Abstract
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.
Highlights
Paul′son, 1875, represent a prime example of obligate symbiotic relationships on coral reefs (Castro, 1976)
Most of our samples were retrieved in clades HM.01 and HM.02; all these specimens were collected from Stylophora
After comparing with results by SETM et al, we found that sequences of specimens of clades HM.03, HM.04, and HM.05 clustered with those of other H. marsupialis individuals associated with Pocillopora corals, whereas specimens of clades HM.01 and HM.02 clustered with those inhabiting host corals Stylophora
Summary
Paul′son, 1875, represent a prime example of obligate symbiotic relationships on coral reefs (Castro, 1976). A remarkable detail of the symbiotic relationship with their host is the modification of the coral tissue resulting in the development of a dwelling or so-called “gall” (Verrill, 1867; Calman, 1900; van der Meij & Hoeksema, 2013). High investment rates were found in pinnotherids (66%–97%), which have a strongly broadened pleon similar to cryptochirids (Hines, 1992; Becker et al, 2011) These results are noteworthy because brood weight is generally constrained in nonsymbiotic brachyuran crabs to about 20% of female body weight due to the limitation in space available for yolk accumulation within the calcified cephalothorax (Kotb & Hartnoll, 2002 and references therein)
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