In 2017, a population of Doris pseudoargus was observed on subtidal rocky ledges at Paddock Rock off Cape Ann, Massachusetts by A. Shure. All animals occurred at a depth below 13m. There were a number of large animals and egg masses that suggested the species had a stable and well established population (Fig. 1). The animals were feeding on the sponge Isodictya palmata (Ellis and Solander 1786) as is evidenced by the feeding wound in the image (Fig. 1a). A large egg mass, a partial one, and a pair of D. pseudoargus mating in the background are visible in the image (Fig. 1b). However, a survey of photographic images from fall 2016 taken by Shure at the same location did not show any specimens present.
Fig. 1 Images of Doris pseudoargus taken at Paddock Rock by A. Shure. Fig 1a large individual of D. pseudoargus crawling on natural substrate. Colony of the sponge Isodictya palmata shows evidence of grazing by nudibranch (arrow). Fig 1b two egg masses (asterisks) of D. pseudoargus on rock ledge; one complete, one partial. A pair of mating D. pseudoargus is visible in background (arrow) among colonies of the sponge I. palmata. Gloved fingers of diver provide scale; animals and egg masses are larger than endemic Gulf of Maine dorid nudibranch species.
Subsequent dives by Shure recorded specimens at a series of additional sites in the GOM, including Halfway Rock and the wreck of the ship/vessel Chester Poling (Table 1). The nudibranchs at most of the deeper sites were observed feeding on I. palmata. Additional specimens were also observed at several locations in 2018 through 2021 (Table 1). In each case the habitat consisted of rocky ledges below the thermocline that supported populations of sponges, particularly the palmate sponge, I. palmata. The only time specimens were observed on artificial substrates was on the wreck of the Chester Poling. Over time, Shure has noticed that there appears to be a decline in the abundance of I. palmata at the dive sites compared to earlier years. In 2020, Shure observed specimens of D. psuedoargus at Graves Light Station, that occurred at depths as shallow as 4m and were feeding on Halichondria panicea (Table 1). Captain J. Sullivan of the charter vessel Keep-ah, told Shure in 2020 that there had been specimens observed at Graves Light Station for the past couple of years.
Table 1. Summary of NW Atlantic observations including dates, locations, relative numbers and egg masses when present.
* 4 specimens collected and transported to University of New Hampshire
Fig. 2 Map of Massachusetts and New Hampshire coastal zone showing location of dive sites indicated by letters annotated in Table 1.
In June, 2019, three specimens were collected off Broad Cove of Appledore Island, Isles of Shoals, ME by divers from the Shoals Marine Laboratory. J. Factor then preserved the animals and delivered them to Harris at UNH. A live specimen was collected at the same site by E. Kintzing and brought to Harris and kept in a recirculating sea water system, photographed and preserved in 95% EtOH. Additional dives in the area reported more specimens present, including egg masses. In November, 2019 and 2020, a breeding population of numerous individuals was observed by the University of New Hampshire Advanced Diving Class at Mingo Rock, a subtidal ledge north of Appledore Island.
In April, 2021, a specimen was observed but not photographed by E. Kintzing on a dive at Cape Neddick, York Beach, Maine. The identification as D. pseudoargus was confirmed by size and morphology (much larger than any native dorid nudibranchs known for the GOM and the observer was familiar with the specimens observed at Appledore Island and Mingo Rock). An identifiable egg mass was observed and photographed at the same site at Cape Neddick a couple of weeks later though the nudibranch was not seen.
In 2019, A. Kuzirian, observed a smaller specimen of D. pseudoargus collected by Marine Biological Laboratory, Woods Hole, MA collectors from the northern end of the Cape Cod Canal. It was photographed and subsequently preserved for genetic analysis.
Recently, A. Shure discovered a web sighting of what is likely to be D. pseudoargus collected off Yarmouth County, Nova Scotia, Canada in 2017 (https://www.inaturalist.org/observations/70940660). This sighting would indicate that the species was already widely distributed when it was first observed off Cape Ann. However, the more recent sightings at the Isles of Shoals and Cape Neddick suggest the species is fairly recent and is expanding its range southward.
On September 27, 2021, W. Grossman, a MBL collector, found three specimens under rocks at an intertidal site at Scusset Beach at the north end of the Cape Cod Canal. The specimens were found coincident with populations of the sea anemone, Metridium senile. However, very few sponges were observed in the vacinity.
A somewhat unique feature of Doris psuedoargus compared to most European nudibranch species is the fact it occurs in a wide range of color patterns, including mottled browns (Harris, personal observations; Thompson and Brown 1984). The specimens photographed by Shure and specimens brought to Spaulding Hall for preservation ranged from somewhat uniform colors of yellow and orange (collected specimens) to more expanded color ranges including mottled brown (Fig. 1a & b; field photographs by Shure).
Our Bayesian and maximum likelihood analyses (Fig. 3a) reveal that the specimen sequenced from Massachusetts (CASIZ 223159A) is nested within a well-supported (pp = 1, bs = 97) cluster of European specimens clearly identifiable as Doris pseudoargus. Within this cluster of D. pseudoargus, there is no clear geographical separation between the European populations and the relationships between all specimens are tentative due to low Bayesian internal node support and the resulting polytomies. The specimen from Massachusetts (MZ389059) groups together with specimens from the North Sea of Scotland (KR084378 & AY345030) and one individual from Heligoland, Germany (KR084586) and Kattegat, Sweden (MG935320) in a weakly supported polytomy; however, this relationship does correspond with the relationships suggested in the TCS hapotype network (Fig. 3b).
The TCS haplotype network of eight specimens of D. pseudoargus suggest seven unique haplotypes with no geographical separation and one shared haplotype between a specimen from Inverness, Scotland (KR084378) and one of two specimens from Heligoland, Germany (KR084586). It also suggests that the Massachusetts specimen, the four European specimens from the shared polytomy, and the specimen from Skagerrak, Sweden (MG935407) are more closely related than the specimen from Esjerg, Denmark (KR084907) and the second specimen from Heligoland, Germany (KR084616).
The ABGD analysis of the mitochondrial COI gene also supports the relationships suggested in the phylogenetic analyses and the TCS haplotype network (Fig. 3b). The Massachusetts specimen has lower genetic variation (0.15-0.31 %) with the five more closely related European specimens (KR084378, AY345030, KR084586, MG935320, MG935407) suggested in the TCS haplotype network. In contrast, there is larger variation (0.76 % in both instances) between the Massachusetts specimen and the specimen from Denmark (KR084907) and the specimen from Heligoland, Germany (KR084586). Since the specimens within European waters exhibit greater genetic variation between themselves (0.0-0.92 %) than to the specimen from the western Atlantic, it strongly supports the hypothesis that the Massacusetts specimen indeed represents an introduction from Europe.
Fig. 3 A. Bayesian phylogenetic tree of D. pseudoargus estimated from COI data. Numbers above / below branches refer to BI posterior probabilities (pp) / ML non-parametric bootstrapping values (bs). Relationships not recovered in ML analysis indicated by a dash. Different colors represent geographical locations. B. TCS haplotype network for D. pseudoargus COI. Each circle indicates a unique haplotype, size indicates number of specimens sharing that haplotype, lines between circles indicates a single substitution, small black circles indicate hypothetical haplotypes, and colors correspond to geographical locations in A.