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 * Diogenes heteropsammicola is a species of hermit crab that lives within "walking corals" of the genera Heterocyathus and Heteropsammia.

These coral have an internal cavity that is typically occupied by a symbiotic sipunculan worm; in the crab's habitat in the Ōshima Straight south of Amami Ōshima, it is often found inside the corals instead. In terms of coloration, this species exhibits distinct features. The shield is predominantly white, while the ocular peduncles are maroon-red. The antennular peduncles have their ultimate and penultimate segments appearing translucent, with the basal segment being maroon-red. Moving to the antennal peduncles, the first to third segments are maroon-red, while the fourth and fifth segments are translucent. The chelipeds are mainly white, with the merus of the left cheliped displaying a slight reddish-brown hue on the mesial face. The dactylus, palm, and fixed finger of the right cheliped exhibit a tinge of reddish-brown or maroon-red on their dorsal surfaces. The ambulatory legs are generally maroon-red, with the dactyli having white distal portions.

Regarding distribution, this species is currently only known to inhabit the Oshima Strait, situated between Kakeroma Island and Amami-Oshima Island in Kagoshima, Japan. It resides at depths ranging from 60 to 80 meters. Additionally, it can be found in Ikomo Bay, along the western coast of Kakeroma Island, at a depth of 31 meters.

In terms of classification, Diogenes heteropsammicola sp. nov. belongs to the D. edwardsii species group. This classification is based on the smooth intercalary rostriform process along the lateral margins, the antennal peduncle clearly extending beyond the distal corneal margin, and the antennal flagellum bearing a pair of long setae on the ventral distal margin of each article.

This new species stands out from all other species in this group due to its remarkably slender chelipeds and ambulatory legs, a symmetrical telson, its distinctive red and white coloration, and its unique symbiotic relationship with solitary corals.

In terms of its name, the new species is named "Diogenes heteropsammicola" to highlight its mutualistic association with solitary scleractinian corals of the genera Heteropsammia. This name also acknowledges its affiliation with Heterocyathus corals.

Ecological and Behavioral Overview

Diogenes heteropsammicola sp. nov. was discovered in relatively shallow waters, at depths ranging from approximately 60 to 80 meters within the Oshima Strait. This area experiences strong periodic tidal currents, even near the ocean floor, where the sediment predominantly consists of shelly sand. This sandy substrate contains numerous fragments of bryozoans, foraminiferans, and molluscan shells, and it is also inhabited by lancelets.

The distinctive behavior of this hermit crab involves its exclusive preference for residing within coiled cavities found in solitary scleractinian corals belonging to the genera Heterocyathus and Heteropsammia. Notably, it does not inhabit empty gastropod shells. When observed in controlled aquaria settings, it was noted that the hermit crab remained mobile even while carrying its host coral. In instances where the coral was overturned, the hermit crab demonstrated the ability to stabilize itself by using its long ambulatory legs and left cheliped to anchor to the seabed. It then skillfully righted the coral using its pleon. Similarly, when the coral was buried under sediment, the hermit crab effectively cleared the surrounding area using its chelipeds and ambulatory legs before moving away while still nestled within the coral.

For sustenance, this hermit crab employs a feeding strategy involving the filtration of organic particles using its antennal flagella and third maxillipeds. It creates an upward current near the third maxillipeds by moving its antennae in a circular motion, with the right antenna rotating clockwise and the left antenna counterclockwise. In case organic particles become attached to the antennal flagellum, the hermit crab utilizes its third maxillipeds to remove them.

Some female specimens were found to carry red, spherical eggs, with major and minor axis diameters measuring approximately 0.5 and 0.4 mm, respectively. In one instance, a female was observed carrying as many as 72 eggs on its pleopods.

This hermit crab coexists within its habitat alongside a symbiotic sipunculan species found within walking corals. Among the walking corals collected from the Oshima Strait between 2012 and 2016, approximately 55% were inhabited by hermit crabs, while the remaining 45% were inhabited by sipunculans. Notably, the structures of the corallum cavities in which they reside were similar, without significant variations between the two symbiotic species.

In terms of the coiled cavities within solitary corals, it was observed that 25% of those inhabited by hermit crabs had dextral coiling, while the remaining 75% displayed sinistral coiling. Conversely, among the cavities inhabited by sipunculans, 10% exhibited dextral coiling, and the majority, or 90%, had sinistral coiling. This suggests a predominance of sinistral coiling within the cavities of solitary corals in the Oshima Strait.