Coralligenous habitat is one of the most important and sensitive habitats of the Mediterranean Sea and several different sampling procedures are currently used in the ecological investigations of coralligenous assemblages. This study aimed to assess the efficacy of different methods in detecting anthropogenic impacts on coralligenous habitat. In particular, the choice of sampling methods, the level of taxonomic resolution, the sampling area, the number of replicates and the spatial scales for detecting possible impacts were evaluated. Results showed that photographic samples larger than 1800 cm2, numbers of replicates larger than 10, the use of taxa and morphological groups as assemblage descriptors, and sampling designs with a high replication at small spatial scales are a valid methodological procedure in impact evaluation studies based on coralligenous assemblages.
El hábitat coralígeno es uno de los más importantes y sensibles del mar Mediterráneo y actualmente se pueden aplicar varios métodos de muestreo en las investigaciones ecológicas de las comunidades macroalgales coralígenas. El objetivo del presente estudio es evaluar la efectividad de los diferentes procedimientos para detectar el impacto antrópico sobre el hábitat de coralígeno. Se evaluaron en particular la elección de los métodos de muestreo, el nivel de resolución a la que los taxones tienen que ser identificados, la superficie de muestreo, el número de repeticiones y las escalas espaciales adecuadas para la investigación de los posibles impactos. Los resultados indican que muestras fotográficas de 1800 cm2, un número de repeticiones mayor que 10, el uso de taxones y grupos morfológicos como descriptores y diseños de muestreo con una alta replicación asociada a pequeñas escalas espaciales pueden representar elementos metodológicos válidos en los estudios de evaluación de impacto basados en las comunidades coralígenas.
The relevance of sampling procedures in marine ecology is widely recognized and determining the sampling methods most responsive to the questions/objectives plays a fundamental role in research success (
Natural variability of marine benthic assemblages is scale-dependent (
Another important consideration concerns sampling procedures in marine habitats. Destructive methods are widely utilized and recognized as suitable for describing benthic assemblages in relation to the assessment of patterns of diversity and detection of rare species (
Identifying suitable assemblage descriptors that are sensitive to human-induced stress is very important in impact evaluation studies, because it optimizes sampling efforts and allows representative patterns of assemblage variability to be obtained (
Coralligenous habitats develop on deep subtidal rocky bottoms in the Mediterranean Sea, where they are one of the most important habitats in size, biodiversity and role in CO2 dynamics (
The aim of the present study was to contribute to the assessment of the most effective procedures for detecting effects of impacts on Mediterranean coralligenous habitats. In particular, the choice of sampling methods, the level of taxonomic resolution, the sampling area, the number of replicates and the proper spatial scales to study were evaluated. To achieve these objectives, multi-factorial sampling designs were used to find spatial scales with high variability and to compare results obtained with different descriptors, different numbers of replicates and different sampling areas.
The study was carried out in the summer months along the coasts of Tuscany (northwestern Mediterranean Sea, Italy), on rocky vertical bottoms at 30-35 m depth. This depth was chosen because macroalgal coralligenous assemblages of this depth range are the most representative of the geographical area considered, in terms of structure and response to alterations of environmental conditions (
Comparison between sampling methods and assemblage descriptors
Two different ecological conditions were considered: a stressed condition consisting of marine areas affected by urban and/or industrial discharges and high sedimentation rates; and a reference condition consisting of areas subjected to absence of, or very minor, stress (Annex I,
Destructive samples were collected by scraping the bottom with a hammer and a chisel, all sessile organisms were identified and the abundance of each sessile species was expressed as percentage cover of the sample area.
In the photographic samples, the percentage cover of the main taxa or morphological groups was evaluated using the “Image J” software (
To compare the efficiency of different methods (destructive vs. photographic) and the suitability of assemblage descriptors (species vs. taxa/morphological groups) in detecting responses to environmental stress, data obtained by analysing photographic samples, data obtained by analysing destructive samples at the species level and data obtained through a taxa/morphological groups analysis of assemblages carried out according to the photographic approach on the same destructive samples were analysed by permutational multivariate analysis of variance (PERMANOVA, Primer v6 program including the add-on package PERMANOVA plus,
In each of the two reference sites, 10 photographic samples of 400 cm2 and 10 photographic samples of 1875 cm2 (fitted with a frame 50×37.5 cm) were collected. Sampling surface and number of replicates were chosen according to pilot studies (
At each of the two reference sites and two stressed sites, 20 photographic samples of 1875 cm2 were collected. To compare the effectiveness of different sampling areas in detecting assemblage responses to stressors and in describing spatial patterns of variability, data obtained with 30, 25, 20, 15, 10 and 5 replicates for each site were analysed by two-way PERMANOVA with Condition (reference vs stressed) as a fixed factor and Site (2 levels) as a random factor nested in Condition.
Spatial variability of coralligenous assemblages
Two pristine or minor stressed sites were selected along the Tuscany coasts and, at each site, two locations several kilometres apart were chosen; at each location, two areas hundreds of metres apart were selected and 15 photographic samples of 1875 cm2 were collected in each area about 1 m from each other.
To determine patterns of variability at each of the chosen spatial scales, data were analysed by a four-way PERMANOVA analysis, with Site (2 levels) as a random factor, Location (2 levels) as a random factor nested in Site and Area (2 levels) as a random factor nested in Location. The pseudo-variance components were calculated for each spatial scale: site, location, area and sample.
A total of 123 taxa were identified by destructive samples (
The studied assemblages were characterized by a stratified structure: encrusting Corallinales, mostly
In reference condition the prostrate layer was mostly characterized by the macroalgae,
At all the stressed sites, several turf-forming macroalgae increased their abundance (
Results of PERMANOVA analyses showed significant differences between reference and stressed conditions for all three approaches used: photographic samples, destructive samples analysed at species level, and destructive samples analysed at taxa/morphological groups level. A significant variability between areas was only detected in the destructive samples (
Destructive species level | Destructive morphological groups level | Photographic morphological groups level | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Source | df | MS | Pseudo-F | P (perm) | MS | Pseudo-F | P (perm) | MS | Pseudo-F | P (perm) |
Condition = C | 1 | 7377.3 | 2.49 | 0.047 | 7823.2 | 3.95 | 0.036 | 7398.4 | 7.54 | 0.022 |
Site(C) = S (C) | 2 | 2956.9 | 0.85 | 0.618 | 1979.3 | 1.10 | 0.388 | 968.8 | 1.11 | 0.392 |
Area(S(C)) | 4 | 3452.6 | 1.65 | 0.039 | 1792.3 | 1.93 | 0.031 | 862.7 | 2.10 | 0.069 |
Residual | 16 | 2082.6 | 928.6 | 408.9 | ||||||
Total | 23 |
PERMANOVA analysis detected no significant differences between samples collected using different areas (
Source | df | MS | Pseudo-F | P (perm) |
---|---|---|---|---|
Area = A | 1 | 4555.9 | 1.72 | 0.211 |
Site = S | 2 | 2642.6 | 1.92 | 0.050 |
Residual | 36 | 1376.2 | ||
Total | 39 |
Taxa/groups | 400 cm2 cover | 1875 cm2 cover | Contrib. % |
---|---|---|---|
Algal turf | 49.04 | 40.03 | 26.5 |
19.6 | 22.91 | 20.59 | |
Erect corticated algae | 12.77 | 0.54 | 12.61 |
Encrusting Corallinales | 4.33 | 12.31 | 11.64 |
0.63 | 4.86 | 4.69 | |
2.43 | 4.23 | 3.78 | |
Encrusting sponges | 1.03 | 3.1 | 3.62 |
3.4 | 0.28 | 3.47 | |
Erect bryozoans | 0.13 | 3.01 | 2.94 |
2.2 | 1.13 | 2.89 |
A similar pattern of spatial variability of assemblages was obtained by analysing 10, 15, 20, 25 and 30 replicates, but the results obtained using the 5 replicates approach gave a different pattern (
Source | df | MS | Ps-F | P(MC) | df | MS | Ps-F | P(MC) |
---|---|---|---|---|---|---|---|---|
5 replicates | 10 replicates | |||||||
Condition = C | 1 | 18991 | 6.45 | 0.024 | 1 | 31967 | 5.16 | 0.030 |
Site(C) | 2 | 2942 | 2.07 | 0.097 | 2 | 6191 | 3.93 | 0.001 |
Residual | 16 | 1415 | 36 | 1572 | ||||
Total | 19 | 39 | ||||||
15 replicates | 20 replicates | |||||||
Condition = C | 1 | 55967 | 11.01 | 0.005 | 1 | 75768 | 7.59 | 0.007 |
Site(C) | 2 | 5079 | 3.21 | 0.008 | 2 | 9975 | 6.32 | 0.001 |
Residual | 56 | 1580 | 76 | 1577 | ||||
Total | 59 | 79 | ||||||
25 replicates | 30 replicates | |||||||
Condition = C | 1 | 83417 | 64.33 | 0.008 | 1 | 92409 | 62.40 | 0.010 |
Site(C) | 2 | 12967 | 79.26 | 0.001 | 2 | 14807 | 88.47 | 0.001 |
Residual | 96 | 1635 | 116 | 1673 | ||||
Total | 99 | 119 |
PERMANOVA analysis showed significant differences in coralligenous assemblages among areas, while differences between sites and locations were not significant (
Source | df | MS | Pseudo-F | P(perm) |
---|---|---|---|---|
Site = S | 1 | 27446 | 3.09 | 0.088 |
Location (A) = L(A) | 2 | 8879 | 0.94 | 0.477 |
Area (L(A)) | 4 | 9376 | 9.78 | 0.001 |
Residual | 112 | 958 | ||
Total | 119 |
The pseudo-variance components showed the highest variability at the smallest spatial scales (area and sample), whereas the variability at the intermediate spatial scales (Location) was undetectable (
The results of this study comparing different sampling procedures commonly used in the ecological investigation of coralligenous habitats provided indications about the method that could be most suitable for detecting changes in the structure of assemblages subjected to different levels of stress.
Both destructive and photographic methods detected significant differences between conditions and the same differences were observed when the destructive samples were analysed at the species level. These results, obtained considering both macroalgae and sessile animals, are in agreement with those highlighted by the comparison of macroalgal species and morphological groups chosen as descriptors of assemblages subjected to different stressors (
These findings suggest that the use of photographic techniques and the taxa/morphological groups approach may be a suitable and cost-effective method for studying coralligenous assemblages, in particular in monitoring programmes and environmental impact assessments; in fact, in these latter cases it is important to detect the early stages of environmental changes using procedures that allow a large number of samples to be examined in a limited time. Moreover, a non-destructive approach is suitable for sampling this particularly sensitive habitat, is the only one applicable in marine protected areas and is surely in line with the recent European Framework Directives.
The minimum area considered for studying rocky sessile assemblages in the Mediterranean Sea is 400 cm2, but this area was obtained through destructive sampling of macroalgal assemblages collected in the shallow subtidal systems (
Differences between locations tens of kilometres apart were low, suggesting that coralligenous assemblages show a homogeneous structure if subjected to similar environmental conditions, at least within the same geographic area. By contrast, coralligenous assemblages showed a high variability at small spatial scales, between plots one metre apart and areas hundreds of metres apart, while variability between sites several kilometres apart was very small. This finding agrees with results of previous studies (
The assessment of the most suitable relation between the sampling area and the number of replicates is an interesting topic for coralligenous ecologists. Large organisms are usually studied using sampling areas (
Summarizing the main results, impact evaluation on coralligenous assemblages may be effectively carried out through photographic samples larger than 1800 cm2, with a number of replicates larger than 10, by using taxa/morphological groups as descriptors and planning sampling designs with a high replication at the small spatial scales. This kind of methodological procedures seems to be a good compromise between habitat conservation, scientific validity and time/cost effort requirements.
Concerning this latter aspect, photographic sampling reduces the time of field work and makes it possible to quickly collect the high number of samples required in a habitat with high variability at small spatial scales. It also involves relatively little laboratory analysis, reducing the time and cost of sorting and taxonomist work. However, photographic samplings require a longer time of image analysis than in situ visual methods (
Although the study covered a limited geographical area, the results could provide basic information that can be integrated with data collected in other Mediterranean areas and in other studies using different approaches, in order to validate a sampling procedure applicable to the whole Mediterranean basin.
We wish to thank Joaquim Garrabou, Julie Deter and one anonymous referee, whose comments and suggestions have improved the manuscript. We also thank Angela Sarni for the English-Spanish translation
OCHROPHYTA | |
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erect corticated algae |
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erect corticated algae |
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erect corticated algae |
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algal turf |
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algal turf |
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* |
CHLOROPHYTA | |
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algal turf |
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algal turf |
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* |
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* |
Microdictyon tenuius Decaisne |
algal turf |
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* |
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* |
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RHODOPHYTA | |
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flattened Rhodophyta |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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erect corticated algae |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
Augier |
algal turf |
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erect corticated algae |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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articulated Rhodophyta |
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erect corticated algae |
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encrusting Corallinales |
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encrusting Corallinales |
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encrusting Corallinales |
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algal turf |
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flattened Rhodophyta |
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encrusting Corallinales |
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encrusting Corallinales |
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algal turf |
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flattened Rhodophyta |
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erect corticated algae |
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Peyssonnelia spp |
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Peyssonnelia spp |
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Peyssonnelia spp |
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flattened Rhodophyta |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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algal turf |
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flattened Rhodophyta |
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erect corticated algae |
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erect corticated algae |
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articulated Rhodophyta |
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algal turf |
PORIFERA | |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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erect sponges |
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erect sponges |
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erect sponges |
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erect sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
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massive encrusting sponges |
CNIDARIA | |
Anthozoa | |
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madrepores |
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* |
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* |
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* |
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* |
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* |
Hydrozoa | |
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hydroids |
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hydroids |
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hydroids |
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hydroids |
BRYOZOA | |
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encrusting bryozoans |
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encrusting bryozoans |
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encrusting bryozoans |
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encrusting bryozoans |
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encrusting bryozoans |
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erect bryozoans |
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* |
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* |
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encrusting bryozoans |
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|
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encrusting bryozoans |
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encrusting bryozoans |
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encrusting bryozoans |
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encrusting bryozoans |
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erect bryozoans |
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* |
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encrusting bryozoans |
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encrusting bryozoans |
CHORDATA | |
Ascidiacea | |
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ascidiaceans |
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ascidiaceans |
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ascidiaceans |