Thuridilla bayeri is common in the Marshalls; we have found at least 113 specimens at Enewetak, Kwajalein, Rongelap and Utirik Atolls. The species was originally described based on a single specimen from Bikini Atoll, also in the Marshalls, by Marcus (1965). While we have no specimens from the type locality, Enewetak is just 347km to the west of Bikini, Kwajalein 412km southeast and Rongelap 157km east, covering the area around the type locality pretty well. Most of our specimens have been on lagoon reefs and pinnacles, where they are usually under or crawling on dead coral from the intertidal to depths of at least 15 meters. According to my notes, in T. bayeri egg masses, the individual eggs measured about 80µm in capsules of about 110µm diameter, and the eggs were piled two deep around an orange extra-capsular yolk string.
A recent paper on the molecular phylogeny of Thuridilla by Martín-Hervás, et al (2021) indicates that there are several very similar species (undescribed except for T. bayeri) that differ in DNA but with blue dots along the sides of the parapodia, and these species can be difficult to differentiate. The authors provide some differentiating criteria including differences in the banding on the rhinophores, relative number and color of blue spots and presence of alternating orange and black spots along the parapodial margins. Our numerous photographs show enough variation in all of those characters that we cannot be certain that all our specimens are indeed T. bayeri, or perhaps that those characters are not sufficient to pinpoint the species. Rhinophores on our photos, for example, range from a pure white upper half with just a trace of orange color at the tips to the upper half distinctly striped red and white. It is unfortunate that it may require a DNA test to positively identify the species.
Numerous photos from Enewetak and Kwajalein are included below to help illustrate the variation. The first three photos below show specimens from Enewetak Atoll with the rest from Kwajalein.
Head shot of the specimen just above.
The specimen below was found on a Kwajalein Atoll lagoon pinnacle on 27 September 2009. It seems to well fit the Marcus (1965) original description of the rhinophores as "black with white stripes, tips of rhinophores orange with white stripes" but the lateral spots are greenish in color and the spots on the head so small as to be almost nonexistent.
The black spots along the parapodial margin are well illustrated below.
The one below was found on 26 May 2013 on a Kwajalein lagoon reef. This one with fewer spots that are colored more sky blue seems to fit the description of undescribed species 4 in Martín-Hervás, et al (2021) but that one is supposed to also show the alternating black and light spots along the parapodial margin and have a shorter white band with reddish rhinophore tip, neither of which are true of this specimen.
Create 22 January 2007
Updated 19 October 2021