Introduction

The 128 species of Antarctic cryonotothenioid fishes are one of the few well-documented examples of an adaptive radiation of fishes in the marine realm (Eastman and McCune 2000; Matschiner et al. 2015; Bowen et al. 2020). The genus Pogonophryne Regan 1914 (Artedidraconidae) is a component of that radiation, and the species are a ubiquitous benthic element of the shelf and upper slope waters of Antarctica (Eakin 1990; Hanchet et al. 2013; La Mesa et al. 2019). Over the past few decades the number of species has increased from 17 to 27, and Pogonophryne is now the most speciose genus of notothenioids (Eakin 1990; Eastman and Eakin 2021). They are medium-sized (predominantly 20–30 cm TL) sedentary benthic fishes found primarily at depths of 500–1600 m (Duhamel et al. 2014; Eastman 2017). Unlike many other cryonotothenioids, they exhibit minimal ecomorphological disparity and are a non-adaptive element of the radiation. They do, however, possess a mental barbel of uncertain functional significance. This structure is subject to intraspecific variation in some species (Eakin et al. 2001). With an age estimate of < 1 Ma, Pogonophryne is also the most recently diverged cryonotothenioid lineage (Near et al. 2012) and, because they exhibit a relatively low degree of interspecific morphological variation, this makes the species of Pogonophryne the most difficult-to-identify cryonotothenioids.

To facilitate identification and based on similarities in external morphology, the 27 species are allocated among three categories and five species groups within the categories (Table 1), with one species group containing three subgroups (Balushkin and Eakin 1998). These divisions, based strictly on morphology, have no taxonomic status, although all five species groups are clades based on mitochondrial gene sequences (Eakin et al. 2009). This has been confirmed by more extensive DNA sequencing that involved genome-wide data (ddRADseq) and morphometric analyses of the body shape axis by Parker et al. (2022), who also identified a previously unrecognized sixth clade. Based on their sample of 18 of the 27 species, Parker et al. propose reducing the number of species of Pogonophryne from 27 to 6 by synonymizing the species in each of the five species groups into a single species bearing the name of the species group. This results in a 78% reduction in the number of species. The newly recognized sixth species from the Ross Sea has not been formally described and named, and there is insufficient information for us to consider it here.

Table 1 Distribution of the 27 species of Pogonophryne among the three categories, five species groups and, within the P. mentella group, three subgroups of species with either short (1), medium (2), or long (3) barbels
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Because explaining diversity is a major focus in organismal biology, it is of general interest when an issue arises involving species diversity within a radiation. This is exemplified by Pogonophryne given that Parker et al. (2022, p. 58) state that their analyses “result in a dramatic reduction in the number of distinct species within” an Antarctic adaptive radiation. We also recognize that there has been taxonomic inflation within Pogonophryne. In the aftermath of the revisions of Parker et al. there will be uncertainty among those confronted with understanding the amalgamations, specifically how species should be identified and assigned to one of the five known species groups. Whether the traditional or the new reduced approach to the taxonomy of Pogonophryne is employed, the placement of a species in a category and then in a species group is the essential step in the identification process. Here we provide a table and an uncomplicated pictorial identification guide for Pogonophryne that dispels confusion by (1) delineating multiple features of external morphology, unrelated to barbel morphology, that define the categories and species groups and (2) identifying the species contained in each species group and subgroup.

Materials and methods

The initial step in the identification of an unknown species of Pogonophryne is the determination of its membership in a category and a species group (Table 1). This can be accomplished by utilizing features visible solely in the heads of species of each of the five groups. Although in Russian and not widely circulated, Figs. 2–5 in Andriashev’s (1967) review of the genus Pogonophryne are extremely useful for this purpose. The species groups were not yet recognized when he wrote the paper, but four of the five groups are represented in his figures. The fifth, or P. albipinna group, was subsequently recognized by Eakin (1981). It should also be noted that the P. scotti “group” consists of a single species.

Figures 2, 3, and 4 are scans of photographic copies of the original figures submitted for publication by Professor Anatole P. Andriashev (1967) for his paper reviewing the little-studied genus Pogonophryne. Andriashev gave photographic copies of his original illustrations to Professor Hugh DeWitt of the University of Maine who passed them on to his, then graduate student, Richard Eakin, and we are using them here. This is significant because these copies of the dorsal and lateral views of the head of P. mentella (Fig. 3c, d) show the large terminal expansion of the mental barbel as originally drawn by Andriashev’s artist. All known reprints and reproductions of this article contain a printing error that omitted the terminal expansion.

We used photographs of a specimen of P. immaculata to represent the P. albipinna species group because the holotype and only known specimen of P. albipinna is a 37.5 mm SL juvenile (Eakin 1981). The photographed P. immaculata in Fig. 1 was a freshly caught 209 mm SL female now preserved and catalogued in the collections of the Museum of New Zealand Te Papa Tongarewa (NMNZ P.043715).

Fig. 1
figure 1

The P. albipinna species group represented by Pogonophryne immaculata. Views of the head (a, b) and trunk (c) of a 209 mm SL female, NMNZ P.043715

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Results and discussion

Categories, species groups, and subgroups of Pogonophryne

The guide is based on the pigmentation patterns, head shapes, and relative sizes of other features easily observed in dorsal or lateral views of the heads of Pogonophryne. The figures show the key features that distinguish categories and species groups: (1) the color pattern of the integument of the dorsal head, (2) the shape of the head, (3) the degree of projection of the lower jaw, (4) the relative volume of the orbit occupied by the eye, and (5) the relative size and shape of the mental barbel.

Each species of Pogonophryne can be assigned to one of three distinct categories (Table 1) based on easily observed differences in spotting patterns of the skin and other less obvious characters not considered here (Eakin 1977; 1981; 1990; Balushkin and Eakin 1998). The species in each category are further separable into five morphologically well-defined species groups (Balushkin and Eakin 1998). The validity of the species groups reflects consistent differences in color patterns, head shapes, jaw prognathism, mental barbel features, and meristic counts that distinguish species in each group, for example, numbers of vertebrae, lateral-line pores or scales, and certain fin rays. There has probably been over reliance on the importance of barbel morphology in some recent descriptions of new species of Pogonophryne, although among all species only the barbels of P. scotti exhibit documented intraspecific variation (Eakin et al. 2001). Parker et al. (2022, p. 71) indicate that their “results provide no genomic or phenotypic support for differentiation among species of Pogonophryne that are diagnosed exclusively using barbel morphology.” However the barbel has not been the sole character used in describing species. For example, six species of the P. mentella group (P. macropogon, P. cerebropogon, P. ventrimaculata, P. orangiensis, P. squamibarbata, and P. bellingshausenensis) are each defined by a unique combination of at least four characters: barbel length, terminal expansion morphology, spotting patterns, and vertebral counts (Eakin 1981, 1987; Eakin and Balushkin 1998, 2000; Eakin and Eastman 1998; Eakin et al. 2008).

Within the diverse P. mentella group there are three subgroups (Table 1) that contain 63% (17/27) of the species of Pogonophryne. These subgroups have no phylogenetic basis and are arbitrarily recognized based on the relative length of the mental barbels. However, arranging and categorizing the 17 species in this group by their barbel lengths facilitates their identifications as species. This does not imply that barbel morphology is the sole diagnostic character for these species, but rather that relative barbel lengths are useful in, for example, taxonomic key couplets (Eakin 1990, pp. 339–340). These subgroups are the focus of much of the consolidation suggested by Parker et al. (2022). Finally, we do not doubt the existence of a sixth lineage identified by Parker et al. and we urge future restraint in the description of new species of Pogonophryne without supporting nucleotide sequence data.

Faunal surveys of Antarctic fishes are now increasingly conducted using images obtained from towed high-resolution camera systems rather than by benthic trawling with its habitat-altering consequences. Although only a dorsal image of the fish is obtained in most survey photographs, it can usually be identified at the species level (Eastman et al. 2013; La Mesa et al. 2019). In addition to presence/absence data, these images also provide insight into other aspects of biology including reproductive behavior and have documented the universality of nesting and nest guarding behavior in cryonotothenioids, including Pogonophryne spp. (La Mesa et al. 2021). Although species identifications are difficult in Pogonophryne, our identification guide makes it possible to determine the species group membership using easily visible features in a dorsal image.