Aeolosoma vietnamicum, Gusakov & Dien & Thanh, 2021

Gusakov, Vladimir A., Dien, Tran Duc & Thanh, Nguyen Thi Hai, 2021, A new species of the genus Aeolosoma (Annelida: Aphanoneura: Aeolosomatidae) with peculiar sigmoid chaetae from fresh waters of Vietnam, Zootaxa 5072 (3), pp. 285-290 : 287-289

Aeolosoma vietnamicum sp. n.

( Figure 1 ; Table 1 )

Holotype. Holotype is deposited in the collection of the Zoological Institute of the Russian Academy of Sciences (St. Petersburg), with slide inventory number 1/48236 .

Paratype. One individual. The worm is stored in the Laboratory of Ecology of Aquatic Invertebrates of the Papanin Institute for Biology of Inland Waters Russian Academy of Sciences (Borok, Yaroslavl Province), in the collection of representatives of meiobenthos from water bodies of Vietnam: slide No. 136 .

Type locality. Vietnam, Khánh Hòa Province, Cam Ranh peninsula, a flooded sand quarry, plant-filled (sedgelike macrophytes) littoral (N 12°04.873', E 109°11.017'), altitude 4 m asl, depth 0.4 m, water temperature 33.1 °С, рН 5.4, conductivity 54 μS/sm, sediment type: sand with a thin layer of silt and plant debris, collected 18 Aug. 2010 (leg. V. A. Gusakov) .

Additional locality. The paratype was found in Đồng Nai Province , Cát Tiển National Park, shallow floodplain lake Bàu Chim, plant-filled (young trees of acacias) residual pool in central part of the almost completely dried up lake’s water area (N 11°28.826', E 107°22.641'), altitude 132 m asl, depth 0.15 m, water temperature 31.3 °С, рН 6.3, conductivity 105 μS/sm, sediment type: grey clay with plant debris and rhizomes, collected 9 Sep. 2010 (leg. V. A. Gusakov) .

Etymology. The specific epithet means “Vietnamese”, “from Vietnam ”.

Description. Both specimens, holotype and paratype, have seven chaetiferous segments, a length of 400 and 350 µm, and a maximum diameter of 62 and 68 µm (in segments III–IV), respectively. No signs of a clitellum or budding zones were detected. Epidermal glands are present, but poorly visible and colorless in fixed state. The structural details of the head (body end to the left in Fig. 1A ) and internal organs are indiscernible.

Dorsal and ventral chaetal bundles bear hairs and sigmoid chaetae. Hair chaetae are present in most dorsal bundles and in the ventral bundles of the first two chaetiferous segments (II, III; the 1st segment is without chaetae in Aeolosoma ). One, rarely two, hairs per bundle, length 49–166 µm, thickness up to c. 1 µm proximally. The longest hairs are in the dorsal bundles of the mid-body. All hair chaetae are almost straight. Most of them are roughly serrated, especially in the distal half; less often, the serration is poorly visible ( Table 1 ; Fig. 1 A,C,D ).

Sigmoid chaetae are one to three per bundle, more numerous on the ventral side, absent in the last segment of the paratype on the dorsal side. Their length varies within 20–34 µm, the shortest ones are noted in the bundles of segment II ( Table 1 ). All chaetae are more or less s-shaped, with a sickle-like distal third. Their greatest width is observed in the middle of the curved distal part. Ventral chaetae are usually somewhat thicker than dorsal ones: up to 1.5 μm vs. 1.0 μm. The tips of all sigmoid chaetae are bifid, with small but clearly distinguishable teeth, and the upper tooth about twice as thin and short as the lower. On the concave side of the chaetae, under the lower apical tooth, there is a row of 3 to 9 roughly equal-sized denticles, comparable in size to the upper apical tooth. As a rule, the dorsal sigmoid chaetae have fewer of these denticles than the ventral chaetae (usually 6–7, but in single cases no more than 3–4 vs. 8–9, respectively) ( Fig. 1 B,D,E–J ).

Remarks. The main feature that distinguishes Aeolosoma vietnamicum sp. n. from all of the 26 currently recognized congeners is the presence of peculiar sigmoid chaetae with a row of 3–9 relatively large, equally sized denticles on the concave side of the distal end and with a bifid tip bearing clearly differentiated distal and proximal apical teeth. Also, the new species stands out by a low number and a peculiar distribution of the hair chaetae in the bundles of the body: it has not more than two hairs per bundle, and on the ventral side hairs are absent in most segments, from IV to the end of the body. Five representatives of the genus have some resemblance with A. vietnamicum sp. n. The presence of denticles (serrations) on the distal concave part of the sigmoid chaetae is known in A. corderoi Marcus, 1944 , A. evelinae Marcus, 1944 , A. sawayai Marcus, 1944 , and A. travancorense , and bifurcated tips of the sigmoid chaetae are noted in A. sawayai and A. tenebrarum Vejdovsky, 1884 (Aiyer 1926; Du Bois-Reymond Marcus 1944; Marcus 1944; Bunke 1967; Brinkhurst & Jamieson 1971; Naidu 2005; Timm 2009). Comparison and characteristics differentiating the listed species from A. vietnamicum sp. n. are given in Table 2 .

Only A. sawayai has sigmoid chaetae with both denticles on the concave side and bifurcate ends, like A. vietnamicum sp. n. (Marcus 1944; Bunke 1967; Brinkhurst & Jamieson 1971; Timm 2009). However, in contrast to the new species, the number of denticles on the concave side of the sigmoid chaetae in A. sawayai does not exceed 2–3 (when present at all), these denticles are very small and may be invisible or completely absent in specimens from some populations ( Brinkhurst & Jamieson 1971). The apical teeth of the sigmoid chaetae of A. sawayai are located on the chaetae's tips next to each other and cannot be clearly identified as distal (upper) and proximal (lower) (Marcus 1944), unlike in A. vietnamicum sp. n. Besides, in specimens of A. sawayai from some regions, the sigmoid chaetae may be not bifurcate [e.g., from Surinam according to Brinkhurst & Jamieson (1971)]. Additionally, A. sawayai differs from A. vietnamicum sp. n. in the starting position of the sigmoid chaetae (from III vs. from II), the presence of hair chaetae also in posterior ventral bundles, more numerous hair chaetae per bundle, and a shorter maximal length of the hair chaetae (see Table 2 ).

A. tenebrarum , according to various sources, does or does not have a bifurcated apex on the sigmoid chaetae ( Bunke 1967; Brinkhurst & Jamieson 1971; Timm 2009). Brinkhurst & Jamieson (1971), summarizing a large body of references, indicate that the species does not appear to have apical teeth but rather a row of one or more small teeth on the convex part of the distal end of the sigmoid chaetae, and that only the first tooth is clearly visible. Thus, there are some disagreements in the structure of the tips of the sigmoid chaetae in A. tenebrarum , in contrast to the distinct bifid type of this chaetae in A. vietnamicum sp. n. Also, A. tenebrarum differs from A. vietnamicum sp. n. in a more posterior starting position of the sigmoid chaetae and their greater length, the absence of any denticles on the concave side of the sigmoid chaetae, the presence of hair chaetae in most ventral bundles, a larger number of hair chaetae per bundle, and a greater maximal length of the hair chaetae (see Table 2 ).

The remaining three species— A. corderoi , A. evelinae , and A. travancorense —all have serrated concave sides of the sigmoid chaetae, but simple-pointed tips of these chaetae, unlike bifid ones in A. vietnamicum sp. n. (see Table 2 ). It should be noted that in A. evelinae , denticles on the concave side of the sigmoid chaetae were noted only in individuals found in Germany ( Bunke 1967), while they were not recorded in the original description from South America (Marcus 1944). Thus, the individuals from Germany were possible misidentified.

In addition to the shape of the apex of the sigmoid chaetae, all three species mentioned above also differ from A. vietnamicum sp. n. in more numerous and shorter hair chaetae in the bundles, and the presence of hairs in most segments on the ventral side of the body (see Table 2 ). A. corderoi additionally differs from the new species in the starting position of the sigmoid chaetae and their greater length, and in a noticeable decrease of the denticles size on the concave side of the sigmoid chaetae towards their proximal end. In turn, A. evelinae additionally differs from A. vietnamicum sp. n. in the presence of sigmoid chaetae exclusively in ventral bundles of the body. Finally, A. travancorense differs from A. vietnamicum sp. n. also in the initial position of the sigmoid chaetae and in unique peculiarities of their distal part, namely two rows of claw-like denticles, noticeably reducing in size proximally, on the concave side of these chaetae, and an elongated, pointed, bristle-like tooth on their tip (see Table 2 ).

Figures

FIGURE 1. Details of the morphology of Aeolosoma vietnamicum sp. n. A. Entire body of holotype, lateral view; head to the left. B. Bundle of ventral sigmoid chaetae. C. Serrated hair chaeta, detail. D. A dorsal bundle with two hair chaetae and two sigmoid chaetae. E. Drawing of ventral sigmoid chaeta, lateral view. F, G. Tips of the ventral sigmoid chaetae, lateral view. H. Tip of the ventral sigmoid chaeta, frontal view from the concave side. I, J. Tips of dorsal sigmoid chaetae, lateral view.

Tables

TABLE 2. Comparison of Aeolosoma vietnamicum sp. n. with morphologically similar congeners (only differentiating characteristics are presented; main differences of the compared species are underlined).

Characteristic	A. vietnamicum sp. n.	A. corderoi	A. evelinae	A. sawayai	A. tenebrarum	A. travancorense
References:	This study	Du Bois-Reymond Marcus 1944; Bunke 1967; Brinkhurst & Jamieson 1971	Marcus 1944; Bunke 1967; Brinkhurst & Jamieson 1971; Timm 2009	Marcus 1944; Bunke 1967; Brinkhurst & Jamieson 1971; Timm 2009	Bunke 1967; Brinkhurst & Jamie- son 1971; Timm 2009	Aiyer 1926; Bunke 1967; Brinkhurst & Jamieson 1971; Naidu 2005; Timm 2009
Sigmoid chaetae:
Location	dorsal and ventral bundles	dorsal and ventral bundles	ventral bundles only	dorsal and ventral bundles	dorsal and ventral bundles, or ventral bundles only, or absent	dorsal and ventral bundles
Starting position	from II	from III (rarely IV)	from II; at times from III or more posteriorly	from III	from IV–V	from III
Length [µm]	20–34	40–46	28–40	28–35	50–60	30–45
Tip type	bifid, with distinct apical teeth clearly dif- ferentiated into distal (upper) and proximal (lower)	simple-pointed	simple-pointed	bifid, with very small (at times not visible) adjacent apical teeth that cannot be clearly identified as distal or proximal	bifid or simple- pointed (according to different sources)	simple-pointed, with an elongate, pointed, bristle- like apical tooth
Denticles on concave side of distal part	one row of 3–9 distinct, roughly equal-sized denticles	one row of 6–7 denticles decreasing in size proximally	side smooth or with one row of 5–7 small denticles (according to different sources)	one row of 2–3 small denticles (may be invis- ible or absent)	absent	two rows of 5–10 clawlike denticles decreasing in size proximally
Hair chaetae:
Location	most dorsal bundles; ventrally only in first two chaetiferous seg- ments (II, III)	most dorsal and ventral bundles	most dorsal and ventral bundles	most dorsal and ventral bundles	most dorsal and ven- tral bundles	most dorsal and ventral bundles
Number per bundle	not more than 2	up to 3–5	up to 5–7	up to 3–4	up to 5–7	up to 4–6
Maximal length [µm]	166	85	110	105	200	100