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

Abstract

Symbiotic relationships contribute considerably to the high biodiversity found on coral reefs. Coral-dwelling gall crabs (Cryptochiridae) represent a prime example of coral-associated invertebrates that exhibit obligate relationships with their host. The induction of a skeletal modification in the coral, used as a dwelling by the crab, is the most remarkable aspect of this close association. Here we examined Hapalocarcinus marsupialisStimpson, 1859, collected from Pocillopora and Stylophora corals in the Saudi Arabian Red Sea. Specimens were DNA barcoded, and five distinct clades were revealed, providing further evidence for the hypothesis that H. marsupialis is a species complex. Divergence (COI) between the clades ranged from 3.2 to 15.7%. The putative species were tested for differences in morphology and reproduction. Crabs were examined regarding morphometric characters (carapace length and width, pleon (abdomen), chelae, and growth patterns) and reproductive traits. The data were pooled and analysed by host genus and putative species, which revealed significant differences for most of the measured variables in female crabs. Specimens retrieved from Pocillopora were significantly larger (up to 49 %) and had higher fecundity than those inhabiting Stylophora. For reproductive output (RO) no differences at species- or host-genus level were detected. The average RO of ~70% over all specimens is high compared to other brachyurans, supporting the assumption that symbiotic brachyurans invest more energy in reproduction than their free-living counterparts. Discrepancies with published data on growth and reproduction of Hapalocarcinus are discussed. Our results demonstrate the usefulness of morphometric traits and fecundity in separating the clades in the H. marsupialis complex, and prepare the ground for further morphometric studies on the genus and other symbiotic brachyurans. Moreover, it highlights the need to check for the presence of cryptic species when studying aspects of the biology of a species.

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

“…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.…”

“…Patterns of host specificity can be confounded by unrecognized diversity. Recent investigations on H. marsupialis, a species distributed from western Indian Ocean to the Central Pacific (Wei et al, 2016;Bähr et al, 2021), showed important implications. 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).…”

“…Recent investigations on H. marsupialis, a species distributed from western Indian Ocean to the Central Pacific (Wei et al, 2016;Bähr et al, 2021), showed important implications. 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).…”

Interspecific coral competition does not affect the symbiosis of gall crabs (Decapoda: Cryptochiridae) and their scleractinian hosts

“…Cryptochirids occur worldwide, mostly on tropical shallow‐water reefs, but they have also been observed at depths below 500 m (Kropp & Manning, 1987 ). Currently, this family includes 53 species in 21 genera (WoRMS, 2023 ), yet several species complexes have recently been uncovered, and more species are awaiting description (Bähr et al, 2021 ; Xu et al, 2022 ). Gall crabs inhabit a broad range of corals from at least 66 genera (Castro, 2015 ).…”

“…Their host specificity was used as a scheme to define cryptochirid genera in the past (Fize & Serène, 1957 ), but this has been overhauled by major taxonomic revisions of both corals and crabs (Fukami et al, 2008 ; Huang et al, 2014 ; Kropp, 1990 ; Terraneo et al, 2017 ). With the exception of the generalist species Troglocarcinus corallicola Verrill, 1908, all cryptochirid species are host specific; some at host species level, whereas other crab species associate with one or more closely related coral genera (e.g., Bähr et al, 2021 ; van der Meij, 2015 ; van der Meij et al, 2015 ; Xu et al, 2022 ).…”