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. 2022 Apr 29;17(4):e0267086.
doi: 10.1371/journal.pone.0267086. eCollection 2022.

Biogeography of the coastal fishes of the Socotra Archipelago: Challenging current ecoregional concepts

Uwe Zajonz  1   2 , Edouard Lavergne  1   3 , Sergey V Bogorodsky  1 , Friedhelm Krupp  1
Affiliations

Biogeography of the coastal fishes of the Socotra Archipelago: Challenging current ecoregional concepts

Uwe Zajonz et al. PLoS One. .

Abstract

The Socotra Archipelago, located in the eastern Gulf of Aden, has a unique marine environment, which combines tropical and 'pseudo-temperate' elements. An updated species inventory recently considered its coastal fish diversity the highest among Arabian ecoregions, necessitating to re-assess the ichthyogeographic position of the island group. The main aim of this study is to describe the distributional biogeography of its coastal fish fauna in relation to contemporary ichthyogeographic and ecoregional concepts. Inferences are drawn with regard to the marine biogeographic arrangement and ecoregional partitioning of the Arabian region. The main datasets comprise eight and twenty selected families including 404 and 898 species, respectively, from Arabian ecoregions. The Socotra Archipelago has close affinities to a putative ecoregion in the eastern Gulf of Aden that extends to southern Oman. It is more closely related to the Arabian Sea coast of Oman than to ecoregions in the Red Sea and a putative ecoregion in the western Gulf of Aden. The Gulf of Aden does not represent a consistent ecoregion in ichthyogeographic terms, because its eastern and western parts are less closely related to one another than to other ecoregions. The Socotra Archipelago and the eastern Gulf of Aden should therefore not be assigned to a joined province with Red Sea ecoregions. The coastal fish faunas of the southern Red Sea have close affinities with those of the western Gulf of Aden. The Arabian/Persian Gulf is least related to the other Arabian ecoregions. The authors posit the Socotra Archipelago as a distinct ecoregion, either on its own or in combination with affiliated mainland areas. This best reflects the ichthyogeographic data and the exceptionally high levels of fish and overall marine diversity. Two alternative ecoregional delineations are proposed, serving as working hypotheses for onward research.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1
North-western Indian Ocean sections of three current global marine “biogeographic” classification schemes: (a) ‘Marine Ecoregions’ (Spalding et al. 2007), (b) ‘Marine/Fish Biogeography’ (Briggs and Bowen 2012), (c-e) ‘Global Reef Fish Biogeography’ (Kulbicki et al. 2013, with three alternatives); as evaluated in the present article. (a-e: Baseline map sourced 2019 from Natural Earth, free vector and raster map data with no copyright restrictions. a, e: Ecoregion boundaries redrawn from GIS data accompanying Spalding et al. (2007), sourced 2012 from , with free permission for scientific use and reproduction (to date only available from ), colours modified.b-d: Redrawn manually by the authors according to the original publications, colours modified).
Fig 2
Fig 2
Overview maps of the study area, showing a) areas in the Arabian Region studied by the authors, including the geographic location of the Socotra Archipelago in the north-western Indian Ocean and the main survey areas included in the subregional analyses (rectangles), with b) fish inventory survey sites including four transect sites in the Yemeni Red Sea of 2003 and 2004, and c) transect sites (T) and ecological monitoring sites (M) in the Socotra Archipelago from 1999 to 2007. The transect sites at Ras Belhaf at the mainland Gulf of Aden coast of Yemen of 2005 are not shown on a separate map. (a: Baseline map sourced 2019 from Natural Earth, free vector and raster map data with no copyright restrictions.b: Baseline map sourced 2019 from under license unported license, made or improved in the German (Map Lab).c: Baseline map owned by the authors, sourced from GIS data based on LandSat TM5 satellite images owned by the authors (map credits: R. Klaus)).
Fig 3
Fig 3
Incidence-based resemblance pattern of 29 subregional semi-standardized fish survey sites (belt transects and detailed fish inventories of near-similar survey effort, data of transects presence-absence transformed), composed of 13 fish transect sites on Socotra Archipelago of 2003 and 2007, 5 fish transect sites on Ras Belhaf, eastern Yemen, of 2005, and 11 survey sites including 7 inventories and 4 transect sites in the Yemen Red Sea from 2003 and 2004, with symbols representing a priori basin group designation overlaid; comparing (a) a dendrogram plot of a hierarchical agglomerative cluster analysis of 403 recorded species according to Hellinger distance (within-cluster data structure without SIMPROF support indicated in red), and (b) a non-metric multidimensional scaling analysis, based on data of the 8 key families only from the same data set. See Fig 2 for locations.
Fig 4
Fig 4
Resemblance pattern of 10 putative Arabian ecoregions and Kenya (8 key families, 404 species) based on Hellinger’s distance, represented as plots of the (a) dendrogram of the hierarchical agglomerative cluster analysis, and the (b) non-metric multidimensional scaling analysis (nMDS), both superposed with symbols representing the statistically (ANOSIM) most valid a priori-province-level Combination U (Table 1); compared with the same nMDS plot superposed with symbols representing the unsupported (ANOSIM) combinations (c) A1 following Spalding et al. (2007) and (d) A2b following Briggs and Bowen (2012) (province-level combinations A2a, A3b and A3a not shown because they are considered invalid according to ANOSIM; see S4 Fig).
Fig 5
Fig 5
Resemblance pattern of 10 putative Arabian ecoregions and Kenya (8 key families, 404 species) based on Hellinger distance, represented as plot of the dendrogram of the hierarchical agglomerative cluster analysis, according to separate checklists for the eastern Gulf of Aden (e Go Aden) and southern Oman (s Oman), verifying the proposed Eastern Gulf of Aden extended (e Go Aden ext.) ecoregion used in the analysis presented in Table 1 and Fig 4.
Fig 6
Fig 6. Resemblance pattern of 10 putative Arabian ecoregions and Kenya (20 core families, 898 species) based on Hellinger distance, represented as plot of the dendrogram of the hierarchical agglomerative cluster analysis, verifying the analysis of 8 key families shown in Fig 4 (within-cluster data structure without SIMPROF support indicated in red).
Fig 7
Fig 7
Comparison of species richness in 10 putative Arabian ecoregions (modified from Spalding et al. 2007), and Kenya as external reference (* surrogate for the eco-regions ’Central Somali Coast’ and northern part of ’Northern Monsoon Coast’ of Spalding et al. 2007), according to (a) 8 key families (modified from Zajonz et al. 2019), and (b) 12 additional core families (this study).
Fig 8
Fig 8. Resemblance pattern of 10 putative Arabian ecoregions, and countries and island groups of the wider Northern, Western and Central Indian Ocean.
The dendrogram (a) of the hierarchical agglomerative cluster analysis of 604 species in 8 key families is compared to the dendrogram (b) of the hierarchical agglomerative cluster analysis of 1292 species in 20 core families; with (a) superposed with symbols representing the statistically (ANOSIM) most significant a priori Combination U of province-level designations for Arabian ecoregions (compare Fig 4a) (within-cluster data structure without SIMPROF support indicated in red).
Fig 9
Fig 9
Provisional alternative delineations of Arabian ecoregions and provinces, modifying Spalding et al. (2007) by (a) tripartitioning ecoregion E89, designating the Socotra Archipelago a distinct ecoregion (e89c) besides the Western (e89a) and extended Eastern Gulf of Aden (e89b), and modifying ecoregion E92 (and a slight modification of ecoregion E93); (b) bipartitioning ecoregion E89 into a Western Gulf of Aden (e89a) and a common ecoregion for the extended Eastern Gulf of Aden and Socotra (e89b2), and modifying ecoregion E92 (amended provinces are indicated by shared colours, and boundary delineations that require further confirmation are indicated by dashed lines). (a, b: Baseline map sourced 2019 from Natural Earth, free vector and raster map data with no copyright restrictions. Ecoregion boundaries redrawn and partly modified from GIS data accompanying Spalding et al. (2007), sourced 2012 from , with free permission for scientific use and reproduction (to date only available from ), colours modified).
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Publication types

Grants and funding

The authors received no specific funding for this particular study. Data upon which the present study is based was first collected under the auspices of the UNDP-GEF project ‘Conservation and Sustainable Use of Socotra Archipelago’ (UNOPS YEM/96/G32, C-972248). The ‘Socotra Conservation and Development Programme’ (SCDP) of UNDP supported subsequent field work between 2006 and 2008. From 2008 onwards field work was primarily conducted at the Socotra Field Research Station of the Senckenberg Biodiversity and Climate Research Centre (SBiK-F) with financial support of the research funding programme ‘LOEWE – Landes-Offensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz’ of Hesse’s Ministry of Higher Education, Research, and the Arts. From 2016 to 2020 the lead author worked for the UNEP-GEF ‘Socotra Project.’ The referenced projects comprise the accumulation of work that made this particular study possible.