Comment on Bang et al. Two New Species of the Genus Longipedia Claus, 1863 (Copepoda: Harpacticoida: Longipediidae) from Korea, with an Update and a Key to Species. Diversity 2021, 13, 590

In the recent description of two new Longipedia species from Korea [1], the authors suggested that Longipedia gonzalezi Schizas et al., 2015 [2], described from Puerto Rico, northeast Caribbean Sea, should be relegated to a junior subjective synonym of L. americana Wells, 1980. This view is based on Onbé’s work [3], where the naupliar and copepodid stages of L. americana are described [1]. The publication by Onbé [3] is not a redescription of the species, but rather a description of the development series of what he considered as L. americana from Massachusetts, northeast U.S.A. In Figures 79, 80 and 81 (p. 626 in [3]), the P2 coxa of copepodid IV, V and adult female L. americana are depicted with what appears to be a sharp projection between the endopod and exopod, an important diagnostic distinction between Longipedia gonzalezi from L. americana [2]. In the original publication of L. americana, the sharp projection on the P2 coxa of L. americana seems to be absent [4]. The second diagnostic character to distinguish Longipedia gonzalezi from L. americana was the absence of ornamentation in the coxa of swimming legs P3 and P4 in L. gonzalezi [2]. Two rows of spinules are present in the coxa of P3 and P4 in L. americana (Figures 87 and 92; p. 627 in [3]). An additional character to differentiate these two species, stated in Table 3 in [2], is the ornamentation of P2 coxa with proximal row of long spinules in L. americana and no proximal row in L. gonzalezi. Other notable differences between L. gonzalezi and L. americana are: (1) The male of L. gonzalezi with three lateral spinules in the telson, but only two in L. americana; (2) Both sexes of L. gonzalezi with a smooth body, but L. americana with dorsal and ventral urosomal spinules; and (3) P6 fused to the urosome in L. gonzalezi, but both P6 swimming legs separated in L. americana.

Finally, and equally importantly, the habitats of the Longipedia specimens from the Onbé [3] and the Schizas et al. [2] studies are vastly different in both types of substrata and environmental variables such as temperature and depth. The material that [3] studied was collected by both surface and vertical plankton net hauls from Vineyard Sound and the pier of the Woods Hole Oceanographic Institution, respectively. Water temperatures from 50 to 70 m depth range approximately between 26 and 29 °C [5]. For comparison, water temperatures at the Woods Hole Pier varied from 0 to 22 °C (data from 1984, the year of the Onbé 1984 publication (http://dlaweb.whoi.edu/water_temperatures/H2O/show_temps.php? assessed on 10 January 2022). The holotype of L. americana (previously identified as L. helgolandica [6]), a species with wide latitudinal distribution, from Massachusetts to the Caribbean, came from Fosforescent Bay in Puerto Rico, a shallow (1–2 m deep) red mangrove embayment near La Parguera, southwest Puerto Rico, approximately 14 km from the locality of L. gonzalezi. Longipedia gonzalezi was collected from the tropical mesophotic reefs of southwest Puerto Rico [2] from >50 m depth. These photosynthesis-dependent deeper reefs are found on the insular slopes of Caribbean islands and are at least as extensive as the shallow coral reefs [7]. Macroalgae, octocorals, sponges, and scleractinian corals dominate the seascape, providing innumerable habitats for meiofauna and small macrofauna. The cryptic biodiversity has barely been studied and numerous new species have been discovered over the last 10 years (e.g., [8,9]). With the application of molecular tools, we have highlighted the presence of cryptic and sibling species in mesophotic reefs of El Seco, Puerto Rico by discovering three divergent haplotypes in the harpacticoid copepod Laophontella armata (Milne Edwards & Haime, 1851) [10]. The potential of finding more cryptic and sibling species in morphologically similar meiofauna taxa from the mesophotic reefs such as Longipedia is high, when examined with DNA sequences.

The opinion of Bang et al. [1] to relegate Longipedia gonzalezi to a junior subjective of L. americana is not supported by the available data. We neither know if the Longipedia specimens that Onbé [3] collected were L. americana nor do we have access to these specimens for examination. Longipedia specimens are notoriously difficult to assign to species, and in most cases, species differentiation is based on subtle differences [4]. Schizas et al. [2] stated that the genus Longipedia is an ideal taxon to include genetic information together with high-quality morphological observations from any new species or collections to independently decipher the number of species present. Bang et al. [1] demonstrated instructively the use of molecular data to differentiate two new species from Korea, L. koreana, Bang et al., 2021 and L. ulleungensis, Bang et al., 2021. Similarly, genetic comparisons between L. americana and L. gonzalezi could establish the amount of DNA divergence between these taxa. Furthermore, Wells [4] identified the Grand Cayman population of Longipedia americana as a possibly distinct but related taxon based on minute leg and body ornamentations; DNA sequence comparisons could contribute to our understanding of species diversity in Caribbean Longipedia.

The wide distribution of L. americana, from the tropical Atlantic Ocean to high temperate latitudes, makes this species a strong candidate for morphological and molecular studies on cryptic and sibling species along a latitudinal cline of seawater temperature. The distribution of L. americana resembles that of another polyarthran harpacticoid copepod, the canuellid Scottolana canadensis (Willey), where genetically based differentiation in growth and reproduction has been observed in northern vs. southern populations [11,12]. Therefore, it would be prudent to collect Longipedia specimens and determine genetically the divergence observed within and between populations along the east coast of USA, including the Caribbean.