial and temporal evolution of numerous vent species. As a consequence, population structure and recruitment of *R. pachyptila* were studied from samples taken at different vent localities during 2 EPR surveys in 1996 and 1997. In 1996, 3 samples were collected within and between the hydrothermal vent fields 13° N and 9° 50′ N to assess the spatial coherence of recruitment. In 1997, a huge number of additional specimens were collected at 9° 50′ N from a single vent site at 3 successive dates over 22 d to determine how growth rate may affect the temporal variability of population structure at a small spatial scale. Size-frequency analyses were performed using the vestimentum width as a biometric index. Inter-site comparisons revealed that size-frequency distributions did not differ significantly within a vent field suggesting the synchronisation of recruitment locally (i.e. a few km). By contrast, population structure strongly differed between the 2 vent fields in response to local variations in time of larval supply and/or reproductive activity. The size-frequency histograms of the 3 samples collected in 1997 displayed a regular polymodal structure reflecting a discontinuous recruitment in time. The presence of a large number of small individuals attached to the tube of conspecific larger ones suggested that biological cues associated with *Riftia* may promote habitat selection by settlers. Differences between the modal vestimentum width of each population 'cohorts' from the 3 temporal samples were significant and could result from either a growth rate heterogeneity in response to local environmental variations or to population ageing. According to this 2nd assumption, growth rate was estimated from the evolution of the 5 'cohorts' found over 22 d. Our data suggested that *R. pachyptila* exhibits: (1) a high yearly tube growth rate (estimated to about 160 cm yr⁻<fnref n="1" />); (2) a high individual mortality rate; and (3) a high population turnover, all of which are the characteristics of an opportunistic species.

**KEY WORDS:** Hydrothermal vents · Populations · Recruitment · Settlement · Growth rate · Vestimentiferans · *Riftia pachyptila*

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## INTRODUCTION

The deep-sea hydrothermal vent fauna consists of dense benthic communities supported by high chemosynthetic primary productivity through endosymbiosis of a few key species. Its habitat is characterised by 3

important environmental attributes which govern communities composition, distribution and dynamics (Tunnicliffe 1991): (1) the harsh physical and chemical conditions experienced by the vent fauna impose specific physiological adaptations to become sulphide specialists and thus lead to the high level of endemism already observed over the ridge systems (Jollivet 1996); (2) vent sites are patchily distributed over oceanic ridges with inter-site distances ranging from a few

<footnote n="1">¹Laboratoire d'Océanographie de Villefranche/mer, CNRS UMR 7093 and ²Laboratoire de Biologie Marine, CNRS UMR 7622, Université Paris VI, Bâtiment A, Case 6, 4 Place Jussieu, 75252 Paris cedex 05, France</footnote>

<footnote n="3">³Station Biologique de Roscoff, CNRS UMR 7127, Université Paris VI, Place Georges Teissier, BP 74, 29682 Roscoff cedex, France</footnote>

<footnote>*E-mail: eric.thiebaut@snv.jussieu.fr</footnote>
metres to 100s of metres within a given vent field, and inter-field distances ranging from 100s to 1000s of kilo-metres between disjunct segments of a ridge; and (3) the high temporal variability of hydrothermal activity due to tectonic events and heat convection through the oceanic crust induces a high site turnover that may vary in duration from years to decades on fast spreading ridges (Watremez & Kervevan 1990, Lalou 1991). Thus, vent fauna have to adapt to an ephemeral and unpredictable habitat, changes of which are responsible for a sequence of extinction and rapid colonisation.

Although the understanding of processes involved in the establishment and maintenance of benthic populations in such an environment has been recognised as a major goal of hydrothermal vent ecology (Tunnicliffe 1991), life histories of vent organisms remain poorly documented due to the lack of long-term data (over yr) that arose from difficult access to deep-sea hydrothermal habitats (Tyler & Young 1999). Larval dispersal has been inferred from (1) the larval development (e.g. Marsh et al. 2001); (2) the size of prodisoconch shells of mollusks adults (e.g. Lutz et al. 1984); (3) the larval physiology (e.g. Pond et al. 1997); (4) the spatial distribution of larvae in relation to hydrodynamic processes (e.g. Mullineaux et al. 1995, Kim & Mullineaux 1998); and (5) indirect measurements of gene flow between distant sites (e.g. Black et al. 1994, Jollivet et al. 1995). On the other hand, discrete benthic samples were used to determine reproductive traits of organisms and to characterise population structure. Population structures have been assessed through size-frequency analysis for polychaetes and bivalves, providing information on the recruitment patterns of hydrothermal species and mechanisms by which populations renew or are founded. Discontinuous recruitment has been reported for most species studied to date including the ampharetid polychaete *Amphisamytha galapagensis* (McHugh & Tunnicliffe 1994), the alvinellid polychaetes *Alvinella pompejana*, *A. caudata*, *Paralvinella grasslei* and *P. palmiformis* (McHugh 1989, Jollivet 1993, Zal et al. 1995), serpulid worms and the vestimentiferan *Tevnia jerichonana* (Jollivet 1993), mytilid bivalves and polynoid commensals (Comtet & Desbruyères 1998, Jollivet et al. 2000). Conversely, McHugh (1989) hypothesised a continuous recruitment of the polychaete *Paralvinella pandorae*. Discontinuous recruitment may be related to the occurrence of reproductive cycles in marine invertebrates. For *Paralvinella grasslei*, Zal et al. (1995) hypothesised that tidal signal, which is clearly periodic in the hydrothermal ecosystem, is the most likely external factor coordinating the reproductive cycle of individuals within vent sites and fields. Conversely, Comtet & Desbruyères (1998) suggested that recruitment of mytilid bivalves on the Azorean triple junction may be syn-

chronised between 2 vent fields separated by a distance of 60 km, and proposed that pelagic fluxes could trigger the emission of gametes and/or the settlement of larvae.

The giant vestimentiferan tube worm *Riftia pachyptila* Jones, 1981 (herafter called *Riftia* for convenience) has been reported from the Guaymas Basin (27°N) to the Galapagos Rift (0° 50′N) in the vigorous diffusive flow zone of hydrothermal vents. As it is one of the most widespread, abundant and ecologically important species at vents of the east Pacific (Jollivet 1993, Shank et al. 1998, Shillito et al. 1999), its life cycle is one of the most studied among vent species in recent years. Van Dover (1994) reported *in situ* observations of discontinuous bursts of gamete emission over *Riftia* thickets at 13°N/EPR. While Cary et al. (1989) hypothesised that fertilisation may take place within the female's vestimentum cavity, Gardiner & Jones (1985) observed individual spermatozoa close to mature oocytes in the female genital tract, raising the possibility of internal fertilisation. However, such fertilisation could occur just by chance, as huge amounts of sperm masses seem to fall down onto the tubes after emission in vestimentiferans (Southward & Coates 1989). Whatever the fertilisation mode, larval development of vestimentiferans includes a free-living, non-feeding trochophore larvae (Young et al. 1996, Marsh et al. 2001). From metabolic rates measurements, the average larval lifespan was estimated to be 38 d (Marsh et al. 2001). These data and the characteristics of current flows suggested that eggs and embryos could be entrained in the neutrally buoyant plumes of active venting sites and subsequently dispersed over distances rarely exceeding 100 km. However, based on gene flow analysis, Black et al. (1994) indicated a high rate of dispersal for *Riftia*. From a temporal survey of the 13°N/EPR communities between 1984 and 1990, Jollivet (1993) suggested that the vestimentiferan *Tevnia jerichonana* is one of the first colonisers of the new vent sites and is subsequently replaced by *R. pachyptila*. From a long-term study of faunistic succession at newly formed vent sites of the 9° 50′N/EPR, Shank et al. (1998) confirmed this hypothesis and highlighted that vestimentiferan tube worms have a very high rate of colonisation; pioneering colonies of *Tevnia jerichonana* were established within 11 mo and subsequently replaced by dense clumps of *Riftia* within 32 mo. While these authors put forward that the vestimentiferan succession was correlated to a temporal modification of geochemical conditions, Mullineaux et al. (2000) argued that the settlement of *Riftia* may be facilitated by the occurrence of *Tevnia jerichonana*. These previous studies on the *Riftia* life cycle raise different questions that are addressed in this study: (1) Is recruitment of *Riftia* discontinuous and if it is, is it periodic? (2) What
are the spatial scales of coherence of the recruitment?
(3) How can growth rate influence population dynam-
ics?

To answer these questions, the aim of the present
work was to examine the population structure of *Riftia*
from samples collected both from different vent locali-
ties and different periods of time on the EPR in 1996
and 1997. The processes involved in the recruitment
and population dynamics of the species are discussed.
Particular attention was given to the influence of
growth rate in shaping population structure at micro-
spatial and temporal scales.

MATERIALS AND METHODS

**Study area and sampling.** As assessing population
dynamics in a patchy environment requires an accu-
rate reference to spatial scales, the terminology
defined by Chevaldonné et al. (1997) has been used in
the present work.

Samples of vestimentiferan tube worms were col-
lected during the February-March 1996 HOT'96 and
November-December 1997 EPR'97 oceanographic sur-
veys conducted on hydrothermal vents at 9°50′N
and 13°N on the EPR. Three previously described
sites were sampled during the 1st survey: Genesis
(12°48.63′N, 103°56.41′W), Parigo (12°48.57′N,
103°56.41′W) and Qvent (9°50.78′N, 104°17.59′W),
while 1 site was sampled during the 2nd survey: Riftia
Field (9°50.75′N, 104°17.57′W). Vestimentiferans were
collected using the arms of the French submersible

'Nautile' (HOT'96) or the American submersible 'Alvin'
(EPR'97) and brought back to the surface in an in-
sulated basket. For the Hot'96 survey, samples from 1
vent site consisted of a pooled mixing of individuals
collected during different dives at the same site. For
the EPR'97 survey, 3 samples were collected at the
same vent site during 3 distinct dives at about 10 d
intervals. On board, vestimentiferans were preserved
with 10% formalin in seawater.

**Determination of a biometric index.** The choice of a biometric index representative of the animal size was determined according to 3 criteria. The variable has to be (1) easily measurable; (2) unaffected by formalin fixation; and (3) related to the animal's growth. The 2nd criterion was defined in order to be applied either to fixed material used in ecological studies or to fresh material used in physiological studies.

Nine easily measurable body dimensions (i.e. total
body length, plume length, vestimentum length, trunk
length, opisthosome length, plume width, vestimen-
tum width, trunk width, opisthosome width) were mea-
sured to the nearest 0.1 mm with a vernier caliper for
widths and to the nearest 0.5 mm with a ruler for
lengths from a subsample of 56 individuals collected in
1997 (Fig. 1). On each individual, measurements were
carried out, first, just after recovery on board and,
second, 6 mo after the fixation. Finally, individuals
from our subsample were dried at 60°C to a constant
weight and subsequently weighed to the nearest
0.1 mg to obtain worm dry weight (DW).

**Size-frequency histograms and sex ratio.** The vesti-
mentum width was measured for a total of 188 fresh or

<figure><img src="image_2.png" /><figcaption>Fig. 1. *Riftia pachyptila*. Biometric measurements carried out on *Riftia pachyptila* individuals. BL: total body length; PL: plume length; VL: vestimentum length; TL: trunk length; OL: opisthosome length; PW: plume width; VW: vestimentum width; TW: trunk width; OW: opisthosome width</figcaption></figure>
fixed individuals and 1608 fixed individuals in 1996 and 1997, respectively. To analyse the population structure, size-frequency histograms were plotted using a 2 mm size-class interval for 1996 data and 0.5 mm for 1997 data. This interval was chosen according to 3 criteria: (1) most size classes must have at least 5 individuals; (2) the number of adjacent empty classes must be minimised; and (3) the interval has to be larger than the error on measurements. Size-frequency histograms were smoothed using a weighted moving average at the 3rd order to rule out spurious peaks (Frontier & Pichod-Viale 1991, Zal et al. 1995, Comtet & Desbruyères 1998, Jollivet et al. 2000). Modal decomposition

of size-frequency histograms was performed using the NORMSEP program (Tomlinson 1971) adapted by Gros & Cochard (1978), assuming a Gaussian distribution of vestimentum width within each cohort. The program discriminates overlapping Gaussian components using the method of Bhattacharya (1967) and calculates the principal parameters of each component (i.e. average, standard deviation and frequency) according to Hasselblad's method (1966).

Individuals were sexed according to the presence of ciliated ridges associated with the paired genital apertures of males which are present at the posterior third of the dorsal surface of vestimentum (Jones 1981). Only individuals with a vestimentum width ≥5 mm were considered. The sex ratios of samples were compared to the expected 1:1 male:female sex ratio using a $\chi^2$ goodness of fit test (Sokal & Rohlf 1981).

# RESULTS

## The biometric index

Linear regressions calculated between fresh and fixed measurements allow us to detect formalin-shrinkage effects as soon as the slope and the intercept of regressions differ significantly from 1 and 0, respectively (Table 1). From these analyses, only 4 body measurements were left unchanged by the formalin fixation and could be used as a biometric index: plume length, plume width, vestimentum width and opisthosome width.

Calculations of allometric equations showed that the best fit was obtained for correlations between the plume length or the vestimentum width and the worm DW (Fig. 2). As body extremities may be partly damaged during sampling or by predation, the vestimentum width was retained as biometric index for the present study.

<table><caption>Table 1. *Riftia pachyptila*. Linear regressions between fresh and formalin preserved body measurements. The levels of significance of the hypotheses $H_0$: Intercept = 0 and $H_0$: Slope = 1 are indicated by asterisks: \*p < 0.05; \*\*p < 0.01; \*\*\*p < 0.001. Changes in the sample size between measurements result from differential alterations of the body in some individuals after the preservation</caption><thead><tr><th></th><th>Sample size</th><th>Intercept</th><th>Slope</th><th>r<sup>2</sup></th><th>p</th></tr></thead><tbody><tr><td>Total length</td><td>47</td><td>0.997*</td><td>0.863***</td><td>0.984</td><td>&lt;0.001</td></tr><tr><td>Plume length</td><td>50</td><td>0.060</td><td>0.965</td><td>0.978</td><td>&lt;0.001</td></tr><tr><td>Plume width</td><td>48</td><td>0.032</td><td>1.046</td><td>0.966</td><td>&lt;0.001</td></tr><tr><td>Vestimentum length</td><td>55</td><td>0.299*</td><td>0.928*</td><td>0.938</td><td>&lt;0.001</td></tr><tr><td>Vestimentum width</td><td>54</td><td>-0.069</td><td>0.949</td><td>0.925</td><td>&lt;0.001</td></tr><tr><td>Trunk length</td><td>56</td><td>1.534***</td><td>0.813***</td><td>0.979</td><td>&lt;0.001</td></tr><tr><td>Trunk width</td><td>54</td><td>0.149*</td><td>1.036</td><td>0.878</td><td>&lt;0.001</td></tr><tr><td>Opisthosome length</td><td>56</td><td>0.078</td><td>0.842**</td><td>0.837</td><td>&lt;0.001</td></tr><tr><td>Opisthosome width</td><td>54</td><td>0.032</td><td>1.020</td><td>0.926</td><td>&lt;0.001</td></tr></tbody></table>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. *Riftia pachyptila*. Logarithm-transformed relationship between (A) the vestimentum width (VW) or (B) the plume-length (PL), and the total dry weight (DW) of the tubeworm</figcaption></figure>

## Data from the HOT'96 survey

Vestimentum width ranged from 0.58 mm for smallest individuals to 31.80 mm for largest ones (Table 2). Due to the small size of samples, no modal decomposition of the size-frequency histograms was performed. However, size-frequency distributions were compared to a normal distribution using a Kolmogorov-Smirnov 1-sample test
<table><caption>Table 2. *Riftia pachyptila*. Sample size and vestimentum width range of *R. pachyptila* individuals collected in February-March 1996 and in November-December 1997 from different hydrothermal vent sites of 13° N/EPR and 9° 50' N/EPR</caption><thead><tr><th rowspan="2">Oceanographic survey</th><th rowspan="2">Date</th><th rowspan="2">Vent area</th><th rowspan="2">Vent site</th><th rowspan="2">Sample size</th><th colspan="3">Vestimentum diameter (mm)</th></tr><tr><th>Range</th><th>Mean</th><th>SD</th></tr></thead><tbody><tr><td>HOT'96</td><td>Feb-Mar 96</td><td>13° N</td><td>Genesis</td><td>52</td><td>0.58-30.00</td><td>12.79</td><td>10.81</td></tr><tr><td>HOT'96</td><td>Feb-Mar 96</td><td>13° N</td><td>Parigo</td><td>64</td><td>0.70-31.80</td><td>14.67</td><td>9.00</td></tr><tr><td>HOT'96</td><td>Feb-Mar 96</td><td>13° N</td><td>Genesis + Parigo</td><td>116</td><td>0.58-31.80</td><td>13.83</td><td>9.85</td></tr><tr><td>HOT'96</td><td>Feb-Mar 96</td><td>9° 50' N</td><td>Q vent</td><td>72</td><td>7.23-28.00</td><td>16.98</td><td>5.41</td></tr><tr><td>EPR'97</td><td>27 Nov 97</td><td>9° 50' N</td><td>Riftia Field</td><td>883</td><td>0.34-21.67</td><td>5.93</td><td>3.94</td></tr><tr><td>EPR'97</td><td>7 Dec 97</td><td>9° 50' N</td><td>Riftia Field</td><td>345</td><td>0.60-16.51</td><td>6.37</td><td>3.90</td></tr><tr><td>EPR'97</td><td>19 Dec 97</td><td>9° 50' N</td><td>Riftia Field</td><td>390</td><td>0.29-21.07</td><td>8.26</td><td>4.98</td></tr></tbody></table>

(Sokal & Rohlf 1981). When pooling samples from Genesis and Parigo, this test demonstrated that the size-frequency distribution of vestimentiferans from the 13° N vent field differed significantly from a normal distribution (p < 0.05) while that of the 9° 50' N vent field (Q vent site) did not (p > 0.05). The sample from the Q vent site had a smaller size range and was exclusively made of individuals having a vestimentum width larger than 7 mm (Table 2, Fig. 3). By contrast, samples from the Genesis and Parigo sites exhibited a greater size range. Samples were mainly subdivided into 2 size groups of small individuals (vestimentum width <5 mm) and large individuals (vestimentum width >15 mm) (Fig. 3). The size-

frequency distributions did not differ significantly between the Genesis and Parigo sites (Kolmogorov-Smirnov 2-samples test, p > 0.05).

## Data from the EPR'97 survey

For the 3 samples of *Riftia* collected from the *Riftia* Field vent site, the vestimentum width ranged from 0.29 to 21.67 mm (Table 2). All the size-frequency distributions significantly differed from the normal distribution (Kolmogorov-Smirnov 1-sample test, p < 0.05) and displayed a regular polymodal structure (Fig. 4).

<figure><img src="image_5.png" /><figcaption>Fig. 3. *Riftia pachyptila*. Size-frequency histograms of the vestimentum width from the 3 *R. pachyptila* samples collected in February-March 1996 on vent sites at 13° N/EPR (Genesis and Parigo) and 9° 50' N/EPR (Q vent). N = sample size</figcaption></figure>
<figure><img src="image_6.png" /></figure>

Fig. 4. *Riftia pachyptila*. Size-frequency histograms of the vestimentum width from the 3 *R. pachyptila* samples collected in November-December 1997 on the Riftia Field hydrothermal vent site (9° 50' N/EPR). N = sample size. ↓ = mean of each modal component determined using the NORMSEP software. 'Attached' and 'Unattached' refer to individuals fixed on a *Riftia* tube or to the basaltic substrate, respectively

Size-frequency histograms are all log-normal shaped
suggesting that a high mortality rate is likely to occur
amongst small individuals. Comparisons between the
3 samples also indicated significant differences in
their size-frequency distributions (Kolmogorov-Smirnov
2-samples test, p < 0.05). The modal decomposition of
the size-frequency distributions indicates the occur-
rence of 6 Gaussian components for the sample from
7 December and 7 components for the samples from
27 November and 19 December. The characteristics of
each component are given in Table 3. Although the
relative importance of each mode varies from one

sample to another, all samples were mainly composed
of individuals belonging to the 4 first modes. Largest
individuals were sparse and formed only a small per-
centage of each sample. One can note that the sample
from 19 December displayed the highest proportion
of newly settled individuals (vestimentum width
<1.5 mm).

Within each sample, all individuals were either
attached to another tube of *Riftia* (hereafter called
‘attached individuals’) or not (hereafter called ‘unat-
tached individuals’) (Fig. 5). Unattached individuals
could have been anchored to the basaltic substrate or
<table><caption>Table 3. *Riftia pachyptila*. Modal decomposition of size-frequency distributions of the vestimentum width in the 3 *R. pachyptila* samples collected in November-December 1997 from the Riftia Field hydrothermal vent site. The modal decomposition was performed using NORMSEP software. The different components are numbered according to the occurrence of a massive recruitment event between 7 and 19 December 1997. See ‘Discussion’ for explanation</caption><thead><tr><th rowspan="2">Date</th><th colspan="8">Component</th></tr><tr><th>0</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th></tr></thead><tbody><tr><td colspan="9"><strong>27 November</strong></td></tr><tr><td>χ<sup>2</sup> = 2.932; p = 0.999; df = 14</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proportion</td><td>–</td><td>13.61</td><td>22.41</td><td>30.52</td><td>21.13</td><td>8.35</td><td>1.67</td><td>2.31</td></tr><tr><td>Average (mm)</td><td>–</td><td>1.29</td><td>2.80</td><td>5.33</td><td>9.08</td><td>11.23</td><td>14.64</td><td>16.58</td></tr><tr><td>SD (mm)</td><td>–</td><td>0.57</td><td>0.94</td><td>1.50</td><td>1.35</td><td>1.65</td><td>0.80</td><td>0.71</td></tr><tr><td colspan="9"><strong>7 December</strong></td></tr><tr><td>χ<sup>2</sup> = 0.892; p = 1.000; df = 12</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proportion</td><td>–</td><td>24.07</td><td>22.04</td><td>24.23</td><td>19.94</td><td>8.28</td><td>1.44</td><td>–</td></tr><tr><td>Average (mm)</td><td>–</td><td>1.97</td><td>4.18</td><td>6.58</td><td>10.20</td><td>13.25</td><td>15.01</td><td>–</td></tr><tr><td>SD (mm)</td><td>–</td><td>0.79</td><td>1.24</td><td>1.33</td><td>1.37</td><td>1.43</td><td>1.15</td><td>–</td></tr><tr><td colspan="9"><strong>19 December</strong></td></tr><tr><td>χ<sup>2</sup> = 3.486; p = 0.999; df = 18</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Proportion</td><td>14.05</td><td>14.85</td><td>31.40</td><td>17.63</td><td>9.78</td><td>7.47</td><td>4.82</td><td>–</td></tr><tr><td>Average (mm)</td><td>0.99</td><td>3.94</td><td>7.45</td><td>10.84</td><td>13.08</td><td>15.68</td><td>18.02</td><td>–</td></tr><tr><td>SD (mm)</td><td>0.50</td><td>1.49</td><td>1.74</td><td>1.33</td><td>1.48</td><td>1.33</td><td>1.59</td><td>–</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>Fig. 5. *Riftia pachyptila*. A young individual that was attached to the tube of a large individual. The arrow indicates the young individual. Scale bar = 2 cm</figcaption></figure>

detached from a tube during the sampling process. Although sampling bias could not be quantified, attached individuals were abundant (i.e. 40.8 to 54.4% of collected individuals within each sample adhered to another tube). Moreover, size-frequency distributions showed that all the smallest individuals were attached (Fig. 4). The chance of being attached decreased as the worm size increased. Thus, no attached individual was observed for a vestimentum width >14.5 mm.

The sex ratios showed significant deviations from a 1:1 male:female ratio for samples from 27 November

and 7 December, and no deviation for the sample from 19 December (Table 4). This deviation was found to vary in favour of females in the 1st sample and in favour of males in the 2nd one. After pooling the 3 samples, no significant difference was detected.

## DISCUSSION

Dispersal is one of the most fundamental issues regarding processes involved in the maintenance of vent-endemic species populations and the colonisation of new vent habitat. For *Riftia*, dispersal ability inferred from population genetics analyses or from larval development and current knowledge about the ridge hydrodynamics is still debatable. Using allozyme data, Black et al. (1994) suggested that larval exchanges are maximal between neighbouring populations, and long-distance dispersal between adjacent

<table><caption>Table 4. *Riftia pachyptila*. Observed deviations from the expected 1:1 male:female sex ratio in the 3 *R. pachyptila* samples collected in November-December 1997 from the Riftia Field hydrothermal vent site. Level of significance: \*p < 0.05</caption><thead><tr><th></th><th>27 Nov</th><th>7 Dec</th><th>19 Dec</th><th>Overall observations</th></tr></thead><tbody><tr><td>Undetermined</td><td>490</td><td>164</td><td>113</td><td>767</td></tr><tr><td>Females</td><td>238</td><td>78</td><td>129</td><td>445</td></tr><tr><td>Males</td><td>196</td><td>106</td><td>149</td><td>451</td></tr><tr><td>χ<sup>2</sup></td><td>4.065*</td><td>4.571*</td><td>1.439</td><td>0.055</td></tr></tbody></table>
ridge axes could occur. Conversely, an estimation of *Riftia* larval lifespan ranging between 34 and 44 d allowed Marsh et al. (2001) to argue that along-ridge dispersal is restricted to 10s of km and rarely exceeds 100 km. However, these studies do not refer to the same temporal scales: while results from the former integrate 1000s of generations and impose effective settlements, results from the latter are restricted to 1 generation and do not take into account the fact that exchanged larvae have to live long enough to reproduce successfully in the home population. As a consequence, such studies are not comparable. Jollivet et al. (1999) indeed demonstrated that long-term dispersal is not a prerequisite to maintain high levels of gene flow at the ridge scale. However, an alternative to testing how larval dispersal affects population structures is to assess the dynamics over a spatial hierarchy, since population dynamics and *in vitro* larval development infer processes over a period of time which is limited to 1 generation.

Despite a limited dataset, our preliminary results obtained from the 1996 samples tend to support the hypothesis of a lack of consequent long-distance transport of larvae. The populations at 13°N/EPR were composed of small and large individuals at both sampled vent sites. Although no modal analysis was performed, the presence of small individuals at the Genesis and Parigo sites suggested that a recent recruitment event occurred at each site and that a synchrony of the recruitment could be considered within a vent field at a scale of km. By contrast, the unique population studied at 9°50'N/EPR included only medium and large individuals. The absence of young recruits at this site seems to indicate a lack of synchrony of reproduction and recruitment between 9°50' N and 13°N vent sectors that suggest no massive exchanges of larvae at this spatial scale.

The size-frequency distributions of the 3 *Riftia* samples collected in 1997 at Riftia Field are clearly poly-modal, which indicates a discontinuous recruitment as previously observed by Jollivet (1993) for the different species of vestimentiferan tubeworms of the EPR at 13°N (i.e. *Riftia*, *Oasisia* spp. and *Tevnia jerichonana*) and by Tunnicliffe (1991) for *Ridgeia* spp. From these data, one striking observation is that the shape of size histograms is greatly skewed towards small individuals suggesting that vestimentiferans undergo high mortality rates after settlement. This could be due to crab and fish predation and/or intra- and interspecific competition.

The modal decomposition of size-frequency histograms shows that each distribution is composed of a mixture of 6 or 7 Gaussian components assumed to represent successive cohorts. Different hypotheses may be formulated to explain differences in the number

and the position of size groups between samples. First, it may result from a bias in the sampling due to either a possible selectivity of the arm of the submersible towards specific size classes or a spatial segregation of the sizes according to their position within a vent site, implying an absence of randomisation over the different size classes. Such a bias may be put forward for the largest modal groups (i.e. vestimentum width >16 mm) which consist of only few tubeworms. Conversely, as younger individuals are attached onto the tubes of older ones, it seems quite improbable that it could affect the smallest modal groups. Second, heterogeneity in the number of size groups may be related to the spatial variation in larval supply. However, according to our previous observations between the Genesis and Parigo vent sites, we can suppose that successive recruitment events should be synchronous at a spatial scale of about 10s of m within a site. Moreover, Kim & Mullineaux (1998) showed that near-bottom currents induced by tides could transport larvae up to 2 km along the EPR during a single tidal excursion in the same vent field at 9°50' N. Finally, from experiments of colonisation on artificial settlement panels, Mullineaux et al. (1998) observed at 9°50' N that the recruitment of the gastropod *Lepetodrilus elevatus* was spatially coherent at a scale of at least 1 km. Thus, it is unlikely that a sampling bias or differing larval supplies at micro-spatial scales induce the lack of 1 cohort for small- and middle-size classes (i.e. vestimentum width <16 mm) despite the aggregative distribution of *Riftia* within a site (Jollivet 1993). However, for these organisms, the different modal components subsequently assigned to cohorts exhibit different characteristics (i.e. mean size, proportion) among samples.

Metabolism and ultimately the growth rate of *Riftia* are dependent upon biosynthesis by endosymbionts and should be closely related to the variability of environmental factors (Goffredi et al. 1997). Along the vestimentum width axis, differences among samples, between the location of the successive modes for small and middle-size individuals, could result from variations of the growth rate in response to local variations of the physical and chemical environment (Johnson et al. 1986, 1988). From the analysis of population structure of the *polychaetes Paralvinella pandorae* and *P. palmiformis*, McHugh (1989) suggested that variation in size among populations from vent to vent on the northern Pacific ridges may partly reflect growth rate variability in response to local habitat variations. Using RNA:DNA ratio measurements on the vestimentiferan tubeworm *Ridgeia piscesae*, De Bevoise & Taghon (1988) observed a significant variation in the organism growth rate from individuals collected at 2 Juan de Fuca vent sites, which were separated by only 2 m but displayed very different environmental conditions.
Finally, from *in vivo* laboratory experiments, Shillito et al. (1999) showed that the growth rate of the *Riftia* tube varied among individuals originating from different vent sites and could be affected by the chemical conditions of each site. In the present study, the influence of habitat characteristics on local variations of *Riftia* growth rates cannot be quantified due to the lack of data on environmental conditions for the 1996 and 1997 cruises.

Although the influence of small-scale spatial varia-
tions of environmental conditions on growth rate can-
not be neglected, a second hypothesis may be pro-
posed to explain the differences of the position of the
successive size groups. As samples were collected over
about 1 mo at a single vent site, modal differences
between the 3 temporal samples could be attributable
to the ageing of the different cohorts of the population.
However, to be valid, this hypothesis implies that the
modal size of each cohort increases between 2 succes-
sive samples. This is verified if we assume that the 1st
component of the 3rd sample, namely from 19 Decem-
ber 1997 (i.e. Component 0 in Table 3) results from a
massive recruitment event occurring between the 2
last samplings. In this case, the 1st component ob-
served on 27 November and 7 December becomes the
2nd component on 19 December. According to this
assumption, growth of the vestimentum width could be
drawn for the 5 main cohorts (i.e. Components 1 to 5 in
Table 3) over a period of 22 d (Fig. 6). *Riftia* displayed
a linear growth of the vestimentum width. Vestimen-
tum width growth rates ranged from 0.121 mm d⁻¹ for
the Cohort 1 to 0.250 mm d⁻¹ for the Cohort 3. The cal-
culation of a mean growth rate equal to 0.193 mm d⁻¹
could then be used to make some assumptions on *Riftia*
population dynamics.

The largest individuals sampled at Riftia Field had a
vestimentum width of 21.67 mm; therefore we may

assume they were about 112 d old according to a mean
growth rate of 0.193 mm d⁻¹. To compare our results
with previous observations on the *Riftia* tube growth
rate, the relationship between tube length (TL) and
vestimentum width (VW) was established on a
subsample of individuals collected on 19 December
1997. This relationship gives the following significant
regression: TL = 11.316VW¹.²⁴¹ (n = 241; r² = 0.926).
Thus, over 112 d, *Riftia* could reach a tube length of
49.3 cm with an impressive tube growth rate of 160 cm
yr⁻¹ which is about twice the previous observations
(i.e. 85 cm yr⁻¹) reported by Lutz et al. (1994) based on
photographic records.

For hydrothermal vent species, the lack of growth
data is generally a main barrier to infer on the
periodicity of recruitment events. In the present
study, the distance between 2 successive size groups
ranged between 1.51 and 3.75 mm in the sample
from 27 November, between 1.76 and 2.21 mm in the
sample from 7 December, and between 2.24 and 3.51
mm in the sample from 19 December (Table 3).
Although the different size groups displayed a non-
equally spaced distribution which varied among
samples, the spatial and temporal variability of
growth rates invalidates any possible conclusion
about periodicity in recruitment. However, these
results suggest that successive recruitment events
occurred at a high frequency, ranging from about 8
to 20 d. This short period could explain the apparent
paradox between a discontinuous recruitment and
the fact that there is no evidence of periodicity in the
reproductive output (Tyler & Young 1999). Along-
ridge flow reversals were reported at both 13°N and
9°50′N field for periods of weeks (Chevaldonné et
al. 1997, Kim & Mullineaux 1998, Marsh et al. 2001)
and may then be involved in the short-term
variability of larval supply.

The proportions of the 1st cohorts varied
greatly within and among samples. Such
variations could be due to differing suc-
cess in the recruitment of *Riftia* in response
to variations of larval supply and juvenile
mortality at small spatial scales. However,
small individuals are the main component
of *Riftia* populations, which is indicative of
an active larval recruitment. By contrast,
largest individuals (i.e. vestimentum width
>15 mm), which are about 80 d old, consti-
tuted only a few percent of the total popu-
lation. Thus, individual survivorship rate is
probably very low as it was already shown
among the vestimentiferans by Tunnicliffe
et al. (1990). During their 26 d experiment,
these authors reported that about 44% of
the observed *Ridgeia* spp. apparently died

<figure><img src="image_8.png" /><figcaption>Fig. 6. *Riftia pachyptila*. Growth curves corresponding to each of the 5 main cohorts shared by the 3 temporal samples of Riftia Field. Growth rates are estimated from the increase of vestimentum width of each Gaussian component mean over the 3 dates. Cohorts are numbered according to Table 3</figcaption></figure>
because of predation and the physical instability of the environment. Assuming that our 3 samples from *Riftia* Field represent the temporal evolution of 1 population, our results suggest that *Riftia* displays a high individual growth rate and a high population turnover, which are characteristic of an opportunistic species.

Following dispersal of offspring, both the success of
recruitment and the maintenance of adult populations
will mostly depend on the ability of larvae to settle over
new vents or on adult populations. Settlement is there-
fore critical and could be driven by the physical, chem-
ical and biological conditions of the hydrothermal
habitat. Concerning *Riftia*, several factors could induce
larval settlement or ensure recruit survival. According
to the physiological tolerance and the nutritional re-
quirements of vent organisms, chemical characteristics
of the habitat could play a significant role in determin-
ing the location of settlers. Thus, following the coloni-
sation of new vents, Shank et al. (1998) reported that
the presence of dense clumps of *Riftia* could be related
to specific levels of hydrogen sulphide. Conversely,
Mullineaux et al. (2000) argued that chemical prefer-
ence is probably not a primary factor governing *Riftia*
recruitment on experimental basalt blocks and sug-
gested that *Riftia* settlement is facilitated by the occur-
rence of the tubeworm *Tevnia jerichonana* through
a biological cue. According to the habitat selection
hypothesis, 1 major result of our study is that most *Rif-
tia* settlers seem to be located on the tubes of older con-
specifics. While several *Tevnia jerichonana* were also
collected in our samples, no *Riftia* settler has been
observed on their tubes. Thus, if the presence of *T. jeri-
chonana* facilitates the initial colonisation of new vent
areas by *Riftia*, our results suggest that a physical or
biological cue associated with *Riftia* adults or their
tubes is also involved in the settlement. One likely
explanation for the vestimentiferan succession is that
both *T. jerichonana* and *Riftia* colonise the same loca-
tions as a result of different specific cues (e.g. vent
chemistry, biological cues), then *Tevnia* dominates the
assemblage as a result of an advantageous biological
strategy and progressively disappears as a result of a
spatial competition for food supply in which the ability
to self-recruit may be crucial. From the numerous stud-
ies dedicated to the habitat selection in shallow-water
marine invertebrates (see reviews Crisp 1974, But-
man 1987, Pawlik 1992), different biological cues
may be proposed to favour *Riftia* settlement that
include: (1) the nature and the texture of the tube;
(2) the presence of natural biofilms; and (3) attractive
substances associated with conspecific recruits and
adults.

To conclude, the present data combined with previ-
ous observations allow us to identify some characteris-
tics of *Riftia* population dynamics: (1) recruitment is

discontinuous and synchronised within a vent site;
(2) recruitment events may occur at a high frequency
according to short-term variability of larval supply;
(3) settlement may result from habitat selection; and
(4) *Riftia* exhibits high growth and mortality rates sug-
gesting a high population turnover.

*Acknowledgements.* We gratefully acknowledge the captains and the crew of the NO 'Nadir' and RV 'Atlantis II', the members of the 'Nautile' and 'Alvin' groups. We are particularly grateful to L. Mullineaux, the chief scientist of the EPR'97 cruise, who allowed us to conduct this work. We are indebted to C. Durif, D. Eveillard and F. Not for their valuable laboratory assistance. This work was supported by the French national programme DORSALES (CNRS, INSU, IFREMER).

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