Introduction

Expeditions to the Southeast Atlantic (DIVA-1 [Balzer et al. 2006], DIVA-2 [Türkay and Pätzold 2009] and part of ANDEEP III [Fahrbach 2006]), the Southern Ocean (ANDEEP I and II [Fütterer et al. 2003]), the South Indian Ocean (CROZEX [Pollard and Sanders 2006]), the central Pacific (NODINAUT [Galéron and Fabri 2004], the North Atlantic (Porcupine Abyssal Plain, PAP [see Kalogeropoulou et al. 2010 for summary] and the Great Meteor Bank [Pfannkuche et al. 2000]) (Fig. 1) provided numerous specimens of the genus Mesocletodes Sars, 1909a. Belonging to the family of Argestidae Por, 1986a, Mesocletodes is considered to be a typical and primarily deep-water dwelling taxon (compare overview in George 2004 and George 2008). The total number of Mesocletodes in deep-sea samples amounts to almost 50% of all Argestidae Por, which in turn form one of the most abundant taxa of harpacticoid copepods therein. Due to the high frequency in deep-sea samples and conspicuous morphological characters, Mesocletodes is informative for chorological, faunistic and biogeographic research. The number of specimens as well as species diversity are substantial, but species are well discernible.

Mesocletodes nowadays comprises 36 species (Menzel and George 2009; Wells 2007). All allied species show characteristic morphological features that allow rapid recognition in metazoan meiofauna samples: body of cylindrical shape, A1 segment 2 with conspicuous protrusion bearing a strong seta, md gnathobase with broad grinding face, P1 exp2 without inner seta, P1 exp3 without proximal outer spine, spines of this segment with subterminal tubular extensions, P2–P4 exp1 without inner seta, P2–P4 enps at most 2-segmented, telson square in dorsal and ventral view and furcal rami long and slender (cf. Menzel and George 2009).

The sex ratio of harpacticoid copepods in the deep sea is strongly biased towards females (e.g. Shimanaga et al. 2009; Shimanaga and Shirayama 2003; Thistle and Eckman 1990) and it is very difficult or nearly impossible to connect males and females of some species (e.g. Menzel and George 2009; Seifried and Veit-Köhler 2010; Vasconcelos et al. 2009; Willen 2006; Willen 2009; Willen and Dittmar 2009), indicating extremely poecilandric populations (Por 1986b). Concerning Argestidae, males could be connected to females for Eurycletodes Sars, 1909b, Argestes Sars, 1910, and Hypalocletodes Por, 1967 (cf. original descriptions). Since the establishment of Mesocletodes early in the 20th century (Sars, 1909a), this has been possible only for two species plus the herein described species. For 32 species of this genus only females are known, while exclusively males are known for two species.

Most of the species descriptions of Mesocletodes are based on few adult specimens (29 descriptions contain one to five type specimens, three descriptions are based on six to ten specimens, four descriptions are based on 11 to 16 specimens). Thus, neither intraspecific variability nor the process of ontogenetic development is reported for any species of Mesocletodes. Expeditions during the DIVA and ANDEEP campaigns yielded 54 out of 66 adults of Mesocletodes elmari sp. n. (more than 80%). The comparatively high frequency of specimens is probably explicable by the greater sampling effort in contrast to the CROZEX, NODINAUT, OASIS expeditions and sampling at the PAP as well as during previous campaigns. Repeated multicorer sampling of the same station (Martínez Arbizu and Schminke 2005; Rose et al. 2005) greatly enhances, for the first time, the opportunity of finding the same species again in one station or region. This implies that more specimens of one species are available, making investigations on intraspecific variability, specification of sexually dimorphic modifications and retracing of the ontogenetic development possible for the first time (cf. George 2008).

The aim of this publication is to convey an initial impression of the extent of sexually dimorphic modifications, ontogeny and intraspecific variability for the genus Mesocletodes, using Mesocletodes elmari sp. n. as an example.

Material and methods

Sediment samples were taken with a multicorer (Barnett et al. 1984) in different oceanic regions: Southeast Atlantic (DIVA-1, DIVA-2 and part of ANDEEP III), Southern Ocean (ANDEEP I and II), South Indian Ocean (CROZEX), central Pacific (NODINAUT), North Atlantic (PAP and Great Meteor Bank) (Fig. 1, Table 1). Adult Harpacticoida were extracted from all samples, whereas copepodid stages are only available from the campaigns DIVA-1, DIVA-2 and ANDEEP.

Altogether 77 specimens (56 adult females, 10 adult males, 2 CV females, 3 CV males, 5 CIV males and 1 CIII) were found. The type material of Mesocletodes elmari sp. n. consists of 7 specimens (2 females plus 1 each of the other discovered stages). The type material was deposited in the collection of the Senckenberg Forschungsinstitut und Naturmuseum Frankfurt (Germany). The remaining 70 specimens are mounted on slides and kept in the collection of the DZMB in Wilhelmshaven (Germany).

The material was mounted on separate slides using glycerol as the embedding medium. Identification at the species level and drawings were carried out using a Leica microscope DM2500 equipped with a camera lucida and interference contrast with a maximum magnification of 1600x.

The CLSM photograph of a Congo-red stained female was taken with a Leica TCS SP5 mounted in a Leica DM5000. Preparations and settings were made according to Michels and Büntzow (2010).

Abbreviations used in the present paper are: A1 (antennula), A2 (antenna), aes (aesthetasc), benp (baseoendopod), CI–CV (copepodid stages 1–5), cphth (cephalothorax), enp (endopod), exp (exopod), FR (furcal rami), GF (genital field), md (mandibula), mx (maxilla), mxl (maxillula), mxp (maxilliped), P1–P6 (pereiopods 1–6), STE (Subterminal Tubular Extension, according to Huys 1996).

I could examine other material for comparison: Type material of Mesocletodes parabodini Schriever, 1983, (1 dissected female, ZMK Cop. No. 1319). Mesocletodes farauni Por, 1967 (1 female, dissected, HUJ Cop no. 69 plus one additional specimen), Mesocletodes glaber Por, 1964a (1 female, dissected, HUJ Cop no. 33) and Mesocletodes monensis (Thompson, 1893) (3 females, dissected, on one slide each, HUJ Cop no. 63, 93, 138).

Taxonomy

Argestidae Por, 1986a

Discussion Allocation of Mesocletodes elmari sp. n. to Mesocletodes and its position within this genus

Allocation of Mesocletodes elmari sp. n. to the taxon Mesocletodes is indisputable since all specimens show the apomorphies recognized by Menzel and George (2009): 1) second A1 segment with a strong protrusion bearing 1 strong, bipinnate seta, 2) proximal outer spine of P1 exp3 reduced, 3) spines of P1 exp3 equipped with STE and 4) blades of md gnathobase forming a strong, grinding tooth.

The phylogenetic relationships within Mesocletodes are still under discussion. However, a first approach is possible: Mesocletodes elmari sp. n. is considered to belong to the “Mesocletodes inermis group” as it lacks the characteristic cuticular processes on cephalothorax and telson that are regarded to be autapomorphic to the Mesocletodes abyssicola-group (Menzel and George 2009). The extreme elongation of the FR is assumed to be convergent in the new species and the Mesocletodes abyssicola-group because several recently observed, but as yet undescribed species of Mesocletodes without cuticular processes on cephalothorax and telson also show elongated FR (personal observation). Future investigations, however, will have to prove the phylogenetic relevance of the elongated FR for the Mesocletodes abyssicola-group.

Mesocletodes elmari sp. n. shows a distinct mxl exopodal segment, and the enp is incorporated into the basis. By contrast, a distinct endopodal segment is described for the mxl of Mesocletodes bodini (Soyer 1964; Soyer 1975) and Mesocletodes irrasus (T. and A. Scott 1894), whereas the exp is considered to be absent. According to Huys and Boxshall (1991) and Seifried (2003), however, the distinct segments of Mesocletodes elmari sp. n., Mesocletodes bodini and Mesocletodes irrasus are homologous to the exp of other Harpacticoida. The description for Mesocletodes irrasus and Mesocletodes bodini is therefore erroneous because they show an articulated exp instead of an articulated enp.

Justification of Mesocletodes elmari sp. n. as a new species

From a morphological point of view Mesocletodes elmari sp. n. is similar to Mesocletodes bodini and Mesocletodes parabodini as these three are the only species of Mesocletodes with elongated P1–P4 enp2. Mesocletodes elmari sp. n., however, shows clear autapomorphies [plesiomorphic states in brackets] that justify it as a new species:

1) mx seta that is fused to the basis, bears a conspicuously strong spinule-like pinna [seta without spinule-like pinna]

2) P2–P4 exp3 proximal outer seta lost [seta present]

3) P1–P4 enp2 extremely elongated [not elongated]

4) FR strongly elongated between setae III and VII [not elongated]

5) female body of a prickly appearance created by setules that are widened at their bases [no prickly appearance]

6) female P2–P4 enp2 proximal inner seta lost [seta present]

Character 1): The mx seta that is fused to the basiscarries a conspicuously strong spinule-like pinna in Mesocletodes elmari sp. n. The corresponding seta in other species of Mesocletodes is usually bipinnate with the pinnae of equal size. The loss of all pinnae except one at the anterior side plus the modification of this pinna towards a spinule-like appearance is not recorded for any other species of Mesocletodes or Argestidae and is therefore regarded here as derived. This modification thus is considered to be autapomorphic to Mesocletodes elmari sp. n.

Character 2): Mesocletodes elmari sp. n. lacks the proximal outer seta on P2–P4 exp3. The reduction of outer pereiopodal ornamentation is considered to be derived according to the rule of oligomerization (Huys and Boxshall 1991), but various harpacticoid taxa, including species of Mesocletodes lack this seta convergently. The loss of the proximal outer seta on P2–P4 exp3 is thus considered to be species-specific and therefore autapomorphic to Mesocletodes elmari sp. n.

Character 3): Endopodal segments of species of Mesocletodes are very short and there are never more than two of them in this genus, many species even have only one single segment. The extreme elongations in P1–P4 enp2 are unique for Mesocletodes elmari sp. n. and are considered to be the result of lengthening of the distal endopodal segment. Ontogenetic stages of males do not show a suture that might indicate a fusion of the distal segment with the preceding. Extreme elongations of P1–P4 enp2 are therefore considered here to be autapomorphic to Mesocletodes elmari sp. n. A less extreme elongation of these segments, however, occurs also in Mesocletodes bodini and Mesocletodes parabodini.

Character 4): The FR of Mesocletodes are longer than wide, with setae IV, V and VI located terminally, whereas setae I, II, III and VII are located closer to or in the proximal part of the ramus. An extreme elongation between setae III and VII has been discussed as an apomorphy for the Mesocletodes abyssicola-group (Menzel and George 2009). However, lacking cuticular processes on cephalotorax and/or telson, Mesocletodes elmari sp. n. does not show the other two apomorphies of the Mesocletodes abyssicola-group. The extreme elongation of FR thus is considered here to occur convergently in Mesocletodes elmari sp. n. and species belonging to the Mesocletodes abyssicola-group.

Character 5): Females of Mesocletodes elmari sp. n. are characterized by the prickly appearance of the body somites dorsally and laterally. Such coverage is absent in other species of Mesocletodes and is therefore regarded here as derived, i.e. an autapomorphic character for Mesocletodes elmari sp. n.

Character 6): Endopodal segments do not seem to be fused in Mesocletodes elmari sp. n. (see character 3). The proximal inner seta on P2–P4 enp2 in males is considered to be reduced in females. The lack of the proximal inner seta on P2–P4 enp2 is therefore considered here to be autapomorphic to females of Mesocletodes elmari sp. n.

Intraspecific variability in Mesocletodes elmari sp. n.

Intraspecific variability in deep-sea harpacticoids has recently been revealed to be extremely high. For instance, George (2008), Seifried and Martínez Arbizu (2008) as well as Gheerardyn and Veit-Köhler (2009) were able to show that neither setation nor segmentation, nor total length of appendages has to be a reliable character for species discrimination in deep-sea Harpacticoida. Variability in Argestidae has only been recorded for the pereiopodal chaetotaxy of Argestes angolaensis George, 2008 (George 2008 and personal observations), and for the shape and number of ventral spinules on the telson in the argestid genus Eurycletodes Sars, 1909b (Menzel in press).

For Mesocletodes intraspecific variability has not yet explicitly been recorded. However, five species were redescribed at least once, indicating that detected specimens deviate minimally from the type specimen: Mesocletodes abyssicola (T. and A. Scott 1901; Sars 1921; Lang 1936), Mesocletodes bathybia (Por 1964b; Soyer 1964), Mesocletodes irrasus (Scott 1893; T. and A. Scott 1894; Lang 1936; Sars 1909; Soyer 1964) Mesocletodes monensis (Thompson 1893; Sars 1921; Lang 1936; Por 1964b;) and Mesocletodes robustus (Por 1965; Menzel and George 2009).

Although clear apomorphies were recognized for Mesocletodes elmari sp. n., careful morphological examination of the 77 specimens revealed high intraspecific variability (cf. Table 1). The total length of FR, the number and the shape of spinules in various parts of the body, the ornamentation of the hyaline frill and the setation of P2–P4 enp2 is variable. Moreover, few specimens bear setular tufts in various positions on the FR. Setular tufts on the FR near seta VII have only been recorded for Mesocletodes bodini (Soyer 1975) and Mesocletodes parabodini (Schriever 1983), but corresponding structures near the basis seem to be unique in Mesocletodes elmari sp. n. Although setular tufts on the FR seem to be species-specific for Mesocletodes bodini and Mesocletodes parabodini, the importance of those cuticular structures for species discrimination or even for unraveling phylogenetic relationships remains unclear.

Sexual dimorphism in Mesocletodes

Many morphological characters of species belonging to Mesocletodes are entirely different in both genders. Nevertheless, the identification keys for Mesocletodes are exclusively based on the morphology of females (e.g. Wells 2007), possibly due to the fact that merely two males have been described to date. With the aid of these keys, it is nearly impossible to connect a male of Mesocletodes to the corresponding female. Consequently the number of species in any deep-sea sample is overestimated, which means faunistic and ecological analyses at the species level are subject to a strong bias. As follows, it appears urgent to quantify the sexually dimorphic modifications in Mesocletodes.

Sexual dimorphism in adults. The descriptions of Mesocletodes contain only females, with the exception of four species: exclusively the male is described for Mesocletodes angolaensis and Mesocletodes fladensis (the latter description is poorly detailed). Both genders are described for Mesocletodes faroerensis and Mesocletodes thielei. However, these two species bear a proximal outer spine in P1 exp3 and 3 inner setae on P3 exp3. Moreover, Mesocletodes faroerensis bears an inner seta on P1 exp2 and 3 inner setae on P3 exp3, and the md gnathobase of Mesocletodes thielei does not form a strong grinding face. Consequently, both species lack autapomorphies of Mesocletodes (cf. Menzel and George 2009). Even though the descriptions are poorly detailed and the type material of both species is not available any more, the characters in question are not to be misinterpreted. Thus, Mesocletodes faroerensis and Mesocletodes thielei have to be excluded from Mesocletodes. Future investigations will have to unveil their generic attribution within Argestidae. Consequently, Mesocletodes elmari sp. n. is the only known species with matching males and females and therefore convenient for investigations on sexually dimorphic modifications in Mesocletodes.

Sexually dimorphic modifications in males of basal Argestidae, such as Argestes (George 2008), and Bodinia George, 2004 (George 2004) include the A1, P5, P6, and the body size, whereas males of Mesocletodes show many more affected characters. The modifications in Mesocletodes elmari sp. n. males are comparable to the ones observed in Mesocletodes angolaensis and numerous undescribed males from deep-sea samples (personal observation) and are therefore considered to be a good representation of male sexual dimorphism in Mesocletodes. 1) The body tapers distally and the setation especially in P1–P4 is very rich and strongly developed in comparison to females. These morphological characters are likely adaptations that help males to stay in the bottom currents once resuspended (cf. characteristics of “typical emergers” [Thistle and Sedlacek 2004; Thistle et al. 2007]) and thus would allow them to explore the sediments for mates. 2) The gut of adult males of Mesocletodes is generally empty (personal observation), but the body is filled with several spermatophores instead of food as is reported for several Harpacticoida (cf. Menzel and George 2009; Shimanaga et al. 2009; Wells 1965; Willen 2005). Since the gut of CIV males and CV males of Mesocletodes elmari sp. n. is well filled with sediment or detritus, feeding seems to be abandoned at the last molt. It has not been investigated yet whether the gut and digestive tissue are present in adult males. However, the abandonment of feeding and the production of extremely large and numerous spermatophores might be an adaptation to the sparsely populated and oligotrophic deep-sea environments and is therefore considered to represent a derived character state. 3) Mouthparts are either absent, strongly reduced or complete, but apparently not utilized for feeding.Along with the complete reduction of mouthparts, the cephalothorax of Mesocletodes angolaensis is slightly depressed in the lateral view and lacks the part that encloses the mouthparts in females. Although the mouthparts of the male of Mesocletodes elmari sp. n. do not differ from the female, the ventral edge of the male cephalothorax is less rounded than in the female, but less reduced than in Mesocletodes angolaensis.

However, not only the empty gut or the reduction of mouthparts indicates the abandonment of feeding in adult males, but also the A1: most setae on the A1 of the adult male of Mesocletodes elmari sp. n. are smooth, merely some in the grasping region of the A1 (segments 3–6) are bipinnate (Fig. 10 A). However, all setae that are smooth in the adult male are strongly pinnate in the two preceding copepodid stages (Figs. 10 B, 14 A). Thus, the loss of pinnae is regarded as another sexually dimorphic modification in adult males since the regression or poorer development of setal elements is typical of non-feeding male copepods (Boxshall and Huys 1998).

Females are generally considered to show the whole character set of a species while the modifications in males are considered to be due to sexual dimorphism (but see George 1998; George 2006a for Ancorabolidae). It is likely, however, that adult females, too, show characters that are connected to the gender because the CV females of Mesocletodes elmari sp. n. do not show characters that are typical of adult females: prickly appearance of the body created by setules that are widened at their bases, coxa of P1 externally widened and basal inner seta arising from a prominent protrusion, strongly bent outwards and overlying the enp, P1 enp exceeding exp in length, all extremities bearing conspicuously numerous and strong spinules, and hyaline frill of body somites ornate.

Sexual dimorphisms in juveniles. Sexually dimorphic modifications expressed in copepodid stages of Mesocletodes elmari sp. n. allow sexing during ontogenetic stages, at least from CV onwards; it is only partially resolved for this species if sexing of CIV is possible because all discovered CIV seem to be of the same gender. A similar constraint applies to the single individual of CIII. This copepodid stage, however, is assumed not to show sexual dimorphism (e.g. Dahms 1990) and is therefore not discussed here.

Sexing of CV. The male CV and the female CV of Mesocletodes elmari sp. n. are distinguishable from the adults by virtue of the overall smaller body size, the lack of the penultimate urosomite and the non-articulated P5 exp. Moreover, the female CV lacks the GF, the male CV lacks the spermatophores and shows strong differences from the adult male in the A1 (Fig. 15 B, C): only six out of nine A1 segments are articulated and several setae are lacking. The position and number of developed setae in these segments, however, resemble the adult male A1 more than the adult female A1 (compare Figs 4 A, 10 A, B, 15 A–C).

Sexing of CIV. Careful examination of the A1 and the P5 suggests sexing of the discovered CIV as males.

The five inner setae on the third segment of the CIV A1 (Figs 14 A, 15 D) are almost evenly distributed as is the case in the CV male (Figs 10 B, 15 C). The CV female A1 (cf. Fig. 4 A) has the aes on the fourth segment, while it is on the third in the CV male (Figs 10 B, 15 C). As follows, if the CIV were females, a separation of the aes-bearing segment from the third segment should happen at the next molt. This does not seem plausible, however, because four setae on female segment 3 (Figs 4 A, 15 A) are close to each other in the middle of the segment, the fifth seta inserts distally. An elongation proximally and distally of the evenly distributed four setae in CIV segment 3, plus shortening of the distances between these setae, is not likely. However, an addition of three inner setae at the molt from CIV (Figs 14 A, 15 D) to CV (Figs 10 B, 15 C) in the distal part of this segment (see solid squares in Figs 10 B, 15 C) and maintenance of the distances between the five setae addressed above appear likely. The A1 of the CIV is therefore considered herein to show male characteristics.

The P5 endopodal lobe of the four CIV (Fig. 8 F) has one short, outer seta, one long medial seta and one inner cuticular protrusion, and is therefore in accordance with the CV male (Fig. 8 E). The setation of P5 exp, however, resembles the CV female. Nevertheless, the small depression on the proximal inner edge of the exp (see asterisk in Fig. 8 F) might indicate the emergence of a seta at the next molt, which is only present in males. It is unclear, however, whether harpacticoid CIV show sexually dimorphic modifications in P5 exp. It seems that the CIV of Mesocletodes elmari sp. n. do, whereas the opposite is reported for the CIV of an undescribed species of Orthopsyllus Brady and Robertson, 1873 (Huys 1990).

P2–P4 enp2 of the discovered CIV bear one inner seta, which is in accordance with female adults and CV. The male adult and CV bear two inner setae in these segments, with the distal seta being homologous to the single seta in the adult female. However, previous studies suggest that endopodal setation is not complete in harpacticoid CIV (Dahms 1990; Dahms 1993; Huys 1990). Thus, the addition of the proximal inner seta at the molt to CV is considered to be likely.

Ontogenetic development of Mesocletodes elmari sp. n.

Although copepodid stages amount to between 30% and more than 50% of the total deep-sea harpacticoid assemblage, they are excluded from faunistic analyses because confident specific allocation is not possible for many families. For investigations on phylogeny, however, juveniles may be the key to plausible theories (e.g. Ferrari 1988; Fiers 1998; Huys and Boxshall 1991).

Many species descriptions contain short remarks on Mesocletodes relationships with other genera and species within the genus. Phylogenetic investigations have been subject to one study to date (Menzel and George 2009), whereas ontogenetic studies on Mesocletodes are pending. However, not all copepodid stages of Mesocletodes elmari sp. n. are available, and a comparison with juvenile stages of other species of Mesocletodes is impossible due to the lack of knowledge. The ontogeny of Mesocletodes elmari sp. n. is therefore presented here in a rather descriptive way, but with the purpose to serve as a background for future studies.

A2, mouthparts and FR of Harpacticoida are complete with respect to segmentation and setation from CI onwards (cf. Dahms 1990; Dahms 1992; Dahms 1993). A1 and pereiopods, by contrast, develop gradually by every molt, which is also the case for the habitus: at each molt from CI to adult, one body somite is added anterior to the telson. CV thus shows seven free trunk segments, CIV shows six, and CIII shows five free trunk segments between cephalotorax and telson. Reproductive organs (GF in females and spermatophores in males) are developed at the molt to adult.

A1. The female A1 of Mesocletodes elmari sp. n. is complete at least at CV, whereas the male A1, which is available from CIV onwards, undergoes extensive modifications at each molt. Segments 3 to 5 of the adult male are part of the third compound segment in CIV males, three setae (marked by solid squares in Figs 10 B, 15 C) are added to this compound segment at the molt to CV. The strongest modifications appear at the molt to adult: the third compound segment is simultaneously separated into segments 3, 4 and 5. Segment 6 of the adult male is distinct at least from CIV onwards, but the proximal seta is added at the molt to CV. Segment 7, directly preceding the geniculation, is not present prior to the molt to adult male.

The characteristic Mesocletodes seta (strong, bipinnate, arising from a conspicuous protrusion, see Menzel and George 2009) and a subterminal seta occur at CV in males (compare setae marked by asterisks in Figs 4 A, 10 A, B, 15 A–C). This is likely the case for females, too, as the second A1 segment does not show sexually dimorphic modification regarding the number and position of setae.

Although sexing of the single discovered CIII was impossible, its A1 provides valuable ontogenetic information for Mesocletodes elmari sp. n. with respect to the first and the last two A1 segments. These segments, moreover, are not sexually dimorphically modified in CIV or later stages.

Segment 1 lacks a seta at least from CIII onwards (Figs 4 A, 10 A, B, 14 A, F). The presence of a seta on this segment in CI and CII, but the loss at the molt to CIII is discussed to be the case for some harpacticoid species (cf. Boxshall and Huys 1998; Dahms 1989). This, however, could not be followed for Mesocletodes elmari sp. n due to the lack of earlier stages than CIII. A similar constraint applies to the development of the last two segments, which are complete at least at CIII (see schematics in Fig. 15), but should also be since CI, as it would be the case in many harpacticoids (cf. Dahms 1989 and references therein).

P1–P5. Copepodid development of CI to CV implies extensive changes in P2–P5 with respect to segmentation and setation at each molt. P1 exopodal setation, however, is complete from CI, endopodal setation from CII (Dahms 1993). Changes from the last copepodid stage to adults are restricted to the increase in size (e.g. Dahms 1993; Ferrari 1988). Although earlier stages than CIII have not been found, the investigations on Mesocletodes elmari sp. n. are considered to provide an adequate insight into postnaupliar development of P2–P4 in Mesocletodes since the progress of the P4 in CIII is comparable to the P2 in CI (Dahms 1993).

Outer elements on the pereiopods of Mesocletodes elmari sp. n. occur earlier during ontogeny than inner setae, exps and enps are affected likewise (see P2–P4 of CIII and CIV, Figs 13 B–D, 14 C–E) (cf. Dahms 1993; Ferrari 1988; George 2001). The development of setae in Mesocletodes elmari sp. n. is complete at the latest in CIII for P1 (however, it should already be complete in CI, see above), or in CIV for P2–P4 respectively. The separation of the second and third exopodal segments of P1–P4, however, occurs at the molt to CV. P1–P3 endopodal segmentation is complete at the latest in CIII of Mesocletodes elmari sp. n., whereas P4 still shows a 1-segmented enp at this stage.

In CIV males the P5 endopodal lobe corresponds to the one in CV and adult, whereas the P5 exp lacks the proximal inner seta (Fig. 8 D, E, F) (see section Sexual dimorphisms in juveniles).

On the basis of adult specimens, Menzel and George (2009) recognized four apomorphies for Mesocletodes (see above). The above addressed ontogenetic development of Mesocletodes elmari sp. n. shows that none of them is characteristic of adults only, but rather appear already during juvenile development.

The characteristic Mesocletodes seta on the second A1 segment is developed from CV onwards of both genders. This segment does not show sexually dimorphic modification, except that the setae of females are bipinnate, whereas males bear bare setae. All investigated stages of Mesocletodes elmari sp. n. lack the proximal outer spine on P1 exp3. According to Ferrari (1988), this is caused by suppression and further indicates pedomorphosis for this character, i.e. the maintenance of juvenile characters in adults. Considering the harpacticoid pattern of leg development (Dahms 1993; Ferrari 1988), the distal part of the single P1 segment in CI or the second segment in CII–CIV is homologous to the third segment in CV and adult. These parts are fully equipped with all elements characteristic of the third segment. STEs arising from spines on P1 exp3 are only traced from CIII on for Mesocletodes elmari sp. n. However, it seems likely that these extensions exist from CI as the setae they are associated with do so. The same applies to the strong grinding tooth at the md gnathobase. This is developed at least at CIII of Mesocletodes elmari sp. n., but according to Dahms (1990), for example, this should be the case from CI onwards.

Brief remarks on the geographic distribution of Mesocletodes elmari sp. n.

Various taxa of benthic harpacticoid copepods show distribution ranges at the species level that extend over thousands of kilometers across Atlantic, Southern Ocean and Pacific abyssal plains: Ancorabolidae Sars, 1909a (George 2006b; Gheerardyn and George 2010), Argestidae (Menzel and George 2009; Menzel in press), Canthocamptidae Sars, 1906 (Mahatma 2009), Ectinosomatidae Sars, 1903 (Seifried and Martínez Arbizu 2008), Paramesochridae Lang, 1944 (Gheerardyn and Veit-Köhler 2009; Plum and George 2009).

In the case of Mesocletodes, as well, the sampling localities known up to now suggest an extremely wide distribution of this genus: the North Atlantic (Scandinavian coast [Lang 1948; Pesta 1927; Por 1964a; Por 1965; Sars 1909; Sars 1921], Irish, English and Scottish coasts [T. Scott, 1900; T. Scott, 1906; Thompson 1893; Wells 1965], Porcupine Abyssal Plain [Gheerardyn 2007; Gheerardyn et al. 2010], Spitzbergen coast [Lang 1936], Arctic Ocean [T. and A. Scott 1901; Smirnov 1946], Icelandic coast and Iceland Faroe Ridge [Schriever 1983; Schriever 1985], Greenlandic coast [Jespersen 1939], off North Carolina [Coull 1973] Nova Scotia Rise [Thistle and Eckman 1990], French Atlantic coast [Bodin 1968], Iberian Basin [Becker et al. 1979], Great Meteor Bank [George and Schminke 2002]), the Mediterranean Sea (Guidi-Guilvard et al. 2009; Por 1964b; Soyer 1964; Soyer 1975), the Red Sea (Por 1967), the Pacific Ocean (Peru Trench [Becker et al. 1979], off Hawaii [Mahatma 2009], off the Californian coast [Thistle et al. 2007], off the Japanese coast [Shimanaga et al. 2004]), the Indian Ocean (Por 1986a), the South Atlantic Ocean (Southwest Atlantic [George 2005], the Southeast Atlantic [Menzel and George 2009]). However, the distribution of Mesocletodes at the species level has been addressed briefly (Menzel and George 2009), and is subject to ongoing studies.

The record of Mesocletodes elmari sp. n. in the North Atlantic Ocean and South Atlantic Ocean, the Southern Ocean, the Pacific Ocean and the South Indian Ocean extends the knowledge on the distribution of Mesocletodes and points a worldwide distribution at the species level. Future studies will have to deal with the means of dispersal as well as ecological and biological needs of species belonging to Mesocletodes to help explain the distributional patterns.