The family Chromodorididae Bergh, 1891 is the most species-rich family of dorid nudibranchs, comprising over 300 described species across 16 genera (Johnson & Gosliner, 2012). Chromodorids are predominantly found in tropical coral reef habitats and subtropical coastal waters and are characterised by their vivid colouration, which may serve as aposematic warning signals to potential predators (Rudman, 1991). The chromodorids sequester defensive chemicals, primarily terpenoid compounds, from sponges upon which they feed, storing these metabolites in specialised mantle glands that act to deter fish predation (Winters et al., 2018a; Cheney et al., 2016). The genus Goniobranchus Pease, 1866 was originally described in the nineteenth century but was subsequently treated as a junior synonym of Chromodorisfor over a century. Molecular phylogenetic analyses by Johnson & Gosliner (2012) revealed that Chromodoris as traditionally circumscribed was non-monophyletic, leading to the re-erection of Goniobranchus as a valid genus for one of the distinct clades recovered. Goniobranchus currently contains approximately 57 described species (Soong et al., 2021) and can be distinguished from other chromodorid genera, in part, by the upright position of their egg ribbons. Within Goniobranchus, several species complexes have been recognised based on shared colour pattern elements, including a 'red-reticulate group' (Yap et al., 2020) and a 'white with marginal bands group' (Soong et al., 2021).

In south-eastern Australia, an assemblage of red-spotted chromodorid nudibranchs has evolved convergent colour patterns, forming a putative mimicry ring (Rudman, 1991). Rudman (1991) first proposed that the similarity among these species, characterised by white mantles bearing red spots and yellow or orange mantle margins, represented collective aposematic colouration that warned predators of chemical defences sequestered from their sponge diets. This hypothesis was subsequently supported by Winters et al. (2018b), who used visual modelling and pattern analysis to demonstrate that the appearance of red-spotted nudibranchs was similar from the perspective of predatory fish, and that the red-spot colour pattern had evolved independently multiple times across distantly related chromodorid lineages. Winters et al. (2018b) further showed that while levels of distastefulness toward palaemonid shrimp remained relatively constant across taxa in the mimicry ring, toxicity levels varied significantly, suggesting a system of Müllerian mimicry with unequal chemical defences.

The south-eastern Australian, red-spotted mimicry ring includes species from multiple genera. Within Goniobranchusitself, the principal members include G. splendidus (Angas, 1864), G. daphne (Angas, 1864), G. hunterae (Rudman, 1983), G. tasmaniensis (Bergh, 1905) and G. loringi (Angas, 1864). Members from other genera comprise Hypselodoris bennetti (Angas, 1864), Mexichromis festiva (Angas, 1864), Mexichromis mariei (Crosse, 1872), Verconia haliclona (Burn, 1957), and Chromodoris thompsoni (Rudman, 1983) (Rudman, 1991; Winters et al., 2018b; Nimbs & Smith, 2017). Although some of these taxa are common and widespread (e.g. G. splendidus andH. bennetti), others are less frequently observed and exhibit restricted geographic ranges (e.g. G. hunterae, G. loringi, and C. thompsoni) (Nimbs & Smith, 2017).

In the wider Indo-Pacific, G. splendidusis the sister taxon to putative species in the G. tinctorius complex (Rüppell & Leuckart, 1828), a widespread group of cryptic red-reticulate taxa (Wilson et al., 2016). Recent molecular analyses have revealed that the red-reticulate Goniobranchus complex contains multiple species-level lineages, and that the non-reticulate, spotted G. splendidusfalls within this complex (Yap et al., 2020). The genus also contains species within a 'G. fidelis colour group' (Rudman, 1985), which are distinguished by orange outer, and red or purple inner, mantle border bands with dark gills and rhinophores.

Observations of animals that appear to be a red-spotted form of Goniobranchus fidelis (Kelaart, 1858) have been recorded from northern New South Wales since the late 1980s (Rudman, 1999) and from Queensland in 2023 (Mullins, 2023). Rudman (1999) considered this variant a strange colour form of G. fidelis, noting that it shared the dark (black) rhinophores and gills typical of that species, but possessed a golden-yellow mantle border and red spots rather than the broad orange-red mantle margin characteristic of typical G. fidelis. Rudman (1999) further suggested that, at the southern extent of its range, this form possessed a large central red spot that may have evolved to mimic the sympatric G. splendidus which, in northern NSW, often displays fewer but larger red spots or a single large central spot rather than the many scattered spots typical of its southern populations (Winters et al, 2017). We hypothesised that this putative colour form of G. fidelis represents a distinct, undescribed species that has converged on elements of the south-eastern Australian, red-spotted mimicry ring. Here, we test this hypothesis using molecular data and describe Goniobranchus solitaryensis sp. nov. from the Solitary Islands, northern New South Wales.