ECOLOGY OF THE FACULTATIVE PHYTOTELMIC CRAB PLATYCHIRARMA BUETTIKOFERI (DE MAN, 1883) (DECAPODA, BRACHYURA, SESARMIDAE)

The mangrove forests surrounding Douala, Cameroon, have a long history of human impact due to rapid urbanization, resulting in patches of true mangrove forest interspersed with areas dominated by the mangrove-associated Pandanus species. A survey carried out to assess the possible shift in crab fauna composition and dominance between mangrove and Pandanus dominated areas revealed an undescribed phytotelmic habitat for Platychirarma buettikoferi . This species was found colonizing the natural reservoirs of fresh water represented by Pandanus spp. Our results show P. buettikoferi to be ecologically dependent upon Pandanus spp., and absent on the mangrove ﬂoor and trees, strongly indicating a phytotelmic adaptation. Our data suggest that this species is a facultative phytotelmic species well adapted to Pandanus spp. dominated areas. Further studies on the reproductive biology and feeding behaviour of P. buettikoferi may shed light on this unique phytotelmic lifestyle and its role in mangrove ecosystem functioning. Les forêts de mangroves entourant Douala, au Cameroun, ont une longue histoire d’impacts humains en raison de l’urbanisation rapide. Celle-ci résulte en des parcelles de forêt de mangrove entrecoupées de zones dominées par Pandanus , espèce associée à la mangrove. Une étude faite pour évaluer le changement dans la composition faunistique de crabes et la dominance entre les zones dominées par les mangroves et les Pandanus a révélé un habitat phytotelmique pour Platychirarma buettikoferi , jamais décrit auparavant. Cette espèce a été trouvée en train de coloniser les réservoirs naturels d’eau douce représentés par Pandanus spp. Nos résultats montrent que P. buettikoferi est écologiquement dépendant de Pandanus spp., et absent du sol et des arbres de la mangrove, ce qui indique fortement une adaptation phytotelmique. Nos données suggèrent que cette espèce est une espèce phytotelmique facultative bien adaptée aux zones dominées par Pandanus spp. D’autres études sur la biologie reproductive et le comportement alimentaire de P. buettikoferi pourraient documenter ce mode de vie phytotelmique unique, ainsi que son rôle dans le fonctionnement de l’écosystème de la mangrove.

The Wouri River Estuary in Cameroon is one of the most thriving mangrove ecosystems in West Africa (UNEP, 2007); but, due to population growth and shipping activity in Douala harbour and to an oil platform close to the Gulf of Guinea, it is also one of the most impacted (Scheren, 2002;Alemagi, 2007). In particular, rapid population growth in Douala has led to large-scale deforestation due to a growing demand for housing and logging cultivation (Saenger et al., 2008;Nfotabong-Atheull et al., 2009, 2013. This urbanization has transformed hectares of forest dominated by mature mangrove trees, such as Rhizophora spp. and Avicennia germinans (L.) Stearn, 1764, into highly degraded areas with a dramatic change in botanical assemblages (Din & Baltzer, 2008;Nfotabong-Atheull et al., 2011). In many of the affected areas, the mangrove-associated screw-pines of the genus Pandanus (Tomlinson, 1986) rapidly took over and are now the dominant species to appear in the vegetation secondary succession. Pandanus spp. (Pandanaceae) are known to shelter animals from predators, such as the micro-mammal Sminthopsis virginiae de Tarragon, 1847 (Braithwaite & Lonsdale, 1987), and to provide a suitable habitat for many birds (Verbeek et al., 1993). They also have an important role in the feeding ecology of terrestrial decapods such as Cardisoma carnifex Herbst, 1796, and Birgus latro Linnaeus, 1767(Grubb, 1971Lee, 1985).
A recent survey of Cameroonian mangroves described a high biodiversity of macrobenthonic brachyuran crabs belonging to the family Sesarmidae (see Fusi et al., 2016). Of the eight species found to inhabit the mangroves and adjacent habitats, Platychirarma buettikoferi (De Man, 1883) appeared to be frequently associated with Pandanus trees. The present study was designed to investigate the possible phytotelmic association of P. buettikoferi with Pandanus trees of the Wouri estuary and the changes in population dynamics resulting from the ecological shift from mangrove to screw-pine dominated forests.

Species studied
Platychirarma buettikoferi (Sesarmidae Dana, 1851) has been recorded along the West African coast from Liberia to Angola (Manning & Holthuis, 1981). The species is easily recognized by its characteristic purplish blue flat chelipeds with red tips (in live animals). Brightly coloured male chelipeds, in particular, are important for signalling females (Manning & Holthuis, 1981). The species is usually found in the landward part of true mangrove forests, beyond the area reached by the highest high tides (see Rathbun, 1921: 450). Ovigerous females were reported in March, June, July-August, and October-December (Rathbun, 1921;Rossignol, 1957;Manning & Holthuis, 1981). In Cameroon, P. buettikoferi has been recorded under the humid leaf litter found on the banks of a riverine forest close to the sea in Kribi and in the Wouri estuary mangrove forests (Fusi et al., 2016).

Study area
This study was carried out in and around the mangrove forests of the Wouri River estuary, one of the largest estuaries in West Africa, surrounding the city of Douala, Cameroon ( fig. 1). The climate is equatorial, and the area receives high rainfall, on average about 4000 mm per year. Most of the rain falls between March-November (rainy season), with a short dry season often punctuated by showers in December-February. The humidity is high year-round (approximately 100%).
The survey was performed in September and October 2009 in two forests: Wouri Bridge, hereafter abbreviated to WB (4°4 19 N 9°42 58 E), and Essengue, hereafter abbreviated to ES (4° 53 N 9°40 39 E). WB is located north of Douala, close to the bridge crossing the Wouri River. Part of this forest was cleared 20 years ago for construction of the bridge and currently the whole forest has regrown with extended patches of Pandanus candelabrum P. Beauv. associated with mature forest dominated by Avicennia sp. and Rhizophora spp. (Din & Baltzer, 2008).
ES forest, located south of Douala at the edge of the harbour, was recently affected by clearing due to rapid development of the commercial port. This forest is colonized by several species of mangrove-associated flora (Nfotabong-Atheull et al., 2013), including Nypa fruticans (Thunb.) Wurmb., Guibourtia demensei Harms, and P. candelabrum.
The locality of Bois des Singes (4°0 49 N 9°40 28 E), hereafter BS, located southwest of Douala, was also considered since there no Pandanus spp. were found. Similarly to WB, BS is a mature forest manly dominated by Rhizophora racemosa Meyer, 1818, Rhizophora mangle L., 1764, and Rhizophora harrisonii Leechman, 1918. We considered this forest as a control to verify the exclusive relationship between P. buettikoferi and Pandanus candelabrum.

Sampling
A total of 49 specimens of Pandanus spp. (39 in WB and 10 in ES; fig. 2) were randomly chosen along two transects in each forest. For every plant, we checked the phytotelmata present and recorded the number of crabs, the crabs' species, each crab's sex and the height above the ground of the inhabited phytotelmic microhabitat. In cases where specimens were small and it was not possible to determine their sex, they were classified as juveniles. We then categorized the phytotelmata into three categories: 0-100, all animals found within the first 100 cm from the ground (the lowest leaf axils were recorded at 10 cm above ground); 101-200, all animals found between 101 cm and 200 cm; and >200 cm, all animals recorded at heights greater than 200 cm. To assess the overall composition of brachyuran fauna, and eventually verify the exclusive association of P. buettikoferi with Pandanus candelabrum, two random transects (100-500 m apart) were established in each vegetation belt (Rhizophora, Pandanus or Avicennia dominated) in each forest. Along each transect, three 2 × 2 m 2 quadrats were randomly sampled to assess the abundance and density of the brachyuran populations. Based on the complexity of the habitat and the varied behaviour of the studied species, different sampling techniques were used to assess the abundance of the various groups of macrofauna (for more information about the sampling methodology see Skov et al., 2002;Fusi et al., 2016). For safety reasons, this survey could not be performed for the ES forest. Sediment water and leaf axil water temperature, conductivity, and pH were recorded by performing in situ measurements with measuring sensors (Acorn pH 6 meter Oakton Instruments ® , Vernon Hills, IL, U.S.A.). These three variables are important because they affect the physiology of the animals. Temperature is one of the most important variables defining the metabolism of animals and is crucial for ectotherms whose body temperature matches the environmental temperature, and therefore it governs the biochemical processes of the animals. Conductivity indicates the presence of solutes in the water. Finally, pH is very important for crabs because it can affect their exoskeleton by dissolving the calcium carbonate structures. The amount of leaf axil water retrieved was 10-20 ml. Crab specimens collected from phytotelmata were identified using Monod (1956) and the nomenclature reviewed following Shahdadi et al. (2019); sex and reproductive stage (ovigerous or non-ovigerous) were recorded and photographed. Ten specimens were stored in alcohol (70%) and deposited in the Zoological Collection of the Museum of Zoology of the University of Florence.
PERMANOVA (Anderson et al., 2008) was used to test the null hypothesis of no differences in P. buettikoferi population characteristics, pH and conductivity across the factors Site (fixed, orthogonal, two levels: WB -Wouri Bridge and ES -Essengue) and Transect (random, nested in Site, 2 levels). Statistical analyses were performed using the software PRIMER 6 (Anderson et al., 2008). RESULTS We recorded a significantly different assemblage of Platychirarma buettikoferi in the two sites (PERMANOVA: F = 7.41; df = 1.46; p<0.01), but no significant differences among the three categories of leaf axil height where the species was found. P. buettikoferi was persistently present in all the 49 leaf axils of Pandanus candelabrum examined.
A similar number of males, females and ovigerous females were found in every leaf cluster analysed from WB forest (average 4.7 ± 0.7 individuals), evenly distributed among sexes, and no statistical differences were detected among different height classes (table I; fig. 3). In ES forest, an average of 9.6 ± 1.37 individuals was recorded evenly distributed among the height classes, with a more pronounced variability of sex ratio distribution. No other species were found in the leaf axils, except for one individual sesarmid Armases elegans (Herklots, 1851) (0-100 cm height in WB) and one individual of a centipede (Scolopendridae, 0-100 cm height in ES). Despite the presence of other crab species, no specimens of P. buettikoferi were found on the forest floor, in either the Pandanus vegetated belt, or in the Rhizophora and/or Avicennia belts (figs. 3, 4).
All water temperatures recorded were around 27.3 ± 0.9°C, with no differences between ground and axil water. Conductivity was significantly higher in phytotelmic water than in sediment water ( fig. 5)    Axil water pH and conductivity, height (= height from the ground), total number of specimens, and sexual assemblage are presented.
role (Fusi et al., 2016). Crabs, in particular, have a wide range of adaptations that allow them to colonize niches created by the complex interaction of vegetation structure and tidal regime (Kristensen, 2008;Nagelkerken et al., 2008;Sousa & Dangremond, 2011;Simoni et al., 2013). Furthermore, they play a pivotal role in mangrove ecosystem resilience (Cannicci et al., 2008;Kristensen, 2008;Lee, 2008Lee, , 2015Dahdouh-Guebas et al., 2011;Ng et al., 2015). Our data described the presence of Platychirarma buettikoferi in all leaf axils of Pandanus candelabrum examined. Although this species has already been recorded in Cameroon (Manning & Holthuis, 1981 and ref. of Mr. Powell within, on p. 351;Ngo-Massou et al., 2012;Fusi et al., 2016), its close association with P. candelabrum described here in detail, and therefore its phytotelmic habit, has not previously been reported. Many animals use P. candelabrum for various purposes, for example shelter and food (Grubb, 1971;Lounibos, 1979;Lee, 1985;Braithwaite & Lonsdale, 1987;Verbeek et al., 1993;Cumberlidge et al., 2005), but P. buettikoferi seems to exclusively exploit the microhabitat formed by the complex leaf clusters. A similar adaptation has been described for another sesarmid species, Scandarma lintou, found to strictly co-occur with Pandanus odoratissimus L. f., 1782, in southern Taiwan, in close proximity to streams or freshwater pools sheltered from strong winds (Schubart et al., 2003).
P. candelabrum leaves provide P. buettikoferi with a freshwater habitat that is physico-chemically different from the pore water of the sediment. Although pH was variable, the leaf axil water had a higher conductivity than the ground water, indicating the presence of more solutes. This reinforces that this aquatic microhabitat can be more stable (i.e., more able to buffer change in pH) than ground water, thereby avoiding potential stress for P. buettikoferi. P. candelabrum leaf axils were also found to collect potentially important food sources for this sesarmid, such as propagules of the taller Avicennia sp. trees as well as leaves and other mangrove litter (Sousa & Dangremond, 2011). Additionally, we consistently found males, females and ovigerous females in every cluster analysed. Faecal pellets were found in almost all of the axils analysed, which suggests a high fidelity to the leaf. The leaf clusters could also provide refuge from the high predation pressure exerted by the mudskipper Periophthalmus papilio (Bloch & Schneider, 1801), found in very high densities in WB on the forest floor (Etim et al., 1996), and other predators (Cannicci et al., 1998). The coincident distribution of P. buettikoferi and P. candelabrum in the Wouri estuary mangroves suggests that this is a tight association. We found no specimens of P. buettikoferi in the mature forests dominated by Rhizophora spp. in the same area, such as Bois de Singes ( fig. 4). This association is corroborated by a previous study that placed P. buettikoferi at the landward side of mangrove forests and did not describe the species as a true mangrove crab (Rathbun, 1921). Furthermore, we found no specimens of P. buettikoferi throughout one month of observation, while other sesarmids were abundant such as Chiromantes angolense (de Brito Capello, 1864), Guinearma alberti (Rathbun, 1921) . 4).
The adaptation to live in a pioneering plant such as P. candelabrum seems to be convergent among different crab species, as documented for Metapaulias depressus in Jamaica (Schubart, 2007), Malagasya goodmani (Cumberlidge, Boyko & Harvey, 2002) and the sesarmid Labuanium gracilipes (H. Milne Edwards, 1854) in Madagascar (Cumberlidge et al., 2005), Geosesarma foxi (Kemp, 1918) and Geosesarma serenei (Ng, 1986)  In the present study, we could not prove that P. buettikoferi is a strictly phytotelmic Fig. 5. The pH and conductivity of ground water (taken at ground level) and phytotelma Pandanus candelabrum P. Beauv. water at the two study sites. Data are expressed as mean ± standard error. N = 14 for Wouri Bridge and N = 10 for Essengue for pH; and N = 12 for Wouri Bridge and N = 10 for Essengue, for Conductivity.
species, but our findings suggest that the vegetation shift induced by human disturbance can at least offer a phytotelmic habitat to this crab species. In fact, the colonization of phytotelmata could also be the proximate factor behind the evolution of its flat chelipeds, unique to P. buettikoferi and very rare in the Sesarmidae ( fig. 2). Indeed, a flat cheliped can help this crab hide within the leaf axils of P. candelabrum and, more generally, better exploit the shelter that this vegetation provides ( fig. 2A). A similar characteristic has been observed in the crab S. lin-tou living in Pandanus odoratissimus, that exhibits conspicuously flattened fingers (Schubart et al., 2003). The permanent presence of water in these microhabitats could represent a physiological advantage for P. buettikoferi, providing a safe habitat in which to excrete catabolites while avoiding desiccation (Anger, 1995) and representing a water source to brood embryos Vogt, 2013).
To confirm the phytothelmic behaviour, the occurrence of ovigerous females is another proof that corroborates our result, although further investigations of egg sizes and the larval development are needed (Anger, 1995;Diesel, 1997).
In conclusion, we reveal a previously undescribed, possibly facultative, phytotelmic habit of P. buettikoferi, which may enable this species to colonize mangrove areas occupied by P. candelabrum.