Tuesday, September 26, 2017

[Crustacea • 2017] Diogenes heteropsammicola • A New Species of Hermit Crab (Decapoda, Anomura, Diogenidae) Replaces A Mutualistic Sipunculan in A Walking Coral Symbiosis


 Diogenes heteropsammicola 
 Igawa & Kato, 2017


Abstract

Symbiont shift is rare in obligate mutualisms because both the partners are reciprocally dependent on and specialized to each other. In the obligate accommodation–transportation mutualism between walking corals and sipunculans, however, an unusual saltatory symbiont shift was discovered. In shallow waters of southern Japan, an undescribed hermit crab species was found living in corallums of solitary scleractinian corals of the genera Heterocyathus and Heteropsammia, replacing the usual sipunculan symbiont. We described the hermit crab as a new species Diogenes heteropsammicola (Decapoda, Anomura, Diogenidae), and explored its association with the walking corals. This hermit crab species obligately inhabits the coiled cavity of the corals, and was easily distinguished from other congeneric species by the exceedingly slender chelipeds and ambulatory legs, and the symmetrical telson. Observations of behavior in aquaria showed that the new hermit crab, like the sipunculan, carries the host coral and prevents the coral from being buried. This is an interesting case in which an organism phylogenetically distant from Sipuncula takes over the symbiotic role in association with a walking coral. The hermit crab species is unique in that its lodging is a living solitary coral that grows with the hermit crab in an accommodation–transportation mutualism.


Fig 6. Diogenes heteropsammicola sp. nov. in life. A, an individual in an aquarium, carrying the coral. 

Fig 6. Diogenes heteropsammicola sp. nov. in life. B, an individual removed from its host coral. Scale bar: 1 mm.

Taxonomic account
Genus Diogenes Dana, 1851

Diogenes heteropsammicola sp. nov.

Fig 7. Behavior of Diogenes heteropsammicola sp. nov.
A–C, sequence of behaviors to recover from an overturned to upright position in which the hermit crab leans out of the overturned coral (A), grasps the bottom with its ambulatory legs and left cheliped (B), and turns the coral upright using the pleon (C); D–F, sequence of behaviors to overcome burial in sediment, whereby the buried hermit crab (D) pushes away the sediment using its chelipeds and ambulatory legs (E), and then crawls away (F). 

Remarks: Diogenes heteropsammicola sp. nov. belongs to the D. edwardsii species group because of the intercalary rostriform process being smooth on the lateral margins, the antennal peduncle distinctly overreaching the distal corneal margin, and the antennal flagellum bearing a pair of long setae on the distal margin of each article ventrally. The new species is readily distinguished from all other species in this group by its exceedingly slender chelipeds and ambulatory legs, its symmetrical telson, red and white coloration, and the unique symbiotic habit with solitary corals.

Etymology: The new species is named after its mutualistic relationship with the solitary scleractinian corals of the genera Heteropsammia, keeping in mind that this hermit crab is also associated with Heterocyathus corals.

Distribution: At present, known only from Oshima Strait, between Kakeroma Island and Amami-Oshima Island, Kagoshima, Japan, depths of 60–80 m, and Ikomo Bay, western coast of Kakeroma Island, depth of 31 m.


Momoko Igawa and Makoto Kato. 2017. A New Species of Hermit Crab, Diogenes heteropsammicola (Crustacea, Decapoda, Anomura, Diogenidae), Replaces A Mutualistic Sipunculan in A Walking Coral Symbiosis. PLoS ONE. 12(9); e0184311.  DOI: 10.1371/journal.pone.0184311

New hermit crab uses live coral as its home

   

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