Occurrence of bioluminescent and nonbioluminescent species in the littoral earthworm genus Pontodrilus

Abstract

Pontodrilus litoralis is a cosmopolitan littoral earthworm known to exhibit bioluminescence. Recently, a congeneric species, Pontodrilus longissimus, from Thailand was described. These species are sympatric, but their burrowing depths on Thai beaches are different. In this study, we examined the in vivo and in vitro bioluminescent properties of P. longissimus and P. litoralis. Mechanical stimulation induced in vivo luminescence in P. litoralis, as reported previously, but not in P. longissimus. In vitro cross-reaction tests between these species revealed the absence of luciferin and luciferase activities in P. longissimus. The coelomic fluid of P. litoralis had strong fluorescence that matched the spectral maximum of its bioluminescence, but the same result was not observed for P. longissimus. These results suggest that P. litoralis has luminescence abilities due to the creation of bioluminescent components (i.e., luciferin, luciferase, and light emitters). The presence of both luminous and nonluminous species in a single genus is likely widespread, but only a few examples have been confirmed. Our findings provide insight into the possible functions of bioluminescence in earthworms, such as avoiding predation by littoral earwigs.

Responses to salinity in the littoral earthworm genus Pontodrilus

“…The established cosmopolitan littoral earthworm Pontodrilus litoralis was found to live in various types of habitats whereas P. longissimus was found only in estuaries with muddy sand. Sympatry of these two Pontodrilus species occurred in some localities, where P. litoralis was repeatedly found in the sand surface, while P. longissimus was found at deeper levels below the surface than P. litoralis 6 , 17 . Pontodrilus litoralis has been reported as a bioluminescence earthworm and uses this to avoid predation by littoral predators because P. litoralis live on the sand surface 17 – 20 .…”

“…Sympatry of these two Pontodrilus species occurred in some localities, where P. litoralis was repeatedly found in the sand surface, while P. longissimus was found at deeper levels below the surface than P. litoralis 6 , 17 . Pontodrilus litoralis has been reported as a bioluminescence earthworm and uses this to avoid predation by littoral predators because P. litoralis live on the sand surface 17 – 20 . We suggest that salinity is one of the key factors affecting the distribution and habitat preference of Pontodrilus in this region.…”

“…Earthworms in general can tolerate limited salinity, but some earthworms, like Eisenia fetida 16 , are highly salt-tolerant. In this research, the salinity tolerance of P. litoralis and P. longissimus , which both occur in saline habitats but in different microhabitats 6 , 17 , was evaluated based upon the following. To verify the distribution pattern of P. litoralis and P. longissimus , it is necessary to further identify the main driving factor behind this habitat selection process.…”