Agaue chevreuxi (Trouessart, 1889)

Bartsch, Ilse, 2016, Commented and illustrated key for identification of Agaue chevreuxi (Trouessart, 1889) and A. panopae (Lohmann, 1893) (Acari, Halacaridae), Ecologica Montenegrina 8, pp. 45-54 : 47-50

Agaue chevreuxi and A. panopae , description and comparison of adults

Agaue chevreuxi (Trouessart, 1889) was first mentioned in Trouessart (1889a) under the name of Leptopsalis chevreuxi , it had been extracted from a sample dredged in 35 brasses (ca 57 m) depths off Croisic, France, North Atlantic. In Trouessart (1889b, c), short descriptions were published; the given records were both from French Atlantic and Mediterranean coastlines. The total length of the species was said to be 800–900 µm (which would equal an idiosomal length of about 600–700 µm). The species is characterized by club-shaped tibiae, a slender gnathosoma with a long rostrum (hypostome trés long), and large claw fossae. A first illustration on the basis of individuals taken in the English Channel was presented a decade later ( Trouessart 1898: plate 12), but according to Trouessart it represents a variety with enlarged lamellae. More details of A. chevreuxi were added by Viets (1928: 57, figs 17–22, 1940: 64, 71, figs 111–115), Mari & Morselli (1990: 247, table I, 1a–c) and Bartsch (1998: 144, figs 1–8). The adults illustrated by Lohmann (1893: plate IV, figs 3–5) were from the south-western Pacific, Sydney, and are most likely not conspecific with the North Atlantic species, they may represent the species now called Agaue galatea Otto, 1999 . The collecting locality of the species of which leg I was illustrated ( Lohmann 1893 text-fig. 6) is not known.

Agaue panopae ( Lohmann, 1893) was described by Lohmann (1893) as Halacarus panopae . The description was based on specimens collected near the Cape Verde Islands, Sydney ( Australia) and off the coast of Brazil. The first-mentioned locality is expected to be the locus typicus and the female illustrated in Lohmann (1893: pl. III, fig. 1) the holotype. The individuals from Sydney are certainly not conspecific with Agaue panopae . The description given by Lohmann (1893) includes specimens from all three localities, their characters are: cerotegumental cover rather inconspicuous, smooth or, if at all, only slightly reticulate; anterior margin of PD truncate; tibiae I to IV club-shaped, tibia I with five, tibia II with four ventral setae; dorsal seta on P-3 short, smooth, seta on P-2 either wide, squamose and bifurcate or slender and setiform. The idiosoma of specimens from the Cape Verde Islands was within a length range of 320–360 µm ( Lohmann 1893). Lohmann (1893) mentioned varieties, one had almost smooth dorsal plates and on P-2 a squamose, bifurcate seta (forma squamifera), the other reticulate dorsal plates and a slender seta (forma setifera). The shape of the seta on P-2 turned out to be an example of sexual dimorphism ( Morselli & Mari 1985). Females of A. agauoides ( Lohmann, 1907) ( Newell 1984: fig. 142; Bartsch 2016: fig. 1D), A. parva ( Chilton, 1883) ( Bartsch 1993: fig. 39B) and A. villosa ( Lohmann, 1907) ( Newell 1984: fig. 171; Bartsch 1993: fig. 43B) have a flattened, furcate or branched seta on P-2, whereas in males of the two first mentioned species the seta on P-2 is smooth and slender ( Bartsch 1993: fig. 39C, 2016: fig. 1E).

Lohmann (1893) mentioned three characters which help to separate Agaue chevreuxi from A. panopae , namely the size (total length 560–780 µm versus 470–590 µm), the shape of the setae on P-2 and P-3 (both slender and smooth versus seta on P-2 often squamose and bifurcate, that on P-3 spiniform) and number of ventral setae on tibia II (five versus four). Viets (1940), Mari & Morselli (1990) and Bartsch (1998) added further discriminating characters but still more exist, as described on the following lines.

Idiosoma. Length of idiosoma of A. chevreuxi female 570–755 µm, of male 567–628 µm ( Mari & Morselli 1990; Bartsch 1998) and of A. panopae female 554–563 µm, of male 493–555 µm ( Morselli & Mari 1985, 1993; Mari & Morselli 1990) (the data given in Trouessart (1889a, b, c) and Lohmann (1893) are omitted, those by Lohmann are from individuals from different parts of the world). Agaue chevreuxi is larger than A. panopae , but the difference in length is generally small. Mari & Morselli (1990: plate I, figs 1a, 2a, 3a) presented the dorsal aspect of the three documented Mediterranean species, the idiosoma of A. panopae is slightly more slender than that of A. chevreuxi and A. adriatica . Such a difference is often negligible, especially in females. Ovigerous females which carry numerous eggs have the striated integument dilated and hence a wider idiosoma.

The outline of AD, OC and PD are similar in the two species A. chevreuxi and A. panopae . The dorsal plates bear longitudinal cerotegumental costae; the AD also an anterior ovate areola beside the two costae which in general do not meet anteriorly ( Figs 1A , 2A). The integument beneath the cerotegument demonstrates a delicate porosity whereas the parts lateral and medial to the lamellae have coarse pores (rarely just faint ones) ( Figs 1C, D , 2C, D). The cerotegumental cover of A. chevreuxi is raised, its surface often smooth, rarely including thin lamellae, the cover of A panopae is not as thick as in A. chevreuxi and always smooth. The AD and dorsal portion of AE are fused anteriorly, in A. chevreuxi the striae between the plates do not reach the gland pores ( Fig. 1D ), whereas in A. panopae the striae extend anteriad beyond the gland pores and the pores are situated on the AD (Fig. 2D). Each OC bears two spherical lenses, between them dark eye pigment; a gland pore is in the lateral margin posterior to the lenses ( Figs 1D , 2D). Slight differences in the outline of the OC illustrated are artefacts, the OC of A. chevreuxi ( Fig. 1C ) is drawn from a somewhat compressed but intact idiosoma, whereas the OC of A. panopae (Fig. 2C) is from a specimen in which the dorsal and ventral plates had been separated. The PD of A. chevreuxi is 1.5–1.6 times longer than wide, has generally a broadly ovate, rarely truncate anterior margin and a delicate, foveate ornamentation (exceptions exist), in A. panopae the PD is 1.8–1.9 times longer than wide, the anterior margin is truncate and the surface of the plate smooth. In many Agaue species the setae ds-2, ds-3 and ds-4 differ in size and shape from the ds-1 and ds-5. In A. chevreuxi these three pairs of setae are short, 32–40 µm long, and surrounded by cerotegument ( Fig. 1B ). In contrast, in A. panopae the ds-2, ds-3 and ds-4 are 57–80 µm long (Fig. 2B), hence generally distinctly longer than the ds-1 and ds-5 which have a length of 20–50 µm. There is no marked difference between females and males in the size and arrangement of the setae.

In the two species, the majority of the specimens studied have uniformly porose ventral plates, but in two individuals of A. chevreuxi , from the south-western coast of Spain (Cadiz), the ornamentation of the cerotegument gives the surface of the plates a foveate aspect. Except for the genital plates, there are almost no differences between females and males, the shape and setation of AE and PE ( Figs 1E , 2E) is almost the same. The GA of females of A. chevreuxi and A. panopae is ovate, the number of pgs of A. chevreuxi is smaller than that of A. panopae (13–17 versus 16–21 pgs) ( Viets 1928; Mari & Morselli 1990; Bartsch 1998), in A. chevreuxi there are two (rarely three) setae on either side of the antero-lateral margin, in A. panopae three to five setae ( Fig. 1F , 2F). Moreover, there is a difference in length of the ovipositor in relation to the anterior margin of GA. In A. chevreuxi the ovipositor extends to or slightly beyond the anterior margin of GA, in A. panopae beyond the GA by almost the length of the GO. The GA of males is wider than that of females. Differences between the species are in the number of pgs, about 81–90 in A. chevreuxi and 100–110 in A. panopae ( Figs 1G . 2G, Mari & Morselli 1990; Bartsch 1998). In A. chevreuxi , the spermatopositor extends to ( Fig. 1G ), but also somewhat beyond the anterior margin of the genital plate, in A. panopae , in the single male studied, it extends somewhat beyond that margin (Fig. 2G).

Figure 4. Agaue panopae ( Lohmann, 1893) . A - gnathosoma, ventral, female. B - tip of palp, dorsolateral, female. C - apical part of palp and rostrum, lateral, male. Scale = 50 µm. (arrows indicate differences in character states; fs, furcate seta; mxs-1, basal pair of maxillary setae; pa, porose areola; s, seta; so, solenidion; pa, porose areola; rs, rostral seta).

Figure 6. Agaue panopae ( Lohmann, 1893) . A - leg I, medial, female. B - tip of tarsus II, medial, female (lateral eupathidia in broken line, lateral claw and fossary seta omitted). C - leg II, medial, female. D - tip of tarsus III, medial, female (lateral claw and setae omitted). E - leg III, medial, female. F - claw of tarsus IV, medial, female. Scale = 50 µm. (arrows indicate differences in character states; acp, accessory process; ce, cerotegument; so, solenidion)

Gnathosoma. The gnathosoma is slender in both species, its length more than 2.0 times the width ( Lohmann 1893; Bartsch 1998) ( Figs 3A , 4A). The cerotegumental cover on the gnathosomal base is thin in A. chevreuxi , inconspicuous in A. panopae . Within a ring-like area, interrupted only by the pharyngeal field, the integument of the gnathosomal base is porose. The tectum is scaliform. The rostrum of A. chevreuxi and A. panopae is slender, extends beyond the middle of P-4, in both species it is about twice as long as the gnathosomal base (1.8–2.1 times) ( Bartsch 1998). In A. chevreuxi the length of the mxs-2 is less than one quarter of that of the mxs-1 ( Fig. 3A ) whereas in A. panopae (Fig. 4A) it is about half of that of the mxs-1 (mxs-2, 50–60 µm, mxs-1, 100–105 µm). The apical rostral setae are short, divaricate. The palps are slender. P-2 and P-3 bear one dorsomedial seta each. The seta on P-2 is slender in A. chevreuxi ( Fig. 3B ), in females about 24 µm long, but in females of A. panopae it is scaliform and furcate, 12 µm long (Fig. 4B). Males of both species have a slender seta on P-2, 20–25 µm long ( Figs 3C , 4C). In females and males of A. chevreuxi the dorsal seta on P-3 is distinct, similar though somewhat shorter than that on P-2 (16–20 µm), in females of A. panopae that seta is spiniform, its length 11–12 µm. The P-4 bears three setae in the basal whorl, a long dorsolateral solenidion and apically one slender setula and two short eupathid setae.

Legs. Both species have club-shaped tibiae and at a first glance the shape of legs is similar but the cerotegumental lamellae of A. chevreuxi are larger than in A. panopae ( Figs 5A, C, E , 6A, C, E). The height of the dorsal lamellae on telofemur I of A. chevreuxi equals at least one third of the segment's height, that of A. panopae is one fifth or less. In A. chevreuxi the telofemora I to III are 3.1–3.2, 2.4–2.5 and 2.2–2.4 times longer than high, in A. panopae 2.9, 2.3 and 2.2–2.3 times, respectively. In both species tibiae I and II are somewhat shorter, but tibiae III and IV somewhat longer than the legs' telofemora. In A. chevreuxi the number of setae on the legs, from trochanter to tarsus (solenidia included) is: leg I, 1, 2, 5, 5, 11, 23; leg II, 1, 2, 5, 5, 11, 11; leg III, 2, 2, 3, 4, 8, 5; leg IV, 0(–1), 2, 3, 3, 8, 5; in A. panopae : leg I, 1, 2, 5, 5, 11, 20; leg II, 1, 2, 5, 4, 10, 9; leg III, 2, 2, 3, 3, 7, 5; leg IV 0, 2, 3, 3, 7, 5. The genua I to IV of A. chevreuxi bear 3/2, 3/2, 3/1, 2/1 dorsal/ventral setae, those of A. panopae 3/2, 2/2, 2/1, 2/1; one of the dorsal setae of both genu II and III is lacking in that latter species. On tibiae I to IV of A. chevreuxi 6/5, 6/5, 4/4, and 4/4 dorsal/ventral setae are found, respectively, on those of A. panopae 6/5, 6/4, 3/4, and 3/4 setae. Of the five ventral setae on tibia II of A. chevreuxi two are ventromedial and three ventrolateral in position, of the four ventral setae on tibia II of A. panopae two are ventromedial and two ventrolateral in position. The tarsi have large fossa membranes. Tarsi I of A. chevreuxi and A. panopae each bear three dorsal setae, a solenidion on the lateral fossa membrane and one ventromedial seta in about the middle of the segment; the species differ in the number of eupathidia, in A. chevreuxi there are about 18 (pas included), in A. panopae only 15 eupathid setae. Tarsus II of A. chevreuxi bears three dorsal setae and on the inside of the medial fossa membrane a 10 µm long solenidion, at the tarsal tip three ventromedial and four ventrolateral eupathidia (doubled pas included) ( Fig. 5B ). The tarsal II chaetotaxy of A. panopae is similar except for the lower number of eupathidia on the tip of the tarsus, namely a single ventral seta and a pair of doubled eupathidia (Fig. 6B). Tarsi III and IV have three dorsal setae and a pair of filiform pas ( Figs 5D , 6D). The claws differ in shape and number of tines. Most A. chevreuxi have 16–18 tines arranged along the concave flank of the claws ( Fig. 5B, D, F ) and distinctly raised accessory processes, each with six tines. In A. panopae the pectines, with 7–8 tines each, are restricted to the middle part of the concave flank (Fig. 6B, D, F), the accessory process is slightly raised and includes three to four tines.

Figures

Figure 1. Agaue chevreuxi (Trouessart, 1889). A - idiosoma, dorsal, female. B - seta ds-3, female. C - ocular plate, female. D - anterior part of AD, female. E - idiosoma, ventral, female. F - genitoanal plate, female. G - genitoanal plate, male.

Figure 3. Agaue chevreuxi (Trouessart, 1889). A - gnathosoma, ventral, female. B - tip of palps, dorsal, female. C - apical part of palp, chelicera and rostrum, lateral, male.

Figure 5. Agaue chevreuxi (Trouessart, 1889). A - leg I, medial, female. B - tip of tarsus II, medial, female (lateral eupathidia in broken line, lateral claw and fossary seta omitted). C - leg II, medial, female. D - tip of tarsus III, lateral, female (medial claw and setae omitted). E - leg III, medial, female. F - claw of tarsus IV, medial, female.