Diversification and distribution of gall crabs (Brachyura: Cryptochiridae: Opecarcinus) associated with Agariciidae corals

Abstract

Coral reefs are home to the greatest diversity of marine life, and many species on reefs live in symbiotic associations. Studying the historical biogeography of symbiotic species is key to unravelling (potential) coevolutionary processes and explaining species richness patterns. Coral-dwelling gall crabs (Cryptochiridae) live in obligate symbiosis with a scleractinian host, and are ideally suited to study the evolutionary history between heterogeneous taxa involved in a symbiotic relationship. The genus Opecarcinus Kropp and Manning, 1987, like its host coral family Agariciidae, occurs in both Indo-Pacific and Caribbean seas, and is the only cryptochirid genus with a circumtropical distribution. Here, we use mitochondrial and nuclear DNA gene fragments of Opecarcinus specimens sampled from 21 Indo-Pacific localities and one Atlantic (Caribbean) locality. We applied several species delimitation tests to characterise species diversity, inferred a Bayesian molecular-clock time-calibrated phylogeny to estimate divergence times and performed an ancestral area reconstruction. Time to the most recent common ancestor (tMRCA) of Opecarcinus is estimated at 15−6 Mya (middle Miocene—late Miocene). The genus harbours ~ 15 undescribed species as well as several potential species complexes. There are indications of strict host-specificity patterns in certain Opecarcinus species in the Indo-Pacific and Atlantic, however, a robust phylogeny reconstruction of Agariciidae corals—needed to test this further—is currently lacking. The Indo-West Pacific was inferred to be the most probable ancestral area, from where the Opecarcinus lineage colonised the Western Atlantic and subsequently speciated into O. hypostegus. Opecarcinus likely invaded from the Indo-West Pacific across the East Pacific Barrier to the Atlantic, before the full closure of the Isthmus of Panama. The subsequent speciation of O. hypostegus, is possibly associated with newly available niches in the Caribbean, in combination with genetic isolation following the closure of the Panama Isthmus.

Diversity of coral-associated pit crabs (Crustacea: Decapoda: Cryptochiridae) from Hong Kong, with description of two new species of Lithoscaptus A. Milne-Edwards, 1862

“…Despite highly conserved morphologies, genetic data based on materials amassed across the distribution range recognize multiple putative species, each biologically distinct (COI divergences range from 3.2% to 15.7%; Bähr et al, 2021), and putative species differ in recorded host (of Pocillopora, Seriatopora, and Stylophora; Wei et al, 2016;Bähr et al, 2021) and early larval characters (Gore et al, 1983). A similar pattern is also observed among Opecarcinus species by Xu et al (2021), who showed the current taxonomic understanding (of nine species; Kropp and Manning, 1987;Kropp, 1989) being a gross underestimation of true diversity (at least 25 OTUs). Both cases illustrate another aspect of "crypticity," revealing a discrepancy between the current species inventory and unrecognized true diversity of species or OTUs.…”

“…These values are comparable with threshold values previously reported for thoracotreme crabs (1.49%-6.25%), from studies of groups in which taxonomy is relatively well resolved, and the examined taxa show discrete morphologies (see Davie et al, 2010;Shih et al, 2012Shih et al, , 2019KJH Wong et al, 2012). However, cryptochirid taxonomy is nowhere well resolved in two sense: numerous previously unrecognized species-level taxa (Wei et al, 2016;Bähr et al, 2021;Xu et al, 2021; see below) and previous morphology-based generic placement insufficient in encapsulate true species richness, thus rendered composite (van der Meij and Nieman, 2016; see Remarks under L. doughnut above). As such, both our intra-and interspecific distance values show considerable range of variation and are probably inflated.…”

Morphology and reproduction in the Hapalocarcinus marsupialis Stimpson, 1859 species complex (Decapoda: Brachyura: Cryptochiridae)

“…Further research, taking the findings of Wei et al (2016) and SETM et al (unpublished data) into account, will help to determine which putative species are closely related. A recent study on the cryptochirid genus Opecarcinus Kropp & Manning, 1987 revealed that most Red Sea species have a sister species in the wider Indo-West Pacific (Xu et al, 2021), hence we cannot assume the five Red Sea putative species are sister species. The within-species variation (p-distance) is low and the smallest difference between groups (3.2%) was found for clades HM.01 and HM.02 (Table 2).…”

New species of Polycystididae (Platyhelminthes: Kalyptorhynchia) from Cuba and the Pacific coast of Panama

“…The rise of the Isthmus of Panama restricted gene flow between marine populations on both sides of the land, triggering vicariance speciation processes (Thacker 2017;Lima et al 2020;Xu et al 2022). However, some species occur on both coast sides today, e.g., some crustaceans (Ferreira & Anker 2021), and recent studies found evidence of transoceanic fish invasions through the Panama Canal (Castellanos-Galindo et al 2020).…”