Sphenopus exilis is a new species of zoanthids that goes against almost everything we thought we knew about this group of reef animals. Not only is Sphenopus solitary, but it is also a free living zoanthid which builds itself up with the sand grains that it lives in.
Not to be confused with Stenopus shrimps, Sphenopus exilis is actually not the first member of this genus as it shares the classification with three other species. What makes the discovery of Sphenopus exilis so exciting is that it’s the first of its kind in over a century with Sphenopus marsupialis being described in the 18th century and two other species in the 19th century.
Earlier this year we were treated to some new species of azooxanthellate zoanthids which live by feeding instead of the more typical symbiotic internal algae. While the newly discovered Sphenophus are still photosynthetic, their free living and solitary nature makes them an oddball among zoanthids.
Picture the luxurious and colorful zoanthid colonies that we all love, in the aquariums you have seen. Now imagine if somehow these colonies lived in the sand, unattached to any substrate, or if there was just one big fat juicy single polyp – any one of these features would be enough to get your attention for its unusual growth form.
Perhaps it is precisely the free-living form that consequently makes Sphenophus zoanthids solitary – perhaps they are capable of budding to some degree but with no behavior of attaching to any substrate, these daughter polyps just wander off to live life by themselves.
In some ways the Sphemopus zoanthids are akin to the Walking Dendro corals, Heteropsammia and Heterocyathus, sans the obligatory commensal peanut worm living inside the base. Interestingly enough, Stemophus has been kept in an aquarium before, and we’re not surprised to learn that it was by the Churaumi Okinawa Aquarium – home to many aquatic exotics.
The newly described Sphemopus exilis was discovered in Japan, in an enclosed silty bay in 15 meters depth. With greater awareness that giant, solitary, free living zoanthids are out there, perhaps the aquarium world could eventually come to learn more about them, and even keep some in reef tanks.
Sphemopus exilis was described by Takuma Fujii and James Davis Reimer in the July 2016 volume of ZooKeys.