Introduction
The Amazon coast extends on the States of Amapá, Pará, and Maranhão, representing about 35% of the total Brazilian shore (Pereira et al., Reference Pereira, Dias, Carmo and Polette2009). The Amazon River mouth is one of the largest discharges of freshwater and sediment into the ocean, creating unique conditions for a massive animal fauna biodiversity, with more than 700 species currently reported (Tosetto et al., Reference Tosetto, Bertrand, Neumann-Leitão and Nogueira Júnior2022; Checon et al., Reference Checon, Costa, Corte, Souza and Pombo2023). The notable biodiversity of the local ichthyofauna is reflected on its socioeconomic scenario, since fisheries represent an important economic, nutritional, and cultural activity, with fish being the main food resource for local populations (Tenório et al., Reference Tenório, Souza-Filho, Ramos and Alves2015; Jimenez et al., Reference Jimenez, Barboza, Amaral and Frédou2019). Despite such an important role in Brazil, the Amazon coast has been constantly impacted by anthropogenic activities, with few effective governmental efforts for conservation, which has resulted in decline of fish stocks and direct impacts on coastal environments (Szlafsztein, Reference Szlafsztein2012; Hayashi et al., Reference Hayashi, Souza-Filho, Nascimento Júnior and Fernandes2019).
Ergasilidae Burmeister, 1835 is a group of cosmopolitan parasitic crustaceans, commonly found on the gills, nostrils, fins, tegument and urinary bladder of fish, and rarely elasmobranchs and bivalve molluscs (Malta, Reference Malta1993; Boxshall and Halsey, Reference Boxshall and Halsey2004; Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023). This family is one of the most species rich within cyclopoid copepods, with 275 species from 30 genera currently known. These copepods represent the commonest taxon infesting fish in Brazil, totalling 77 species from 17 genera reported in the country. Despite its notable diversity, recent studies have stated that the knowledge related to the richness and distribution of Ergasilidae may be inaccurate, due to a low number of fish species investigated for parasitic copepods in Brazil (Luque et al., Reference Luque, Vieira, Takemoto, Pavanelli and Eiras2013; Couto et al., Reference Couto, Nunes, Rincon, Paschoal and Pereira2024a).
The genus Acusicola Cressey, 1970 was originally proposed to allocate Acusicola cunula Cressey, 1970, from the Needlefish Pseudotylosurus angusticeps (Günther, 1866) (Actinopterygii: Belonidae) in Brazil, and Acusicola tenax (Roberts, Reference Roberts1965), a parasite of the White crappie Pomoxis annularis Rafinesque, 1818 (Actinopterygii: Centrarchidae) collected in the USA, which was first assigned to Ergasilus. Currently, Acusicola comprises 17 species in which ten were reported from Brazil, mainly in the Amazon River Basin (Cressey and Collette, Reference Cressey and Collette1970; Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023). This taxon belongs to a group of three genera within Ergasilidae that share a five-segmented antennule, and a latching mechanism on its antennae, which allow the copepod to completely encircle the gill filament of its host. This group is composed of Acusicola, Miracetyma Malta, 1994, and Amplexibranchius Thatcher & Paredes, 1835, which was believed to form a unique lineage in the family, supposedly supporting a subfamily named Acusicolinae Thatcher, Reference Thatcher1984; however, such hypothesis was rejected because of the lack of robust phylogenetic evidence (Thatcher, Reference Thatcher1984; Thatcher and Paredes, Reference Thatcher and Paredes1985; Boxshall and Halsey, Reference Boxshall and Halsey2004). Despite these similarities, Acusicola spp. have a two-segmented endopod armed with at least six elements on the first leg, which represents a particular pattern that distinguishes it from the latter two genera (Boxshall and Halsey, Reference Boxshall and Halsey2004).
During a survey of parasitic copepods from the Brazilian Amazon Coast, specimens of Acusicola were collected on the gills of the largescale foureyes Anableps anableps (Linnaeus, 1758) (Actinopterygii: Anablepidae). A detailed morphological study of these specimens revealed that they represent a new species, which is described herein.
Material and methods
Fish were caught in the São Marcos Bay (2°31′48″S, 44°20′28″W) (Figure 1), State of Maranhão, Brazilian Amazon Coast, and kept frozen at −20°C, prior to parasitological examination. Copepods were collected through washing of the gill filaments in flowing water, or detached using a needle, fixed and preserved in 70% ethanol. For observation using light microscopy, parasite specimens were cleared in 85% lactic acid, and the appendages were dissected and examined using the wooden slide procedure described by Humes and Gooding (Reference Humes and Gooding1964). Drawings were made using a drawing tube attached to an Olympus CH2 microscope. Measurements were performed using an ocular micrometer and are presented as range, followed by mean and standard deviation in parentheses, all in micrometers. The descriptive terminology and classification of copepods followed Boxshall and Halsey (Reference Boxshall and Halsey2004). Prevalence and mean intensity were given according to Bush et al. (Reference Bush, Lafferty, Lotz and Shostak1997). Host identification was based on Marceniuk et al. (Reference Marceniuk, Caires, Carvalho-Filho, Rotundo, Santos and Klautau2021) and their nomenclature and classification were updated according to Eschmeyer's Catalog of Fishes (Van der Laan et al., Reference Van der Laan, Fricke and Eschmeyer2023). To avoid ambiguity of some generic names, the following abbreviations were used: ‘A.’ for Acusicola and ‘An.’ for Anableps. Type-specimens were deposited in Coleção Carcinológica do Museu de Zoologia da Universidade de São Paulo (acronym MZUSP), Brazil. Access to genetic heritage was registered in the Sistema Nacional de Gestão do Patrimômio Genético e do Conhecimento Tradicional Associado (acronym SisGen), under the number A03E910, according to Brazilian Federal requirements.
Figure 1. Map indicating the sampling site of the present study (modified from Google Earth): (A) Brazil, showing position of the State of Maranhão (at the centre of the white rectangle); (B) State of Maranhão, showing the municipality of São Luís (delimited by the white rectangle); (C) São Marcos Bay, with the white dot, georeferenced, showing sampling site.
Results
Sixty-two specimens of An. anableps were analysed, in which 21 were parasitized at least by one specimen of Acusicola. A total of 178 female copepods were collected, showing prevalence of 66% and mean intensity of 4.34 copepods per infected fish (range 1–16).
Systematics
Class Copepoda Milne Edwards, 1840
Order Cyclopoida Burmeister, 1834
Family Ergasilidae Burmeister, 1835
Genus Acusicola Cressey, 1970
Type-species: Acusicola cunula Cressey, 1970 by original designation.
Acusicola rochai n. sp.
ZooBank registration: urn:lsid:zoobank.org:pub:AD08F121-4561-4D0A-A620-37CA67A4EDD0
(Figures 2–4)
Material examined
Holotype female (MZUSP-45941) and nine paratype females (MZUSP-45942) collected on the gill filaments of the largescale foureyes An. anableps (Linnaeus, 1758) (Actinopterygii: Anablepidae) (type host) from São Marcos Bay (2°31′48″S, 44°20′28″W), State of Maranhão, Brazil (type locality).
Etymology
The new species is named ‘rochai’ in honour of Dr Carlos Eduardo Falavigna da Rocha for his contribution to the knowledge about the richness and diversity of the genus Acusicola.
Description
Adult female [based on 10 specimens]. Body length from anterior margin of prosome to posterior margin of caudal rami 595–889 (697 ± 101.4). Body comprising prosome and urosome (Figure 2A, B). Prosome consisting of cephalosome, with antennule visible in dorsal view, and four pedigerous somites. Cephalosome and first pedigerous somite fused (=cephalothorax), with boundary almost indistinct (Figure 2A, B). Cephalothorax (Figure 2A, B) bullet-shaped, longer than wide, 283–360 (319 ± 37) × 129–261 (192 ± 47), not inflated and slightly constricted, representing 45% of body length; dorsal surface with anterior naupliar eye bearing five sensilla on each side, inverted T-shaped mark with two sensilla and circular mark posteriorly; three sensilla on each lateral edge (Figure 2A). Second pedigerous somite with two dorsal and two lateral sensilla; third and fourth pedigerous somites with three dorsal and two lateral sensilla each (Figure 2A, B). Second pedigerous somite bearing pair of rounded integumental windows laterally (Figure 2A, B). Third and fourth pedigerous somite with three anterior protrusions (Figure 2A, B). Urosome consisting of fifth pedigerous somite, genital double-somite, and three free abdominal somites; third abdominal somite (= anal somite) bipartite (medially incised). Fifth pedigerous somite short (Figure 2C). Genital double-somite globular, slightly longer than wide 61–77 (72.6 ± 4.6) × 60–75 (68.6 ± 5) (Figure 2C). Free abdominal somites wider than long (Figure 2C); first somite longer than second; anal somite shorter than previous two. Caudal rami 1.5× longer than wide and longer than anal somite, with row of spinules on posterolateral margin near insertion of minor seta; each ramus armed with large apical seta, two medial apical setae and minor ventral seta (Figure 2C). Paired egg-sacs (Figure 3D) longer than wide, each composed of 1–2 rows of eggs.
Figure 2. Acusicola rochai n. sp., adult female: (A) habitus, dorsal, pr = protrusion; (B) habitus, lateral, a1 = antennule, a2 = antenna, mp = mouthparts, p1 = leg 1, iw = integumental window, p2 = leg 2, pr = protrusion, p3 = leg 3, p4 = leg 4, p5 = leg 5; (C) urosome and caudal rami, ventral; (D) rostral area, ventral; (E) antennule, ventral, arrows pointing aesthetascs. Scale bars: A–B, 150 μm; C, 30 μm; D, 35 μm; E, 20 μm.
Figure 3. Acusicola rochai n. sp., adult female: (A) antenna, ventral, black arrow pointing third vestigial segment, white arrow pointing groove; (B) mouthparts, ventral, mb = mandible, me = maxillule, sy = syncoxa, ma = maxilla; (C) interpodal plates of legs 1–4, ventral, arrow pointing protrusions; (D) egg sac, dorsal; (E) leg 5, dorsal. Scale bars: A, 60 μm; B, 20 μm; C, 35 μm; D, 200 μm; E, 10 μm.
Rostrum (Figure 2D) with two anterior and four posterior blunt elements. Antennule five-segmented (Figure 2E), tapering distally, aesthetascs present on fourth and fifth segments; setal formula as follows: 11: 4: 4: 2 + ae: 4 + ae: all setae naked. Antenna (Figure 3A) comprising coxobasis and three-segmented endopod with terminal claw. Coxobasis, first endopodal segment and the first half of the second endopodal segment with hyaline processes on inner margins and enclosed by membranous sheath. Coxobasis short, proximally longer, armed with distally naked seta; membrane between coxobasis and first endopodal segment not inflated. First endopodal segment longest, nearly 6.9× longer than wide, armed with one posterior blunt element and three spiniform elements: one anterior, one medio-lateral, and one posterior, near insertion of the second segment; all elements inserted on cuticular elevations; second endopodal segment longer than wide, representing 35% of previous segment length; third endopodal segment vestigial bearing short, curved claw with fossa on inner margin near tip.
Mouthparts (Figure 3B) include mandible, maxillule, and maxilla; maxilliped absent. Mandible unsegmented bearing palp, anterior, mid, and posterior blades; palp small and naked; anterior blade with small spinules on outer margin; mid blade with long spinules on outer margin; posterior blade with smooth teeth along posterior margin. Maxillule small, bearing inner minute spiniform element and two outer setae. Maxilla comprising large syncoxa with two small setae, one on posterior outer margin and one on inner margin, and naked seta near teeth; second segment (basis) bearing long and sharp anterior teeth with long spinules along anterior, ventral, and apical margins.
Swimming legs 1–4 biramous (Figure 4A–D), each with two-segmented protopod comprising coxa and basis; interpodal plates (Figure 3C) smooth; first intercoxal sclerite with a pair of protrusions anteriorly. Armature of legs (spines, Roman numerals; setae, Arabic numerals) as follows:
Figure 4. Acusicola rochai n. sp., adult female: (A) leg 1, ventral; (B) leg 2, ventral; (C) leg 3, ventral; (D) leg 4, ventral. Scale bars: A–D, 20 μm.
Leg 1 (Figure 4A) coxa unarmed. Basis with outer naked seta and row of spinules on posterior margin, near endopod insertion. Exopod three-segmented, with rows of spinules on outer margin of all segments; first segment with small outer spine; second segment with inner plumose seta; third segment with two unequal subapical spines, long apical semi-pinnate seta, and four plumose setae. Endopod two-segmented, both segments with rows of spinules on outer margin; first segment representing 70% of exopodal ramus length, with plumose inner seta; second segment with two apical pectinate spines, innerspine falciform, and five plumose setae.
Leg 2 (Figure 4B) coxa with protrusion on posterior margin. Basis with outer naked seta. Exopod three-segmented, all segments smooth; first segment longest, with small outer spine; second segment with inner plumose seta; third segment, with six apical plumose setae and small subapical outer spine. Endopod three-segmented, all segments smooth; first segment with plumose inner seta; second segment with two plumose inner setae; third segment with apical curved spine, and five plumose setae.
Leg 3 (Figure 4C) similar to leg 2, except for absence of outer spine on last exopodal segment.
Leg 4 (Figure 4D) coxa with protrusion on posterior margin. Basis with outer naked seta. Exopod two-segmented, both segments smooth; first segment unarmed; second segment with five plumose setae. Endopod three-segmented, all segments with row of spinules on posteroventral margin; first segment with inner plumose seta; second segment with two plumose setae; third segment with long apical spine, representing 62% of endopodal ramus length, and three plumose setae.
Leg 5 (Figure 3E) represented by two unequal naked setae carried on rounded papilla.
Remarks
Representatives of Ergasilidae are characterized by the second antenna modified in a robust prehensile organ, mandibles with two or three spinulate blades, lack of maxilliped in adult females, and leg 4 with one or two segments, or rarely absent (Boxshall and Halsey, Reference Boxshall and Halsey2004). Among the 30 genera hitherto described in the family, Acusicola can be identified based on a five-segmented antennule, a groove on the second endopodal segment of the antennae that latches the claw of the opposite side, allowing a complete encircling of the gill filament, and leg 1 with two-segmented endopod armed with at least six elements (and rarely three) (Amado and Rocha, Reference Amado and Rocha1996; Boxshall and Halsey, Reference Boxshall and Halsey2004; Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023; Walter and Boxshall, Reference Walter and Boxshall2024). Therefore, the present parasitic copepods have all the characters previously mentioned, clearly justifying their allocation in Ergasilidae and Acusicola.
Currently, Acusicola comprises 17 nominal species, but only the following six have the first leg with one spine on the first exopodal segment and five setae on the last endopodal segment as in the new species: Acusicola brasiliensis Amado & Rocha, Reference Amado and Rocha1996, Acusicola joturicola El-Rashidy & Boxshall, Reference El-Rashidy and Boxshall1999, Acusicola margulisae Santacruz, Morales-Serna, Leal-Cardín, Barluenga & de León, Reference Santacruz, Morales-Serna, Leal-Cardín, Barluenga and Léon2020, Acusicola mazatlanesis El-Rashidy & Boxshall, Reference El-Rashidy and Boxshall1999, Acusicola pellonidis Thatcher & Boeger, Reference Thatcher and Boeger1983, and Acusicola spinuloderma El-Rashidy & Boxshall, Reference El-Rashidy and Boxshall1999 (Thatcher and Boeger, Reference Thatcher and Boeger1983a, Reference Thatcher and Boeger1983b; Amado and Rocha, Reference Amado and Rocha1996; El-Rashidy and Boxshall, Reference El-Rashidy and Boxshall1999; Santacruz et al., Reference Santacruz, Morales-Serna, Leal-Cardín, Barluenga and Léon2020; Walter and Boxshall, Reference Walter and Boxshall2024). However, A. rochai n. sp. can be easily differentiated from these congeners by the presence of three anterior protrusions on the dorsal margins of third and fourth pedigerous somites (vs absent on the congeners listed above) and by the smooth urosome (vs with rows of spinules on A. brasiliensis, A. joturicola, A. margulisae, A. mazatlanensis, and A. spinuloderma; and small posterolateral spine on the last abdominal segment in A. pellonidis) (Thatcher and Boeger, Reference Thatcher and Boeger1983a, Reference Thatcher and Boeger1983b; Amado and Rocha, Reference Amado and Rocha1996; El-Rashidy and Boxshall, Reference El-Rashidy and Boxshall1999; Santacruz et al., Reference Santacruz, Morales-Serna, Leal-Cardín, Barluenga and Léon2020).
The new species also differs from A. brasiliensis because its cephalosome is fused to the first pedigerous somite (vs first pedigerous somite free in the latter species); from A. brasiliensis and A. spinuloderma by the absence of a spine on the last exopodal segment of leg 3 (vs spine present in the latter two species); from A. joturicola, A. spinuloderma, A. margulisae, and A. mazatlanensis because its proximal endopodal segment of antenna is smooth (vs with spinules in A. joturicola, A. spinuloderma, and A. mazatlanensis; and striations and setules in A. margulisae); and from A. pellonidis because it has six setae on the last exopodal segment of leg 2 (vs four setae in the latter species) (Thatcher and Boeger, Reference Thatcher and Boeger1983a, Reference Thatcher and Boeger1983b; Amado and Rocha, Reference Amado and Rocha1996; El-Rashidy and Boxshall, Reference El-Rashidy and Boxshall1999; Santacruz et al., Reference Santacruz, Morales-Serna, Leal-Cardín, Barluenga and Léon2020).
Additionally, there is only an additional species of Acusicola that has dorsal protrusions on the pedigerous somites as in the new species, i.e. Acusicola iamarinoi Couto, Pereira, Luque, Paschoal & Pereira, Reference Couto, Pereira, Luque, Paschoal and Pereira2022. Nevertheless, A. rochai n. sp. has three protrusions on the third and fourth pedigerous somites, while in A. iamarinoi the third pedigerous somite is smooth and the fourth pedigerous somite has only two protrusions (Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023). Furthermore, A. rochai n. sp. can be distinguished from A. iamarinoi because its cephalosome is fused to the first pedigerous somite (vs first pedigerous somite free in the latter), it has five setae on the last endopodal segment of leg 1 (vs four setae in the latter), the second and third interpodal plates are smooth (vs with row of spinules in the latter), and its urosome is smooth (vs with rows and patches of spinules in the latter) (Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023).
Discussion
In South America, the family Anablepidae is represented by only two species, namely, An. anableps and Anableps microlepis Müller & Troschel, 1844. Species of this genus are commonly known as ‘foureyes’ fish due to their unusual morphology: eyes divided into two portions, each with an individual pupil, enabling surface-swimming individuals to simultaneously focus on images above and below water. Anableps anableps is commonly found inhabiting freshwater and mangrove coastlines in Brazil, primarily in the Amazon River Delta, Northeast coast, and is frequently used as a subsistence resource by some populations (Nelson et al., Reference Nelson, Grande and Wilson2016; Figueiredo et al., Reference Figueiredo, Nunes, Almeida, Paz, Piorski and Reis2019; Rodrigues et al., Reference Rodrigues, Machado, Oliveira and Andrade2021; Froese and Pauly, Reference Froese and Pauly2024). Despite its local importance and peculiar aspect, only five species of parasitic crustaceans have been reported on An. anableps: Gnathia sp. (praniza larvae) in the State of Pará, Excorallana longicornis Lemos de Castro, 1960 (Corallanidae), and Nerocila acuminata Schiödte & Meinert, 1881 (Cymothoidae), both from the State of Amapá, Cymothoa curta Schioedte & Meinert, Reference Schioedte and Meinert1884 in an unspecified locality, and Cymothoa sp. (both Cymothoidae) off the State of Pará, Brazil (Schioedte and Meinert, Reference Schioedte and Meinert1884; Diniz et al., Reference Diniz, Varella, Guimarães, Santos, Fujimoto, Monfort, Pires, Martins and Eiras2008; Esteves-Silva et al., Reference Esteves-Silva, Oliveira, Gentil-Vasconcelos, Costa-Campos and Tavares-Dias2020; Loureiro et al., Reference Loureiro, Trindade, Diniz, Vallinoto, Diniz and Giarrizzo2021). In this sense, the present study represents the first report of a parasitic copepod infesting an Anablepidae fish in Brazil and, consequently, the host An. anableps. Additionally, considering the potential for parasitic crustaceans observed in this host species, it is reasonable to consider this fish as a potential host for other copepods, which demands further investigations to better understand its parasitic fauna and ecological interactions. Moreover, this work also underscores the high biodiversity potential of parasitic copepods in fish with little commercial importance, which are frequently neglected due to the historically uneven research, widely documented in Brazil (Luque and Poulin, Reference Luque and Poulin2007; Luque and Tavares, Reference Luque and Tavares2007; Luque et al., Reference Luque, Vieira, Takemoto, Pavanelli and Eiras2013; Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023, Reference Couto, Nunes, Rincon, Paschoal and Pereira2024a, Reference Couto, Potes, Feitosa, Pereira and Paschoal2024b).
When analysing the descriptions of new species within Acusicola, it is common to use armature of legs, proportion of segments in the antennae and leg 1, body size, and other morphometric data of females for diagnosing the species (Roberts, Reference Roberts1965; Cressey and Collette, Reference Cressey and Collette1970; Thatcher and Boeger, Reference Thatcher and Boeger1983a, Reference Thatcher and Boeger1983b; Thatcher, Reference Thatcher1984; Amado and Rocha, Reference Amado and Rocha1996; El-Rashidy and Boxshall, Reference El-Rashidy and Boxshall1999; Araújo and Boxshall, Reference Araújo and Boxshall2001; Santacruz et al., Reference Santacruz, Morales-Serna, Leal-Cardín, Barluenga and Léon2020; Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023). Boxshall (Reference Boxshall2016) asserted that in many genera of Ergasilidae, the armature of swimming legs should be used with caution due to imprecise descriptions, particularly in older studies. Although we agree with this author in respect to many genera of ergasilids, in regard to Acusicola, this feature appears to be well-documented and reliable for species differentiation. Therefore, we encourage authors to use these characteristics for species diagnosis in the particular case of Acusicola.
Morphometric data and ratios of body to appendage segment lengths have proven to be very useful across many families of cyclopoid copepods, including Ergasilidae (Suárez-Morales et al., Reference Suárez-Morales, Santana-Piñeros and González-Solís2008; Oliveira et al., Reference Oliveira, Corrêa, Adriano and Tavares-Dias2021; Uyeno and Nagasawa, Reference Uyeno and Nagasawa2021; Paschoal et al., Reference Paschoal, Couto, Pereira and Luque2022, Reference Paschoal, Couto, Pereira and Luque2023). Despite such an importance, the way that the information is described varies considerably in Acusicola, with numerous species lacking complete information about their measurements or proportions (Cressey and Collette, Reference Cressey and Collette1970; Amado and Rocha, Reference Amado and Rocha1996; El-Rashidy and Boxshall, Reference El-Rashidy and Boxshall1999; Araújo and Boxshall, Reference Araújo and Boxshall2001; Santacruz et al., Reference Santacruz, Morales-Serna, Leal-Cardín, Barluenga and Léon2020). Therefore, it is highly recommended that further studies provide descriptions with most detailed morphometric data possible, to facilitate the intraspecific comparisons among congeners.
The use of body size as a diagnostic feature for parasitic copepods is also contradictory, since it can be heavily influenced by host–parasite interactions, as highlighted by Araújo and Boxshall (Reference Araújo and Boxshall2001) and that we agree. Therefore, authors should prioritize other characters whenever feasible, as previously commented. Additionally, in the present study, the ornamentation of body segments (presence and absence of spinules, protrusions, etc.), interpodal plates and appendages, has proven to be useful for supplementing the specific diagnosis of the new species, and differentiates it from the closest congeners. This approach is common regarding other ergasilid genera (e.g. Ergasilus), but not that frequent in Acusicola (Boxshall, Reference Boxshall2016; Taborda et al., Reference Taborda, Paschoal and Luque2016; Marques et al., Reference Marques, Clebsh, Córdova and Boeger2017; Couto et al., Reference Couto, Nunes, Rincon, Paschoal and Pereira2024a). In this sense, despite the small size and occasional difficult visualization, it is fundamental for future studies not to neglect these features and provide more detailed descriptions, in order to enhance the knowledge on the morphological diversity of Acusicola spp., as well as of ergasilid copepods in general.
Most of the evolutionary modifications observed in parasitic copepods are reflected in their attachment apparatus, which influences the interaction with their hosts and the pathology they cause (Boxshall and Halsey, Reference Boxshall and Halsey2004; Pádua et al., Reference Pádua, Jerônimo, Menezes-Filho, Taboga, Martins and Andrade Belo2015). In Ergasilidae, the prehensile antenna is usually the main appendage responsible for attachment, and varies greatly among genera. A judicious morphological analysis of the antennae from the species of Acusicola indicates great diversity of adaptations, which can be extremely informative for taxonomists. For example, the antennae of the three species described by El-Rashidy and Boxshall (Reference El-Rashidy and Boxshall1999) differ considerably in length, width, proportion and ornamentation of the first endopodal segment, and in relation to the membranous sheath: in A. mazatlanensis, this segment is 6.5× longer than wide, with small conical spinules, and the membranous sheath reaches the half of the second endopodal segment; in A. joturicola, it is also 6.5× longer than wide and share small conical spinules, but the membranous sheath reaches the first half of the outer margin of the claw; and A. spinuloderma has this segment 5.5× longer than wide, with large conical spines that decrease in size proximally and distally, and the membranous sheath reaches the vestigial third endopodal segment. In the study by Amado and Rocha (Reference Amado and Rocha1996), the species also shows variation in the length-to-width ratio of the first endopodal segment, in the proportion of the second endopodal segment compared to the anterior segment, and in the ornamentation of both segments. Moreover, other notable variations can be observed in the descriptions of A. pasternakae and A. minuta (Araújo and Boxshall, Reference Araújo and Boxshall2001; Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023). Nevertheless, as the antennae represent the main attachment structure in Acusicola, it is reasonable to assume that it exhibits many of the adaptations acquired during its evolutionary history and holds taxonomic value, as previously discussed. In conclusion, it is recommended to consider the modifications of the antennae together with the leg armature, as important diagnostic features when dealing with these parasitic copepods.
Since the erection of Acusicola, only one dichotomous key has been provided for species identification (Amado and Rocha, Reference Amado and Rocha1996). Amado and Rocha (Reference Amado and Rocha1996) relied primarily on the armature and proportion of leg elements of the ten known species at that time. Subsequently, eight additional species have been described, and new morphological traits have been documented in the genus, highlighting the necessity of updating this important taxonomic tool (Couto et al., Reference Couto, Pereira, Luque, Paschoal and Pereira2023; Walter and Boxshall, Reference Walter and Boxshall2024). Recently, Couto et al. (Reference Couto, Potes, Feitosa, Pereira and Paschoal2024b) observed that mounting copepods in permanent slides can compress and distort body shape. Although the shape of cephalosome seems to be taxonomically informative in Ergasilidae, we are cautious when using this feature here, since A. tucunarense, A. pellonidis, and A. lycengraulidis were described based on permanent slides and could suffer from the same problems observed in previous works (Thatcher and Boeger, Reference Thatcher and Boeger1983a, Reference Thatcher and Boeger1983b; Thatcher, Reference Thatcher1984; Couto et al., Reference Couto, Potes, Feitosa, Pereira and Paschoal2024b). Morphometric data and proportions of body, and leg segments and elements were used only as supporting data for identifying only certain species, since not all species of Acusicola has this information available. The prehensile antenna was also used here as an important diagnostic feature, since it appears to be highly informative in the genus (see above). The key provided herein was primarily developed in accordance with the features used by Amado and Rocha (Reference Amado and Rocha1996), and supplemented with other reliable, easily observable and well-documented characters. The objective of this key is to be reliable and straightforward, as well as to be used by specialists on copepod taxonomy and other parasitologists alike. The key is provided as follows:
Key for species of Acusicola
(1) Cephalosome fused to first pedigerous somite ……… (2)
Cephalosome not fused to first pedigerous somite ……… (8)
(2) Second endopodal segment of leg 1 with two spines and five setae; third exopodal segment of leg 2 with outer spine ……… ……… (3)
Second endopodal segment of leg 1 with two spines and three setae; third exopodal segment of leg 2 without outer spine ……… ……… A. lycengraulidis
(3) First exopodal segment of leg 1 with outer spine; third exopodal segment of leg 1 with two spines and five setae ……… (4)
First exopodal segment of leg 1 unarmed; third exopodal segment of leg 1 with three spines and four setae ……… A. tenax
(4) Third and fourth pedigerous somites with dorsal surface smooth ……… (5)
Third and fourth pedigerous somites with three anterior protrusions on dorsal surface each ……… A. rochai n. sp.
(5) Claw of antenna not enclosed by membranous sheath ……… (6)
Claw of antennae enclosed by membranous sheath on outer margin ……… A. joturicola
(6) First endopodal segment of antenna smooth or ornamented with small conical spinules ……… (7)
First endopodal segment ornamented with large conical spinules decreasing in size proximally and distally ……… ……… A. spinuloderma
(7) First endopodal segment of antenna smooth; this segment is about 4.9× longer than wide ……… A. pellonidis
First endopodal segment of antenna ornamented with small cone-shaped spinules; this segment is about 6.5× longer than wide ……… A. mazatlanensis
(8) Second endopodal segment of leg 1 with at least six elements ……… (9)
Second endopodal segment of leg 1 with three minute spine ……… A. spinulosa
(9) First exopodal segment of leg 1 with outer spine ……… (10)
First exopodal segment of leg 1 unarmed ……… A. tucunarense
(10) Third endopodal segment of leg 1 with two spines and four or five setae; first endopodal segment of leg 4 with one seta ……… ……… (11)
Third endopodal segment of leg 1 with one spine and four setae; first endopodal segment of leg 4 unarmed ……… A. rogeri
(11) Leg 5 reduced to single seta ……… (12)
Leg 5 reduced to two setae ……… (13)
(12) Third exopodal segment of legs 2 and 3 with outer spine; second endopodal segment of antenna not enclosed by dark membrane ……… A. cunula
Third exopodal segment of legs 2 and 3 without outer spine; second endopodal segment of antenna enclosed by dark membrane ……… A. paracunula
(13) Second endopodal segment of leg 1 with two spines and four setae ……… (14)
Second endopodal segment of leg 1 with two spines and five setae ……… (16)
(14) Fourth pedigerous somites with dorsal surface smooth ……… (15)
Fourth pedigerous somites with two anterior protrusions on dorsal surface ……… A. iamarinoi
(15) Cephalosome inflated, with antennal area projected forwards; first exopodal segment of legs 2 and 3 unarmed ……… ……… A. rotunda
Cephalosome not inflated, antennal area not projected forwards; first exopodal segment of legs 2 and 3 with outer spine ……… ……… A. pasternakae
(16) Second endopodal segment of antenna without inner membranous expansions; first endopodal segment of leg 1 longer than whole exopod ……… (17)
Second endopodal segment of antenna with two inner membranous expansions; first endopodal segment of leg 1 shorter than whole exopod****A. minuta
(17) Second exopodal segment of legs 1–3 with small inner process near setae ……… A. margulisae
Second exopodal segment of legs 1–3 without small inner process ……… A. brasiliensis
Data
The authors confirm that the data supporting the findings of this study are available within the article.
Acknowledgements
We wish to thank Filipe Ribeiro Menks and Saturno de Sousa Dias from Universidade Federal do Maranhão in São Luis, State of Maranhão, Brazil, for helping with fish collection and parasitological analysis.
Author contributions
J. L. S. N. and F. P. performed field collections and the parasitological survey. J. V. C., F. P., and F. B. P. analysed the copepods, prepared the illustrations, and wrote the first draft of the manuscript. All reviewed the manuscript and approved the final version. F. P. and F. B. P. supervised the study.
Financial support
J. V. C. was supported by Coordenação de Aperfeiçoamento de Pessoal do Ensino Superior (CAPES) (Financial Code 001), Brazil. F. P. was supported by Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) (84516/2022), Brazil. F. B. P. was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Universal 404083/2021-8), Brazil.
Competing interest
None.
Ethical standards
All applicable institutional, national, and international guidelines for the care and use of animals were followed.
