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Keywords:

  • Indeterminate growth;
  • modular animals;
  • population dynamics;
  • resource allocation

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

1. A trade-off was predicted between investment in defence and growth rate in the encrusting sponge Crambe crambe (Schmidt), while survival rates were expected to correlate positively with the production of defences. Previous studies have demonstrated that this sponge is better defended (chemically and physically) in shaded animal-dominated habitats than in well-illuminated habitats, dominated by algae. It was determined whether these habitat-associated differences in investment correlate with differences in growth, regeneration and mortality rates of small specimens (initial average area less than 100 mm2) of this sponge.

2. In the 2 years of the study the sponge grew slowly (size increased 2·5 times on average), and showed high interindividual variation in growth rates. A seasonal pattern was evident, with marked size increases from May to October. Significantly higher growth rates (on a monthly basis) were found in the well-illuminated habitat during the second year of study. There was also a negative relationship between monthly growth rate and toxicity (measured in previous studies). No differences were found in sponge regeneration rates between communities.

3. Mortality was significantly higher in individuals from the well-illuminated habitat, and mainly affected the smallest sponges (< 150 mm2).

4. This sponge was significantly more abundant in the shaded habitat, but the mean size of the sponges was greater in the well-illuminated habitat. The size distributions in the two communities were also significantly different. Sponges of the smallest size classes were more abundant in the shaded community.

5. It is concluded that sponges in the shaded habitat, in which investment in defence was greatest, featured higher survival but grew more slowly than those in the well-illuminated habitat. Differential mortality and growth observed in the study were sufficient to explain the patterns of abundance and size distributions of the established sponge populations from these two habitats.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We aimed to assess whether the between-habitat differences in investment in defence, structural materials and reproduction reported in previous studies in the sponge Crambe crambe (Schmidt) could be related to differences in growth rates and mortality. A trade-off between allocation to defence (chemical and physical) and to reproduction has been reported for this species (Uriz et al. 1995). We predicted a similar trade-off between investment in defences and growth, while survival rates were expected to correlate positively with the production of defences. We also wanted to determine whether differences in growth rates and mortality could explain differences in the abundance and size-structure of the established sponge populations.

Crambe crambe is one of the most widespread littoral sponges in the north-western Mediterranean, where it has a wide ecological distribution (Uriz, Rosell & Martin 1992a). It is a red encrusting form, which can reach surface areas of 0·5 m2 in the study zone. At the same time, C. crambe featured strong bioactivity in general bioassays (Jares-Erijman, Sakai & Rinehart 1991; Berlinck et al. 1992; Uriz, Martin & Rosell 1992b), suggesting the production of chemical defences. These defences appear to perform multiple ecological roles (Becerro et al. 1994a; Uriz et al. 1996; Becerro, Turon & Uriz 1997). In addition, this species is a thinly encrusting form and it is therefore highly surface-dependent, which implies that growth and mortality rates are influenced by strong space competition with neighbours. Here we seek to build on previous knowledge of the variability of other traits of the biology of C. crambe, namely, investment in chemical and physical defences, production of larvae, reproductive periods and filtration rates (Becerro, Uriz & Turon 1995; Uriz et al. 1995; Turon, Becerro & Uriz 1996a; Turon, Galera & Uriz 1997). Our aim was to complete the picture of the biological strategy of this sponge.

Ayling (1983) rightly claimed that almost no data on growth and regeneration rates in encrusting sponges existed, and few studies have contributed information since then (e.g. Pomponi & Meritt 1985; Plucer-Rosario 1987; Pansini & Pronzato 1990). The same can be said for sheet-like invertebrates in general. Yet encrusting invertebrates are a major part of many epifaunal assemblages in rocky bottoms, and they are believed to be efficient competitors for space because of their morphology (Buss 1979; Jackson 1979). Studies on population dynamics of this type of organism, especially if they are coupled with data on energy allocation, population structure and other traits of their life history, are needed to explain processes and adaptive trends in epifaunal assemblages (Sebens 1987).

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

STUDY DESIGN

Growth of C. crambe was studied by monitoring live specimens at the locality of Blanes, northeast Spain (western Mediterranean; 41°40·4’N, 2°48·2’E). It was expected that small sponges would grow faster than large ones, according to previous observations in several sponge species (Dayton 1979). Large specimens usually occupy all available space (Turon et al. 1996b) and their growth and shrinking may reflect interactions with their neighbours rather than their own physiological state. Large sponges, on the other hand, are more likely to suffer partial losses of material and subsequent regeneration. It was therefore decided to study growth rates in small specimens (less than 100 mm2), which may be assumed to be settlers from one of the two preceding years, while regeneration rates were studied in large specimens (more than 40 000 mm2).

Differences in growth and mortality were analysed by comparing two contrasting habitats, in which previous studies on other biological parameters of this species had been performed. A thorough description of these habitats was given in Becerro, Uriz & Turon (1994b). Briefly, the sampling site consisted of two parallel vertical walls between 6 and 12 m in depth, and 3 m apart. These walls were similar in all respects except for the orientation: one faced north, the other south. The former received less irradiance than the latter, and the assemblages that developed on these walls differed markedly: on the northward-facing wall a space-limited community (hereafter called the sciaphilous assemblage) was dominated by encrusting animal species, mainly sponges. On the southward-facing wall, erect macroalgae and patches of bare rock dominated the landscape (hereafter the photophilic assemblage). Growth form and relative allocation to structural materials, reproduction and chemical defences of C. crambe were studied in the same walls (Becerro et al. 1994b, 1995; Uriz et al. 1995). Moreover, seasonal fluctuations in the toxicity of this species had been monitored in the same site (Turon et al. 1996a).

MONITORING

In November 1994 small specimens (less than 100 mm2) were selected on each wall. Every month their outlines were traced in situ onto acetate sheets. The outlines were then digitized and their surface areas were calculated. Since it was apparent from the beginning that mortality was higher on the well-illuminated wall, new individuals from this wall were included in the monitoring during the first 4 months of study. Fusions and divisions of sponges over time were also recorded. When fusion occurred, the areas of the sponges before merging were added together to give a single value per survey. When fission occurred, the areas of the resulting clonemates were summed in subsequent measurements. Final numbers of monitored sponges were 24 in the shaded wall and 51 in the well-illuminated wall. At each sampling time the water temperature at a depth of 10 m was recorded. The survey was terminated in January 1997.

For the study of regeneration rates, circular holes (about 450 mm2) were scraped down to the rock in each of 10 large (more than 40 000 mm2) specimens on each wall and their regeneration was monitored after 2 and 3 weeks by drawing the outlines of the holes on acetate paper and calculating their areas as described above. This study was conducted in October 1995 (outside the reproductive season of this species) and lasted only 3 weeks because it was assumed that true regeneration would occur only shortly after damage. In fact, after 3 weeks, a film of filamentous algae was already established in the holes and subsequent growth could not be considered simple regeneration.

In February 1997, the abundance and size of specimens of C. crambe on each wall were also studied. To this end, three transect lines, 5 m long, were laid on each wall. The transects were placed at 6, 7 and 8 m of depth, with their origins laid haphazardly on the walls. A 0·25 m stick was then run above and below the transects and all sponges larger than 500 mm2 touched by the stick at each metre of transect were counted. Specimens larger than those monitored in the 2-year survey were selected because our aim was to ascertain the effects of early mortality and growth on the structure of the population at later stages. Individual sizes were estimated by taking two measurements on each individual, one along its longest axis and one along the longest dimension perpendicular to the first one. Both measures were then treated as the axes of an ellipse whose area was taken as an approximation to the sponge’s area.

STATISTICAL METHODS

From the changes in area over time, a monthly growth rate was computed by the formula:

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where Am and Am– 1 are the areas at month m and at the previous month, respectively. This growth rate is the monthly change in area, relative to area at the beginning of the month interval.

Cross-correlation analyses were used to check for relationships between monthly growth rates and temperature. Spearman rank correlation was used to assess relationships between size and growth rates in the month in which the highest mean growth rate was recorded.

For the comparison of growth rates between walls, a repeated-measures analysis of variance (Potvin, Lechowicz & Tardif 1990; Von Ende 1993) was first tried with habitat as the between-individual factor and time as the within-individual factor. However, the circularity assumption, necessary for the use of the univariate repeated-measures analysis (ANOVAR), was badly violated by our data (Mauchly’s sphericity test). The multivariate equivalent of this analysis (MANOVAR, also called profile analysis) requires the less stringent assumption of homogeneity of the variance–covariance matrix, which was nevertheless not met in our case (Box M-test). No transformation succeeded in rendering the data amenable to these analyses. A randomization method was therefore used, based on Manly (1991), which consisted of a two-stage permutation of the data: first, individuals were randomly reassigned to the two habitats, and then readings for each individual were randomly rearranged among observation times. With this two-level randomization, the overall total sum of squares, as well as the total between- and within-subject sums of squares, remain unchanged (Manly 1991). The sum of squares associated with each factor and their interaction is therefore a suitable statistic on which to test the significance of each effect. The whole series of data was randomized 4999 times (plus the observed one) to approximate the null hypothesis distribution of the sum of squares for each factor and their interaction, and then how extreme were the observed values in this distribution was examined. An effect was judged significant when the observed sum of squares was exceeded by less than 5% of the corresponding values in the randomization series. An important advantage of this method, aside from being non-parametric, is that it can use all the data set, not just the sponges that survived until the end of the experiment (ANOVAR/MANOVAR procedures require measurements for each individual at each time interval). For sponges that were not present at all observation times, the permutation between times was done only within the period in which the sponge was in the study.

Survival of sponges was analysed by the life-table method, and hazard functions were computed for both walls. Survival on both walls was compared by a Wilcoxon-type test (Fox 1993). The relationship between the area attained and survival was analysed with the Cox proportional hazards model (Fox 1993). The packages Systat version 5·0 and Statistica version 4·0 were used for the analyses, and the randomization routine was written in TURBOPASCAL version 6·0.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The time courses of the mean surface area and the monthly growth rates of the sponges studied, both habitats pooled, are presented in Fig. 1. The monthly growth rate was smoothed to avoid noise fluctuations by a weighed moving average (current value × 0·5 + previous value × 0·25 + following value × 0·25). Water temperature readings during the study period are also shown.

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Figure 1. . Mean values of area and monthly growth rate of C. crambe during the study period (both habitats pooled). Bars are standard errors. The water temperature at – 10 m is also indicated.

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The periods of growth in this species fluctuated seasonally. Gains in area were recorded from May to October (roughly late spring and summer), while in late autumn and winter there was no growth of the colonies, even a small shrinkage relative to the values in October. The growth rates were negative or close to 0 from October to the beginning of spring, and their peaks occurred in August 1995 and June 1996.

Cross-correlation studies between monthly growth rates (both habitats pooled) and temperature revealed a significant positive correlation between both parameters at time lags of 0, 1 and 2 months, and significant negative correlations at time lags of – 3, – 4 and – 5 months. This indicated that growth rates covaried positively with the temperature measured in the two preceding and the current month, and negatively with temperature recorded in the subsequent third, fourth and fifth months.

The relationship between growth rate and size was analysed in the month with the highest monthly growth rate (August 1995). The size at the beginning of the month interval was correlated with the growth rate. The Spearman correlation coefficient was negative, indicating a tendency towards lower growth in larger specimens, but not significant (rs = – 0·119, P = 0·41).

When considered separately, the values of monthly growth rates in the two habitats (Fig. 2) revealed a trend towards increased growth in the well-illuminated assemblage, which was especially clear during the second year of the study. The results of the randomization test performed on the monthly growth rates are shown in Table 1. Both time and the interaction between time and habitat were significant. A significant interaction term means that the time course of growth was different in sponges from the two habitats. Figure 2 reveals that the differences between habitats occurred mainly during the second growth season. The first and second years of study were therefore analysed separately, and neither the habitat nor the interaction was found to have a significant effect during the first year, while only the habitat effect was significant if the second year of study was considered (Table 1). This difference in the habitat effect over time led to a significant interaction term when both years were considered together. The pattern of significant results was the same when the analyses were repeated with just the survivors from each habitat (not shown).

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Figure 2. . Time course of the mean values of monthly growth rate (bars are standard errors) and percentage survivorship of C. crambe in the two habitats studied.

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Table 1.  . Significance levels obtained by randomization for the repeated measures analyses of the growth rates Thumbnail image of

Mortality was higher in the well-illuminated habitat (Fig. 2). By the end of the study, only 16 (31%) sponges survived in this habitat, while 15 (62%) sponges initially included in the study in the shaded wall were still alive. The data were analysed by the life-table method, and differences in mortality were significant (Gehan’s Wilcoxon test, P < 0·001). There was no clear temporal pattern in the hazard rates (probability of a sponge dying in any one interval, provided it has arrived alive at the beginning of this interval), except that hazard was high in the photophilic assemblage during the first part of the study, resulting in many losses.

The Cox’s regression model between hazard rate and maximal size attained showed significant negative relationships between mortality and size in both habitats (P < 0·0001 in both cases). Dead individuals were mostly sponges smaller than 150 mm2 in surface area. Few deaths occurred once individuals had attained sizes above 150 mm2, and none occurred in both habitats in individuals that attained sizes above 450 mm2.

The abundance of sponges larger than 500 mm2, measured in the transects laid on each wall, was 40·80 ± 3·22 individuals m–2 (mean ± SE) in the sciaphilous assemblage, and 22·66 ± 2·30 individuals m–2 in the photophilic one. The mean size of the sponges, on the other hand, was 6886 ± 1033 mm2 in the former habitat and 4118 ± 340 mm2 in the latter. The between-wall differences were significant for both variables (Mann–Whitney U-test, P < 0·0001 in both cases). Figure 3 shows the size–frequency structure of the sponges in the two habitats. Sponges belonging to size-classes below 5000 mm2 were more abundant in the sciaphilous habitat. At the other extreme, some sponges from the photophilic habitat featured sizes up to 100 000 mm2, while the maximal sponge size recorded on the shaded wall was close to 50 000 mm2. The two frequency distributions were significantly different (Kolmogorov–Smirnov two-sample test, P = 0·015).

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Figure 3. . Size–frequency distributions of Crambe crambe in the two habitats surveyed. Numbers in the abscissa indicate the upper limits of the size-class intervals (× 103). Note change in interval-width for sizes above 20 000 mm2.

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There were relatively few fusions or fissions during the study; 14 fission and 12 fusion events occurred throughout the study in the photophilic assemblage, 5 fissions and 7 fusions in the sciaphilous one (note that the number of sponges monitored was double in the former).

Regeneration rates in the two habitats considered, estimated by the regeneration of holes scraped in sponge individuals in each wall, did not differ significantly between walls (Fig. 4). The holes had closed slightly more in the sciaphilous sponges after three weeks, but differences were not significant (t-test, P = 0·5794).

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Figure 4. . Time course of the regeneration of holes scraped in C. crambe specimens in the two habitats.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The 2-year monitoring revealed that the growth rate of small specimens of C. crambe was extremely low, especially if compared with that reported for massive demosponges (Dayton 1979) or calcareous sponges (Johnson 1979). Sizes increased on average about 2·5 times in 26 months (Fig. 1), which is surprising given the very small initial size as compared with the large individuals common in this habitat (see Fig. 3). Bryan (1973) and Ayling (1983) similarly reported very slow growth in several species of encrusting sponge. A high interindividual variability was also apparent in our study, as seems to be the rule in modular organisms (Brunetti & Copello 1978; Dayton 1979; Bak, Sybesma & Van Duyl 1981; Ayling 1983; Wulff 1985; Todd & Turner 1988; Stocker 1991; Turon & Becerro 1992). Some specimens of C. crambe remained small throughout the study period. Other individuals were ‘successful’ in that they increased in size throughout the study. In some cases, individuals that had attained a moderate size regressed to a very small surface area. These cases were, however, rare in our study. Although some fission and fusion of sponges did occur, the fragment dynamics of small individuals of this species is much less relevant to its demography than that reported for other encrusting invertebrates (Bak et al. 1981; Stocker 1991) or for branching sponges (Wulff 1991).

The growth of C. crambe fluctuated seasonally; in both years sponges increased in size from May to October, coinciding with the rise of sea-water temperatures. Seasonal patterns are common in temperate habitats (Elvin 1976; Osman 1977; Sutherland & Karlson 1977; Sebens 1986). For sponges, seasonal cycles with higher growth rates in winter–early spring (the reverse of what we have found) have been reported by Johnson (1979). In contrast, Ayling (1983) found fluctuations in sponge sizes not related to seasonal or reproductive cycles, and no seasonal pattern of growth has been observed in other Mediterranean species (Pansini & Pronzato 1990). Alternating cycles of allocation to reproduction and growth are usually proposed to explain seasonal fluctuations in growth rate. However, as small sponges were selected, reproductive activity could not explain the cycles found. Larvae were not observed in the colonies monitored and, in a previous survey, it was found that only 10% and 20% of sponges smaller than 1000 mm2 were reproductive in the sciaphilous and photophilic assemblages, respectively (Uriz et al. 1995). Turon et al. (1996a) reported seasonal differences in toxicity levels and thus investment in chemical defence of C. crambe in this area. Although the study was performed on different individuals and in different years, if the monthly mean values of toxicity found by Turon et al. (1996a) are tentatively analysed against the monthly growth rates (both years averaged) found in the present survey, a significant negative relationship appears (Pearson correlation coefficient = – 0·702, n = 12, P = 0·019).

It was found that growth rates were not significantly correlated to size in the month in which the highest growth rate was recorded. This was as expected for organisms featuring indeterminate growth (Jackson 1979). In particular, in sheet-like forms, no allometric adjustment of surface/volume ratio is necessary (Ryland & Warner 1986), so growth rate can be maintained, at least theoretically, indefinitely. However, it was found that the correlation coefficient was negative (even if not significant), indicating a tendency towards declining growth rates with size. Dayton (1979) found higher growth rates in small sponges than in larger ones, while Fell & Lewandroski (1981) did not find the growth of Halichondria sp. related clearly to size. Turon & Becerro (1992) found significant negative relationships between growth rate and size in four encrusting ascidian species. Attenuation of growth with size may reflect habitat constraints as sponges occupied the free space available (Sebens 1987).

Life spans of benthic invertebrates are poorly known (Jackson 1986). Our sponges might have been 3·5- or 4·5-years-old at the end of the experiment, if we were right in assuming that they came from the 1992 and 1993 recruitments. However, by the end of the study, none had reached a size comparable even to medium-sized specimens in the area (see Fig. 3). This suggests that C. crambe is a remarkably long-lived species, although calculating life spans from the growth rates observed is not advisable owing to the decoupling of age and size in these modular organisms.

As for the between-habitat comparisons of growth rates, a different time course was found in the two habitats (significant interaction). This was due to the growth of sponges during the second year (Fig. 2), in which growth rates were high in the well-illuminated wall from April to September, while they remained close to zero in the shaded habitat. Accordingly, a significant habitat effect was found when only the second year was analysed. Mortality rates, on the other hand, featured clear differences between habitats; survival in sponges from the sciaphilous habitat was double that of sponges from the photophilic assemblage. On both walls there was a clear inverse relationship between mortality and size (Cox regression model). Very rarely had a large sponge disappeared from one observation to the next, but it was not uncommon to find a regression in size during the few months preceding the death of the individual. Size may act as a refuge against the causes of mortality in the zone (Sebens 1982). Johnson (1979) reported that the periods of high mortality in calcareous sponges coincided with the season of slow growth. In our case, aside from a high mortality during the first months of the study (especially in the photophilic sponges), no clear seasonal trend in mortality was apparent.

Why mortality was higher in the well-illuminated wall remains unclear. Damage by passing sea-urchins during grazing excursions may explain the loss of small C. crambe colonies, but sea-urchins were equally abundant on both walls. The number of echinoids measured in five 5 × 1 m2 transects on each wall was 3·48 ± 0·46 individuals m–2 (mean ± SE) on the well-lit wall, and 3·36 ± 0·55 on the shaded wall. Overgrowth by fast-growing algal species may be responsible for the higher mortality rates. Experimental work is needed to address this question.

Crambe crambe was twice as abundant in the shaded habitat; here they suffered lower mortality, but were smaller as a result of slower growth rates. The size frequency distributions revealed a larger number of sponges smaller than 5000 mm2 in the sciaphilous habitat (Fig. 3). These findings are consistent with the idea that mortality is high on the photophilic wall during the first years of the life. On the other hand, the specimens that survive on this wall grow at a faster rate and attain a larger size. For example, large sponges, in excess of 50 000 mm2, had been recorded in the photophilic habitat, but not in the sciaphilous one. The patterns of sponge abundance and size distribution described from these two habitats match the pattern expected if mortality and differential growth during the first years of life determined the structure of this population. Other factors, such as differential settlement preferences, need not be invoked to explain the patterns observed. Indeed, we have found evidence that larvae of this sponge do not respond to light gradients (M. J. Uriz, unpublished data).

Regeneration rates were similar on both walls, and were quite fast. The rate at which the area of the holes produced in the sponge surfaces decreased as a result of sponge regeneration was – 0·456 (SE = 0·077) during the 3 weeks of the experiment. Consequently, regeneration rates were much faster than growth rates in this species, as noted also by Ayling (1983) in another encrusting sponge. Rapid regeneration has been considered a mechanism whereby sponges can maintain their dominance in cryptic reef environments (Jackson & Palumbi 1978). Similarly, rapid regeneration may contribute in our case to the ability of sponges to recover lost space (originating from local perturbations) in the presence of opportunistic colonizers of bare substratum. Fast regeneration and long life span may contribute to explain why a species with such a slow growth as C. crambe is an important member, in terms of cover, of sublittoral assemblages.

In previous studies, differences in biological parameters in this species between the same two walls studied have been substantiated: toxicity and thus investment in chemical defences is higher on the shaded wall (Becerro et al. 1995). Likewise, investment in supporting, structural elements (mainly collagen and fibres) is higher in sciaphilous sponges (Uriz et al. 1995). The pattern was therefore one of higher investment in both chemical and physical defences on the shaded wall. Becerro et al. (1995) and Uriz et al. (1995) suggested (although proper experimental confirmation has not yet been possible) that this pattern may be due to the different competition pressures in the two habitats; space was a limiting resource in the sciaphilous assemblage dominated by slow-growing animal species. A strategy of strong space competition with physical and chemical defences may develop there, while the photophilic habitat was dominated by fast-growing, erect algae, with patches of bare substratum being freed continuously. A more opportunistic strategy may be suitable here.

If photophilic specimens invest less in defence, we expected to find a higher allocation in this habitat to growth and reproduction. Reproductive investment is indeed higher in the photophilic sponges, as shown by Uriz et al. (1995). As for growth, the results obtained here of the monthly growth rates and the negative correlation between growth rate and toxicity on a temporal basis indicated that growth is slower on the wall in which more energy is devoted to defence, and that growth rates are depressed during the periods of higher toxicity. The data support the existence of a trade-off between production of defences and growth rate. It should also be pointed out that between-habitat differences in allocation are more distinct in medium- to large-sized individuals (Uriz et al. 1995), so the effect of differential investment in these diverse functions might have been more apparent as specimens grow to larger sizes than those considered here, which is consistent with the fact that the between-habitat differences in growth rate appeared especially in the second year. All this suggests that longer-term studies are needed to understand better the interaction between demography and habitat in this species. Interpretation of our results need some caution owing to the brevity of the present study. Differences in mortality, on the other hand, were as expected. Sponges in the high-mortality site invested less in structural, long-lasting physical defences (which may be useless in the competition with fast-growing algae).

In conclusion, the between-habitat comparison showed that specimens from the habitat in which the highest investment in defence had been reported survived better. A trade-off between growth rate and production of defence could also be substantiated. Such a trade-off may be more evident in larger specimens, especially at sizes where the reproductive activity is high. Differential growth and mortality can explain the contrasting population structure of this species in the two habitats. The results are consistent with the idea of a cost associated with investment in defence, which results in better survival but at the price of lower reproductive output and slower growth.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank A. Davis (University of Wollongong, Australia) for critically reading the manuscript. M. Maldonado and R. Martí (CEAB, Spain) helped in the field work. This research was funded by projects DGCYT PB94–0015 and MAR95–1764 of the Spanish Government, and also benefited from the support of the Catalan Government (GRQ93–8042).

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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