Feeding behaviour of the clown crab Platypodiella picta upon two species of the genus Palythoa
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1
Universidad de La Laguna, Spain
Platypodiella (Decapoda, Xanthidae) is a genus of relative small crabs, called clown crabs, mainly known for living in close association with Zoantharia (Hexacorallia, Cnidaria), especially with those belonging to the genus Palythoa (Den Hartog and Holthuis 1984). Different kinds of associations are frequently found among marine organisms, such as mutualism, symbiosis or commensalism, including species that become poisonous only after feeding from their prey (Cheney et al. 2016). For example, it has been found that specimens of Platypodiella spectabilis exhibited huge concentrations of palytoxin in their body tissues, an extraordinary toxic compound of zoantharian, mainly of the genus Palythoa (Gleibs et al. 1995). Direct observations of Platypodiella crabs feeding on zoantharians, which could help elucidate whether specimens are able to acquire these palytoxins by feeding on Palythoa, have not been up to date reported.
Palythoa spp. are organisms very common in tropical and subtropical benthic ecosystems, where some species are currently expanding their populations and even replacing altered hard-corals ecosystems (Cruz et al. 2016). Given the inherent features of zoantharians, such as their prolific mat-forming growth and asexual reproduction, these population outbreaks can alter the structure of whole ecosystems, so there is a need to improve our understanding of any ecological forces that can directly or indirectly control populations. The increased dominance of Palythoa spp. in the habitat may also lead to an increase in associated Platypodiella spp. populations. In this sense, the aim of this study was to provide new information about the nature of this particular relationship between clown crabs and Zoantharia, in order to establish if there is any predatory control of colonies by the crab and a potential to buffer such zoantharian populations.
In order to do so, colonies of the two most common Palythoa spp. found in the Canary Islands, P. caribaeorum and P. aff. clavata (López et al. 2019), were collected and immediately transferred to the laboratory in humid and dark conditions. Fragments of 6–8 cm2 of P. caribaeorum and 7 to 10 polyps of P. aff. clavata were glued in artificial substrates and placed in tanks with running filtered sea water and 12 h light / 12 h dark cycle under LED lighting. Four colonies of P. caribaeorum and four of P. aff. clavata in a total of four tanks were separately exposed to one individual of Platypodiella picta. We included crabs of different gender and sizes to avoid any possible bias related to sexual dimorphism and/or ontogenetic changes. To standardize feeding status between individuals, crabs were not fed during two days previous to experiment commencement (Scheibling and Robinson 2008). Respective controls without predatory crabs were established using another set of four colonies per species and tank. Susceptibility of zoantharians to the predatory crab P. picta was assessed by means of calculating predation rates upon experimental specimens as the difference in the weight of colonies between the commencement and the end of the experiment (5 days). A distance-based permutational analysis of variance (ANOVA) (Anderson 2001) was used to evaluate differences in predation rates and feeding behaviour among crabs due to their intrinsic characteristics of gender and size. Additionally, a total of 60 P. picta were collected for morphometric measurements of the carapace width and claw lengths to determinate if there are in fact any differences between males and females.
Results of this study showed that Platypodiella picta is an active predator of Palythoa species (Fig. 1). Experimental colonies showed a significant decrease in weight in the presence of P. picta (F= 15.48, p= 0.001 and F= 15.04, p= 0.001 for P. aff. clavata and P. caribaeorum, respectively), being more noticeable in the case of P. caribaeorum (-1.0879 ± 0.75 g) (Fig. 1). We found a positive linear relationship between carapace width and claw length in both sexes of P. picta, without differences between males and females (13,70 ± 2,16 and 13,91 ±2,51 in average, respectively, Fig. 2a). In fact, no significant effect of either crab sex or size were detected in analyses and several bites were observed in the experimental colonies, as well as crabs directly eating pieces of both Palythoa spp. Platypodiella specimens have been observed associated to zoantharians in several sites around the Canary Islands and, although they usually remain cryptic in their habitats, high abundances of P. picta are easily observed in large colonies of P. caribaeorum. This fact may be related the greater predation activity of the crab observed upon this species in the present study. Results suggest that crabs may not only find a source of food in these kinds of habitats but also shelter from predators, due to the intrinsic characteristics of this species with polyps immersed in a well-developed coenenchyma (Pax 1910). Additionally, preferences of P. picta could be related to concentrations of palytoxins, which is specie-specific, being P. aff. clavata 90% more toxic than P. caribaeorum (Sawelew et al. 2018). Nevertheless, feeding on the high biodiversity of mesoinvertebrates that also live in association with zoantharians (González-Delgado et al. 2018) cannot be excluded and further multiple choice experiments should be considered. P. caribaeorum has high environmental tolerance and northward expansion of this species is expected to occur under future mild climate scenarios of change (Durante et al. 2018). Given the nature of the close relationship reported here, changes in population structure and distribution of P. caribaeorum could be also joined to changes in Platypodiella spp. abundances.
Fig. 1. Results of predation experiments with P. aff. clavata and P. caribaeorum, showing mean differences in colony weight (±SD) after 5 days of exposure to the crab Platypodiella picta (E: experimental colonies) and respective control conditions without crab (C: control colonies). Specimen of P. picta with a polyp of P. aff. clavata in its claw is also shown.
Fig. 2: Relationship between carapace width and claw length of the clown crab Platypodiella picta (a).
Acknowledgements
CL was co-funded by the Canarian Agency for Research, Innovation and Information Society of the Ministry of Economy, Industry, Trade and Knowledge and by the European Social Fund (ESF) integrated operational program of the Canary Islands 2014–2020. This research was conducted within the framework of the ‘INDICATROP’ project conducted with the financial support of ‘CajaCanarias’ Foundation.
References
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Keywords:
predation,
Zoantharia,
commensalism,
P. aff. clavata,
P. caribaeorum
Conference:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.
Presentation Type:
Poster Presentation
Topic:
Ecology, Biodiversity and Vulnerable Ecosystems
Citation:
Batista
CL and
Clemente
S
(2019). Feeding behaviour of the clown crab Platypodiella picta upon two species of the genus Palythoa.
Front. Mar. Sci.
Conference Abstract:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) .
doi: 10.3389/conf.fmars.2019.08.00160
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Received:
15 May 2019;
Published Online:
27 Sep 2019.
*
Correspondence:
Miss. Cataixa L Batista, Universidad de La Laguna, San Cristóbal de La Laguna, Spain, kattaissa@gmail.com