Introduction

Derogenids are hemiuroid digenean gut parasites, occurring in fishes. Throughout most of their taxonomic history, they were accommodated within a broad concept of the family Hemiuridae Looss, 1899 [1]. The Derogenidae Nicoll, 1910 was first used at full family rank by Dollfus [2] but was initially erected at the subfamily level by Nicoll [3] as the Derogeninae Nicoll, 1910 (referred to as the Derogeninae Dollfus, 1950 by Skrjabin and Guschanskaja [4]). The latter authors included the subfamily within the Halipegidae Poche, 1926, but the Derogenidae has priority [1].

Gibson and Bray [5] established the initial foundational classification of the Derogenidae, offering identification keys for its subfamilies and genera. Within this context, Gibson [1] acknowledged the presence of three subfamilies: Derogeninae Nicoll, 1910, Halipeginae Poche, 1926, and Gonocercinae Skrjabin & Guschanskaja, 1955. At present, the Derogenidae comprises only two subfamilies: Halipeginae Poche, 1926, and Derogeninae [1]. This adjustment in classification occurred due to a molecular study conducted by Sokolov et al. [6], who elevated the Gonocercinae to the status of a full family.

Five valid genera are included in the Derogeninae: Derogenes Lühe, 1900, Gonocercella Manter, 1940 [7], Leurodera Linton, 1910, Progonus Looss, 1899, and Derogenoides Nicoll, 1913 [1, 8].

Records of derogenine derogenids in the Mediterranean are rare [9]. Thus, previous records of Derogenes spp. in this region include D. adriaticus Nikolaeva, 1966, D. crassus Manter, 1934, D. fuhrmanni Mola, 1912, D. latus Janiszewska, 1953, D. minor Looss, 1901, D. ruber Lühe, 1900, and D. varicus (Müller, 1784) [10, 11]. However, most of the records lack morphological information justifying correct species identification and making the assessment of host–parasite associations difficult if not impossible. For example, only in the Mediterranean, D. varicus has been reported in 15 hosts of 13 unrelated fish families [11] indicating that this “generalist” species may represent a species complex. This has been suggested by Bray et al. [12] and Køie [13], and a recent study based on multigene sequence data supported this suggestion by providing evidence for the existence of four genetic lineages of D. varicus [14, 15].

Derogenes ruber, the type-species of the genus was less frequently encountered and reported. The type-material of D. ruber was described by Lühe [16] from the gall-bladder of the streaked gurnard Trigla lineata Gmelin, 1789 (a junior synonym of Chelidonichthys lastoviza (Bonnaterre, 1788)) off Rovinj, Croatia, Adriatic Sea. This trematode is known from the short original description that lacked illustrations, and a subsequent general illustration, based on a record and identification from a different host, the piper gurnard Trigla lyra L. from a close locality, off Split, Croatia [17]. Bouguerche et al. [15] redescribed this species based only on two specimens found in Arthur Looss’s collection and did not thus provide any molecular data. Other reports of this derogenid are from the North-East Atlantic (off Azores, Canary and Cape Verde Islands [18], and off Spain [19].

During parasitological surveys of helminths of fishes from off the southern coasts of the Western Mediterranean off Algeria, we collected representatives of D. ruber from the gall bladder of its type-host, C. lastoviza. The aim of the present study is to provide a formal redescription of D. ruber and to characterise the species genetically based on partial 28S ribosomal RNA gene (28S rRNA), internal transcribed spacer ITS2, and a fragment of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene sequences.

Materials and Methods

Collection and Sampling of Fish

A total of 168 specimens of C. lastoviza were collected during 2017–2019, from local fishermen immediately after capture in different regions off the Algerian coast: Ghazaouet (35° 06′ 0′′ N, 1° 51′ 0′′ W), Cherchell (36° 36′ 31′′ N, 2° 11′ 50′′ E), Bouharoune (36° 37′ 24′′ N, 2° 39′ 17′′ E), Alger (36° 45′ 8′′ N, 3° 2′ 31′′ E), Bordj el Bahri (36° 47′ 26′′ N, 3° 14′ 59′′ E), Ain Taya (36° 47′ 30′′ N, 3° 17′ 20′′ E), Reghaia (36° 43′ 60′′ N, 3° 21′ 0′′ E), Cap Djinet (36° 52′ 37′′ N, 3° 43′ 23′′ E), and Dellys (36° 54′ 48′′ N, 3° 54′ 51′′ E). Fish specimens were kept on ice and transferred immediately to the laboratory, identified using the key [20, 21], and examined on the day of purchase. Viscera were placed in separate Petri dishes containing seawater and observed under a Zeiss microscope for the presence of digeneans.

Morphological Methods

Live digeneans were killed and fixed in near-boiling water. Specimens for morphological analysis were fixed under cover-glass pressure in Bouin’s fluid [10], then preserved in 70% ethanol, stained with acetic carmine, dehydrated through a graded alcohol series, cleared in clove oil, and mounted in Canada balsam as permanent mounts. Five specimens were preserved immediately in 96% ethanol for molecular characterisation and were processed as hologenophores (sensu Pleijel et al. [22]).

Permanent mounts of the hologenophores, consisting of 2/3 of the body (posterior third excised and used for sequencing), stained and mounted in Canada balsam. Drawings were made using a Zeiss microscope (Université des Sciences et de la Technologie Houari Boumediene, USTHB) and a Nikon Eclipse i80 microscope with DIC (differential interference contrast) (Swedish Museum of Natural History, SMNH) equipped with a drawing tube, and scanned and redrawn with Adobe Illustrator 2023, version 28.0.

Measurements are in given in micrometres and presented as the range followed by the mean in parentheses. Voucher material was deposited at the Swedish Museum of Natural History (SMNH), Stockholm, Sweden under accession numbers SMNH 218781–SMNH 218 805.

Molecular Methods

Genomic DNA was extracted from a total of five hologenophores, and genetic sequence data were generated for three genetic markers: a partial region of the mitochondrial cytochrome c oxidase subunit 1 gene (cox1), the second internal transcribed spacer region (ITS2 rDNA), and the large (28S) ribosomal RNA gene. A small fragment of each hologenophore (posterior third) was placed in a 1.5 ml microcentrifuge tube containing 20 μL buffer ATL (Qiagen, Hilden, Germany). For extraction of genomic DNA (gDNA), 20 μL buffer ATL and 20 μL proteinase K were added to each sample, followed by vortexing and incubation in an incubating microplate shaker at 56 °C and 300 rpm overnight. The lysed samples were processed to obtain gDNA following the manufacturer’s instructions for gDNA extraction using the Qiagen QiAmp DNA Microkit. Polymerase chain reaction (PCR) amplification was performed in 25 µl reaction mix using Illustra Hot Start Mix RTG (0.2 µl) reaction kit (GE Healthcare Life Sciences, Uppsala, Sweden). The reaction mix consisted of 1 µl (0.4 µM) of each primer, 2 µl template DNA, and 21 µl nuclease-free water. The primer set JB3 (5′-TTT TTT GGG CAT CCT GAG GTT TAT-3′) and COI R-Trema (5′-CAA CAA ATC ATG ATG CAA AAG G-3′) were used to amplify a fragment the cox1 gene [23]. The thermocycling profile consisted of an initial denaturation step at 94 °C for 5 min, followed by 35 cycles of denaturation at 94 °C for 30 s, annealing at 45 °C for 30 s, and extension at 72 °C for 1 min, with a final extension step at 72 °C for 10 min [14]. Primers, amplification, and sequencing protocols for the 28S rDNA region followed Pérez-Ponce de León et al. [24] and García-Varela and Nadler (2005) [25]. The thermocycling profile consisted of an initial denaturation step at 94 °C for 3 min, followed by 35 cycles of denaturation at 94 °C for 60 s, annealing at 54 °C for 60 s, and extension at 72 °C for 1 min, with a final extension step at 72 °C for 7 min. ITS2 rDNA spacer was amplified using the primers 3S [26] and ITS2.2 [27] and the following thermocycling profile: an initial denaturation step at 94 °C for 3 min, followed by 35 cycles of denaturation at 94 °C for 1 min, annealing at 54 °C for 1 min, and extension at 72 °C for 1 min, with a final extension step at 72 °C for 7 min. PCR products were purified (Ampure XP Kit, Beckman Coulter, Indianapolis, USA) and sequenced in both directions on a 3730 l DNA Analyzer 96-capillary sequencer (Applied Biosystems, Foster City, CA, USA). We used CodonCode Aligner version 3.7.1 software (Codon Code Corporation, Dedham, MA, USA) to edit sequences and compared them to the GenBank database content using BLAST. The newly generated sequences are deposited in the GenBank database under the accession numbers OQ919798-OQ919804, OQ919806, OR245546, and OR245386.

Phylogenetic analyses were performed using the newly generated sequences of D. ruber and those for Derogenidae species available in GenBank (Table 1). Alignments for each gene region were constructed in AliView [28] and trimmed to the length of the shortest sequence. Nucleotide substitution models for phylogenetic analyses using the maximum-likelihood method were estimated using MEGA11 [29]. The best-fit models selected were the Kimura 2-parameter model with gamma distributed amongst-site rate variation (K2 + G) for the 28S rDNA alignment, Kimura 2-parameter (K2) model for the ITS2 alignment, and Tamura-Nei model (TN93) with estimates of invariant sites and gamma distributed amongst-site rate variation (HKY + I + G) for cox1. All trees were constructed in MEGA11, with 500 replications. Genetic distances [uncorrected p-distance model (Kimura 1980)] were computed with MEGA11.

Table 1 Hosts, locality, and GenBank accession data for the sequences of Derogenes spp. and halipegine derogenids analysed in this study
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Results

Family Derogenidae Nicoll, 1910

Subfamily Derogeninae Nicoll, 1910

Genus Derogenes Lühe, 1900

Derogenes ruber Lühe, 1900 (Fig. 1 A-E)

Fig. 1
figure 1

Derogenes ruber from Chelidonichthys lastoviza. A Hologenophore, ventral view, SMNH 218782. B Whole-body, ventral view, SMNH 218789. C Posterior extremity showing ends of caeca. D Egg, SMNH 218789. E Anterior extremity showing details of terminal genitalia, SMNH 218789. Abbreviations: C.: caecum; E.: egg; G.a.: genital atrium; G.p.: genital pore; M.d.: male duct; Ph.: pharynx; P.P.C.: prostatic cells; S.: sphincter; S.s.: sinus-sac; S.o.: sinus-organ; S.v.: seminal vesicle; U.: uterus

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Type-host: Streaked gurnard Chelidonichthys lastoviza (syn. Trigla lineata) [16].

Other reported host: Piper gurnard Trigla lyra [17].

Type-locality: Off Rovinj, Croatia, Adriatic Sea [16].

Other localities: Off Split, Croatia, Adriatic Sea [17]; off Azores, Canary and Cape Verde islands [18] and off Spain [19], North-East Atlantic; Off Trieste, Italy, Western Mediterranean [15]; off Algeria, Western Mediterranean, present study.

Site in host: Gall bladder.

Other sites in host: Intestine [15].

Voucher material: A total of 25 voucher specimens are deposited in the collections of the Swedish Museum of Natural History, Stockholm (SMNH 218781- SMNH 218805) including 5 hologenophores (SMNH 218785, GenBank OR245386, OQ919806, OQ919799; SMNH 218786, GenBank OR245546, OQ919798; SMNH 218782, GenBank OQ919801, OQ919800; SMNH 218785, GenBank OQ919804; SMNH OQ919802, OQ919803).

Redescription

[Based on 20 specimens mounted in toto and 5 hologenophores, metrical data are provided in Table 2.] Body stout, fusiform (Fig. 1A, B), widest at ventral sucker level. Tegument smooth. Pre-oral lobe present. Oral and ventral suckers well developed; oral sucker ventro-subterminal, subglobular-to-globular, wider than long; ventral sucker larger than oral sucker, spherical, located in posterior half of body. Forebody somewhat longer than hindbody. Prepharynx absent. Pharynx well developed, subglobular, muscular. Oesophagus short, barely visible, opening posteriorly via sphincter (Fig. 1E) to join intestinal bifurcation in anterior half of forebody, immediately posterior to pharynx. ‘Drüsenmagen’ not observed. Caeca broad, thick walled, extending into hindbody, reaching beyond gonads, terminating close to posterior extremity (Fig. 1C). Termination of caeca often obscured by eggs.

Table 2 Metrical data for Derogenes ruber from Chelidonichthys lastoviza and Trigla lyra
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Testes two, entire, rounded, symmetrical, pre-ovarian, posterior to ventral sucker and separated by uterine coils. Seminal vesicle external, tubular, thin-walled, in forebody. Pars prostatica long, tubular, surrounded by numerous gland cells, extends between distal end of seminal vesicle and sinus-sac. Metraterm protruding along with ejaculatory duct into sinus-sac forming hermaphroditic duct (Fig. 1E). Sinus-sac muscular. Sinus-organ muscular, conical, projecting into genital atrium. Genital pore ventro-median, posterior to pharynx, at level of intestinal bifurcation (observed only in five specimens).

Ovary transversely-oval, sinistral, post-testicular, at 1110 from posterior extremity. Oviduct, oötype, and Laurer’s canal not observed. Uterus well developed, coiled throughout much of hindbody and in forebody as far as level of sinus-sac. Vitellarium comprises two symmetrical, subglobular, multi-lobed, post-ovarian masses; right vitelline mass composed of 8–10 lobes; left vitelline mass composed of 7–9 lobes. Eggs numerous, small, tick-shelled, without opercular spines or filaments (Fig. 1D).

Excretory vesicle Y-shaped; bifurcation not observed; arms unite dorsally to oral sucker in forebody (Fig. 1B); excretory pore terminal.

Molecular Characterisation of the Digeneans

Four sequences (∼841 bp) for the nuclear 28S rRNA gene were obtained for D. ruber. The tree built using the newly generated sequences plus 20 sequences for species of Derogenes and the subfamily Halipeginae and Prosogonotrema bilabiatum Vigueras, 1940 as the outgroup yielded the topology shown in Fig. 2. There were a total of 688 positions in the final dataset. The general topology of the ML tree agreed with the taxonomic classification of the included species and distinct lineages. Species/lineages of Derogenes were recovered in five strongly supported reciprocally monophyletic clades: (i) D. ruber from C. lastoviza off Algeria; (ii) D. lacustris Tsuchida, Flores, Viozzi, Rauque et Urabe, 2021from Galaxias maculatus (Jenyns) off Argentina [31]; (iii) Lineage “D. varicus DV1” from fish hosts in the White and Barents seas [14]; (iv) Lineage “D. varicus DV2” from mollusc hosts in the White Sea [14]; and (v) Lineage “D. varicus DV3” from Eumicrotremus fedorovi Mandrytsa. in the Pacific Ocean [32]. All Derogenes spp. lineages (Derogeninae) clustered in a strongly supported clade distinct from that of the representatives of the Halipeginae.

Fig. 2
figure 2

Tree inferred using the maximum-likelihood method based on the 28S rDNA sequence data; only bootstrap values higher than 70 are indicated. The newly generated sequences are indicated in red. Lineages “Derogenes varicus DV1, DV2, DV3” and Derogenes lacustris are highlighted in differently colored boxes

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The four newly generated 28S sequences of D. ruber were identical. They differed from Lineage “D. varicus DV1” from various fish hosts in the White and Barents seas (see above) by 2% (16 substitutions); from Lineage “D. varicus DV2” from Hippoglossoides platessoides (Fabricius) from North Sea and from a mollusc Buccinum scalariforme Møller. from the White Sea by 3% (20 substitutions); and from lineage “D. varicus DV3” by 2% (16 substitutions). Sequences of D. ruber differed from those of D. lacustris from G. maculatus (Jenyns) off Argentina by 9% (68 substitutions). Intraspecific/intralineage divergence for Derogenes spp./lineages ranged between 0 (for D. varicus lineages DV1, DV2, and DV3) and 1 substitution (for D. ruber and D. lacustris).

Five ITS2 sequences (∼566 bp) were obtained for D. ruber. The tree built using the newly generated sequences aligned with 12 sequences for Derogenes spp. and Prosogonotrema bilabiatum as the outgroup is shown in Fig. 3A. Derogenes ruber and the lineages “D. varicus DV1” from various fish hosts in the White and Barents seas and “D. varicus DV2” from the molluscs B. scalariforme, Amauropsis islandica (Gmelin) and Euspira pallida (Broderip & Sowerby) from the White and Barents seas clustered in reciprocally monophyletic groups with a maximum nodal support.

Fig. 3
figure 3

Trees inferred using the maximum-likelihood method based on the ITS2 rDNA and cox1 sequence data. A, ITS2 rDNA tree; only bootstrap values > 70 are indicated. The newly generated sequences are indicated in red. Lineages “D. varicus DV1 and “D. varicus DV2” are highlighted in differently colored boxes. There were no ITS2 sequences available for D. lacustris. B, cox1 tree; only bootstrap values > 70 are indicated. The newly generated sequences are indicated in red. Derogenes lacustris and lineages “D. varicus DV1”, “D. varicus DV2” are in different colors. There were no cox1 sequences available for the lineage “D. varicus DV3"

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The five newly generated ITS2 sequences for D. ruber were also identical and differed from those for the lineage “D. varicus DV2” by 4% (16 substitutions) and from those for the lineage “D. varicus DV1” by 5% (21 substitutions). None of the taxa included in the analysis showed intraspecific/intralineage variation.

The two newly generated cox1 sequences of D. ruber (∼898 bp) were identical. We also included in the analysis four sequences of D. varicus (sensu stricto) from Merlangius merlangus (L.) from off Sweden [15]. The tree built using the newly generated sequences aligned with 22 sequences for Derogenes spp. and Didymocystis wedli Ariola, 1902 as the outgroup is shown in Fig. 3B. The species/lineages of Derogenes formed four reciprocally monophyletic groups with maximum support: (i) Derogenes lacustris from salmonids off Argentina; (ii) D. ruber from C. lastoviza off Algeria; (iii) Lineage “D. varicus DV1” from M. merlangus off Sweden, and fish hosts in the White and Barents seas [14]; and (iv) Lineage “D. varicus DV2” from mollusc hosts in the White Sea [32]. The intraspecific divergence between the newly generated cox1 sequences for D. ruber was 0.8% (7 substitutions). Sequences of D. ruber differed from the sequences for lineages “D. varicus DV1” and “D. varicus DV2” by 19% (158 substitutions) and 17% (135 substitutions), respectively. The largest genetic divergence was found between D. ruber and D. lacustris (23%; 186 substitutions). Intraspecific/intralineage divergence for Derogenes spp./lineages ranged between 0% (Lineage “D. varicus DV2”) and 2% (D. ruber: 0.8%; D. lacustris: 0.1–0.2; Lineage “D. varicus DV1”: 1%).

Discussion

Derogenes ruber was described from the gall bladder of the streaked gurnard C. lastoviza off Rovinj, Croatia, Adriatic Sea [16]. Although the original description of D. ruber was detailed, it lacked illustrations. The only subsequent illustration of this species is that of Sey [17], which barely shows any internal organs and omits any details of the terminal genitalia. Sey (1968) examined three specimens of a distinct host, T. lyra, and redescribed briefly D. ruber based on two specimens. Although the geographical distribution of the type-host, C. lastoviza, is wide, D. ruber has been reported only from the Central Mediterranean (Adriatic Sea off Croatia, type-locality in the original description [16] and later, from a different host [17] and recently from the type-host off Italy, based on A. Looss’s material) [15]. The latest record despite providing few morphometrical data and illustration did not include any genetic data. Derogenes ruber was reported from the type-host in the North-East Atlantic, off Azores, Canary and Cape Verde islands [18] and off Spain [19]. Consequently, this paper provides a detailed illustrated description of D. ruber and Algeria as a new locality for this digenean. Additionally, we genetically characterised for the first time D. ruber using the partial fragments of the nuclear 28S rRNA gene and ITS2, and the mitochondrial cox1 gene. Most sequences for Derogenes spp. available to date are those provided in an extensive study by Krupenko et al. [14] and Tsuchida et al. [31] who provided abundant data, corresponding to the “candidade” D. varicus species complex and D. lacustris, respectively. Krupenko et al. [14] have shown the existence of four groups (labelled as DV1-DV4) within the “candidade” D. varicus species complex; of these, they considered that two (DV1 and DV2) may belong to distinct species [14]. Recently, Bouguerche et al. [15] demonstrated that DV1 is in fact D. varicus sensu stricto.

Herein, the 28S rDNA analysis recovered D. ruber in a clade distinct from lineages “D. varicus DV1, DV2, and DV3” and the well-established species D. lacustris. The ITS2 analysis supported the monophyly of D. ruber, and lineages “D. varicus DV1” and “D. varicus DV2” and the cox1 tree yielded a similar topology. Although the sequences obtained herein were short affecting thus the alignment’s length, the analysis led to results similar to those of Krupenko et al. [14].

More importantly, the genetic distance for the cox1 gene between D. ruber and lineages “D. varicus DV1” and “D. varicus DV2” was 19% and 17%, respectively; D. ruber also differed from D. lacustris by 23%. These levels of genetic divergence agree well with previously reported interspecific divergence based on cox1 within the closely related halipegine derogenids ranging between 10.5–15.1% for Genarchopsis spp. [23] and 16.9–20.4% for Genarchopsis Ozaki, 1925 and Allogenarchopsis Urabe & Shimazu, 2013 [33]. Furthermore, the levels of interspecific genetic divergence are more than ten times greater than those for the intraspecific divergence for the mitochondrial “barcode” marker, thus supporting the recognition of D. ruber as a valid distinct species. The molecular data generated herein advance our knowledge on interspecific genetic variations within Derogenes and will help further efforts to untangle the D. varicus species complex and delimit the potentially cryptic species hidden under the single name “D. varicus”. Additionally, the morphometrical data of D. ruber from the type-host (Table 2) will help accessing interspecific morphological differences.

A problem arises when comparing D. ruber to a closely related species, D. latus Janiszewska, 1953, first described based on a single specimen in the intestine of Mullus barbatus Linnaeus from the same Adriatic locality as that of D. ruber, off Split, Croatia [34]. Derogenes latus was redescribed from the intestine of M. barbatus and Trisopterus capelanus (Lacépède) in the North Adriatic Sea [35] and from the gall bladder of M. surmuletus off Corsica (France), Western Mediterranean [10]. The redescription provided by Bartoli and Gibson [10] (based on accessible voucher material and serial sections) should undoubtfully be referred to as the most detailed modern redescription of D. latus. Derogenes latus has been frequently reported from its type-host in the Western Mediterranean, off Spain [36] and off France [37], and from a closely related host, M. surmuletus, in the Western Mediterranean (off France and Algeria) [37,38,39].

This species has also been reported on hosts other than Mullidae, mainly from S. scrofa (Scorpaenidae) in the Western Mediterranean, off Spain [40] and off France [41]; from L. mormyrus (Sparidae) off Montenegro, Adriatic Sea [42] and off Algeria, Western Mediterranean [43]. It was furthermore recorded from Sardinella aurita Valenciennes. (Dorosomatidae) off Algeria, Western Mediterranean [44] and from Phycis phycis (Linnaeus) (Phycidae) from the Western Mediterranean (off France) [41].

The taxonomic status of D. latus is uncertain. The distinction D. ruber and D. latus has been questioned [10], and the two species share a stout body, post-testicular vitellarium composed of two multi-lobed masses and a uterus occupying almost the entire body [10, 35, 42]. The type-hosts are, however, different: C. lastoviza for D. ruber [16] and M. barbatus for D. latus [34]. Overall, all morphometric data for D. ruber and D. latus overlapped (Tables 2, 3) except for specimens of D. latus from M. surmuletus and S. scrofa from the Western Mediterranean having larger eggs (see Table 3) and the two species clearly share the deeply loped shape of the vitelline masses. It is worth noting that a comparison of the present specimens of D. ruber with those of D. latus provided by Bartoli and Gibson [10] in the most detailed modern description based on accessible voucher material and serial sections and providing metrical data, revealed that, despite some overlaps, D. latus is generally larger than D. ruber (means 5581 × 2180 vs. 4348 × 1443 µm) with a longer forebody (mean 2523 vs. 1966 µm) and longer hindbody (mean 1926 vs. 1364 µm). Derogenes latus also differs from D. ruber in having a broadly longer pre-oral lobe (mean 126 vs. 39 µm), larger oral sucker (means 787 × 789 vs. 469 × 538 µm), larger ventral sucker (means 1136 × 1110 vs. 952 × 1022 µm), and larger pharynx (means 291 × 254 vs. 153 × 157 µm). Additionally, D. latus differs from D. ruber in having a longer pars prostatica (mean 790 vs. 565 µm), considerably larger testes (means 480 × 366 vs. 233 × 170 µm for right testis, 488 × 400 vs. 247 × 170 µm for left testis), larger ovary (means 511 × 341 vs. 293 × 183 µm), and larger vitelline masses (means 823 × 500 vs. 432 × 412 µm for right vitelline mass, 963 × 608 vs. 452 × 424 µm for left vitelline mass).

Table 3 Metrical data for Derogenes latus from different hosts and localities in the Mediterranean
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Bartoli and Gibson [10] convincingly highlighted the striking morphological similarity between D. latus and D. ruber and indicated that the egg size given by Sey [17] for D. ruber is probably an inaccuracy. They refrained from synonymising the two species formally until further studies of material from the type-hosts and localities are available. Although we found morphometric differences between the present material of D. ruber from the type-host and the material of D. latus described by Bartoli and Gibson [10], and given that the occurrence of a single Derogenes species in various hosts has been challenged by molecular data [14, 15, 31], and both D. lacustris and D. varicus sensu stricto (D. varicus lineage DV1 of Krupenko et al. [14]) had been genetically proven to occur in various hosts (see Fig. 3A), it is possible that D. ruber and D. latus are indeed synonymous, thus transforming D. ruber to a euryxenic species. However, since molecular data for D. latus are still lacking, we also refrained from synonymising the two species. The genetic data generated herein for D. ruber from its type-host will be certainly valuable for a future investigation of the synonymy of these two species.