Rhyacodrilus alcyoneus, Rodriguez, Pilar & Fend, Steven V., 2013

Rodriguez, Pilar & Fend, Steven V., 2013, New species of Rhyacodrilus (Annelida: Clitellata: Rhyacodrilinae) of North America, with re-description of R. sodalis (Eisen, 1879), Zootaxa 3664 (1), pp. 1-44 : 24-28

Rhyacodrilus alcyoneus sp. n.

( Figs 12–13 )

Holotype. USNM 1202066, mature but unmated specimen, dissected and stained in hematoxylin; mounted in Canada balsam.

Paratypes. USNM 1202067-69, 1 dissected specimen stained in hematoxylin, from the type locality (20 June 2011). 2 partially mature, dissected worms from the type locality (7 June 2008 and 25 March 2007).

Type locality. Guadalupe Creek, above Guadalupe Reservoir, in a short (ca. 50 m) reach below the confluence with Rincon Creek, N37.1831° W121.8716°, Santa Clara Co. California ( USA) (21 June 2011).

Other material. From the type locality. Slide-mounted, partially mature specimens: 6 dissected (25 March 2007, 6 April 2008, 11 May 2009, 7 June 2008). 20 unmounted, partially mature specimens: 2 from 22 March 2008, 4 from 6 April 2008, 3 from 17 April 2008, 10 from 27 March 2009, 1 from 8 March 2010.

Etymology. alcyoneus , for Alcyoneus , the eldest of the gigantes ("the earth born" in Greek) slain by Heracles. The name refers to the very large size of the worms in this species, relative to congeners.

Description. Whole body dimensions in the description are based on 24 worms, all with at least obvious sperm sacs or partially developed genital pores. Number of segments in complete specimens about 100–135, length up to 63 mm (most specimens about half this length; median 41 mm). Diameter of the body in VIII 0.9–1.3 mm; maximum diameter to 1.6 mm. Short, round prostomium (315–486 µm long, 504–630 µm wide at base) ( Fig. 12 A). Epidermis glandular, 15–33 µm high in anteclitellar region, where it is more darkly stained in haematoxylin than in postclitellar region. Secondary annulation usually visible in anterior segments as a narrow anterior ring; median and posterior furrows often present, dividing each anterior segment into 2–4 apparent sections ( Fig. 12 A, B); apparent segmentation in posterior segments varies with fixation, but additional rings may be visible. Mid-dorsal pores at the anterior secondary furrow in III–VII ( Fig. 12 B, E).

Clitellum saddle-shaped (thickest dorsally), maximum thickness 70–100 μm, from level of chaetae in X to mid-XIII. One pair spermathecal pores at the beginning of segment X, slightly lateral to the line of ventral chaetae, on the secondary annulation; actual pore is at the outer edge of an inconspicuous transverse groove ( Fig. 13 B). One pair male pores in segment XI, in line of ventral chaetae, externally visible as transverse folds or pits. One pair female pores, inconspicuous at 11/12, about midway between the lateral line and the line of ventral chaetae.

Dorsal bundles in anterior segments with 1–3 smooth hair chaetae (maximum length 530–764 µm, shorter in segment II up to 240–370 µm) and (1)3–6 pectinate chaetae. Pectinate chaetae up to 124 µm long in II, 166–224 µm in other segments, very slightly sigmoid, teeth about equally long, and pectinations shorter than or equal to outer teeth ( Fig. 12 F, G). Posterior dorsal chaetae 3–6 per bundle, not pectinate, much like ventral chaetae posterior to about XV (155–237 µm long), upper tooth thinner and of the same length or slightly shorter than lower ( Fig. 12 H). Hair chaetae absent posterior to segment XIII or XIV. Ventral chaetae bifid. Anterior ventral chaetae 4–6 per bundle (141–200 µm long), sigmoid, with nodulus about 1/3 the distance from the tip; distal tooth 1.5 to 2 times the length of proximal. In mature worms ventral chaetal bundles of IX and X on low, round tubercles, 170–220 μm wide ( Fig. 13 B cht); ventral chaetae in X reduced to 2(3) per bundle, slightly longer (245–288 μm) than those in anterior segments, and with distal tooth up to 4 times longer than proximal ( Fig. 12 M). Postclitellar ventral chaetae 1–4(6), commonly 2–3, per bundle (133–230 µm long); teeth about equal in length, distal tooth thinner. Modified penial chaetae unequal in length (221–327 µm) in segment XI, 5–7 per bundle, simple-pointed or minutely bifid in the most mature specimens (or bifid in some of the less-mature worms), and arranged fanwise at male pore ( Fig. 12 K, L).

Very abundant nucleated and granulated coelomocytes (diameter 24–48 µm) ( Fig. 12 C). Pharynx in II–III (IV) with a well-developed dorsal pad of glandular tissue interspersed with muscle strands. Pharyngeal glands from IV to V, or restricted to IV. Chloragogen layer covering the gut from segment V backwards, up to 108 µm high, densely granulated ( Fig. 12 D).

Descriptions of reproductive organs are based on dissections of 3 unmated specimens with mature eggs in the egg sacs, collected on 20–21 June 2011, and supplemented with less-mature material collected on other dates. One pair testes in segment X and one pair ovaries in segment XI. Large eggs with yolk granules visible in 3 individuals, egg sac to XX–XXVI; sperm sac back to segment XVIII–XXIV.

Spermathecal duct joins a wide ectal vestibule at lateral edge of outer fold (135–175 μm long); the duct is very short (45 μm long by 100–175 μm wide), with narrow-columnar epithelium and a very narrow lumen ( Fig. 13 A, B). Spermathecal ampulla without sperm in all examined individuals (i.e. unmated), up to 340 by 490 μm.

Sperm funnel opens in the ventral part of septum 10/11, up to 300 μm wide. Vas deferens ciliated, with columnar epithelium in middle portion; diameter 51–66 µm, narrowing to 27–45 µm close to the atrial junction, and to 30–54 µm at the sperm funnel. Vas deferens longer (520–700 µm) than atrium, joining atrium subapically ( Fig. 13 A, B). Atrium elongated and roughly tubular, narrowed at ends, 405–510 μm long in the 3 most-mature worms, 360–469 µm in the other material, and 82–125 µm wide. No clear separation between atrial ampulla and duct. Ental 1/2 to 2/3 of atrium ampulla-like, with somewhat columnar epithelium, lumen ciliated, muscle layer about 3 μm thick and covered by a layer of prostate cells (70–120 µm high) in tightly-packed bundles in more mature worms, or appearing diffuse in less-mature worms. Ectal 1/2 to 1/3 of atrium without prostate, 180–220 μm long, to 122 μm wide entally, narrowing to 78 μm ectally; with oblique muscle strands extending from the atrial muscle layer (5 μm) to the body wall near the male pore. A mass of loosely packed cells (possibly glandular) among the muscle bands, associated with the male pore. Several dorso-ventral muscle strands are associated with the bundles of penial chaetae at the male pores.

Distribution and habitat. Guadalupe Creek occupies a small drainage in the Coast Ranges of central California. The stream is regulated, but the alcyoneus type locality is upstream of the reservoirs, and has a relatively undisturbed watershed with natural flow regime. The collection site is a third-order stream with gravelcobble sediments; summer temperatures are generally low (below 20° C), and salmonids are present (Carter & Fend 2000; Jae Abel, Santa Clara Valley Water District, pers. comm. 2012). Surface flow is very low from late summer through autumn, and is intermittent in some years. The section of the river Guadalupe where the population has been sampled has localized sulfide upwellings.

Remarks. The size of Rhyacodrilus alcyoneus sp. n. relative to other Nearctic Rhyacodrilus is one of the most striking characteristics of this species. Two other very large Rhyacodrilus species, R. intermedius Semernoy, 2004 and R. multispinus (Michaelsen, 1905) , have been reported from Lake Baikal, although both of them are devoid of hair chaetae. The former has been reported as 50 mm long by 1.5 mm wide (Semernoy 2004) and the latter up to 57 mm long by 4.0 mm wide (Chekanovskaya 1962). The extremely large dimensions (80-150 mm long) reported in the description of R. sinicus Chen, 1940 (and repeated in the literature, e.g. Ohtaka 1995) are surely an error in the original description, as Chen (1940) also stated that the species is similar in size to other tubificids, the number of segments (42–56) is quite modest, and and hair chaetae are stated to be about the diameter of the body (240–250 μm).

All examined individuals are from the type locality in Guadalupe Creek. Collections were made on several dates between 2007 and 2011. Although sampling was done at other times during the year, Rhyacodrilus alcyoneus sp. n. has only been collected from the period of January to June. The reproductive organs appear completely developed in only 3 specimens of the available material. These individuals had mature eggs with yolk granules in the egg sac, but all of them lacked sperm in the spermathecal ampulla and in the sperm funnel, suggesting that this population fails to reproduce sexually and is parthenogenetic. According to Edwards and Bohlen (1996), the association between parthenogenesis and high polyploidy in earthworms is an advantageous condition that provides resistance to environmental stress (Cosin et al. 2011).

The structure of the male duct is very similar to that described below for R. longichaeta sp. n. found at another site on the same stream. However, R. alcyoneus sp. n. never develops the characteristic long hairs in segment II that are diagnostic for R. longichaeta . Other characters that consistently distinguish R. alcyoneus sp. n. from R. longichaeta are the lack of hair chaetae in postclitellar segments, the posterior dorsal chaetae resembling the ventral chaetae, the distinct spermathecal vestibule, the larger number (more than 5) of penial chaetae, the large chaetal tubercles in IX and X, and the pharyngeal glands not extending posterior to segment IV or V.

Two Siberian species, Rhyacodrilus brevis Semernoy, 2004 and R. sibiricus Semernoy, 1971 , also have hair chaetae restricted to anterior segments, although atria of these species are more or less globular instead of tubular. Based on the presence of hair chaetae in dorsal bundles, modified penial and spermathecal chaetae and a tubular atrium, R. alcyoneus sp. n. is closer to R. svetlovi Sokolskaya, 1976 from the Chukchi Peninsula (East Siberia), but the vas deferens in that species is shorter than the atrium, and joins it apically instead of subapically. Compared with other Rhyacodrilus species having modified penial chaetae and tubular atria but unmodified spermathecal chaetae, R. alcyoneus sp. n. is well-distinguished by several features. The subapical junction of the vas deferens with the atrium distinguishes R. alcyoneus from R. korjakovi Semernoy, 2004 , R. vasalatus Semernoy, 2004 and R. suputensis Timm, 1990 , where the vas deferens joins the atrium apically. It is also well-separated from the Baikalian species R. korjakovi and R. vasalatus by the absence of a clearly separated atrial duct. It is also worth mentioning that R. korjakovi loses the hair chaetae when it matures (while keeping the pectinates), and R. vasalatus only has bifids.

Among Rhyacodrilus species, the modified spemathecal chaetae are usually very different from somatic ventral chaetae (i.e. long or grooved distal teeth, like in R. saelonae sp. n. or R. carsticus Košel, 1980 ); however, in R. alcyoneus sp. n. they are larger with longer distal teeth, but not very different in shape.

The presence of dorsal pores in this species is an interesting feature, since after Caramelo and Martínez- Ansemil (2012) the only microdriles (apart from enchytraeids) known to have dorsal pores belong to the subfamily Rhyacodrilinae : Monopylephorus aucklandicus (Benham, 1909) , Bothrioneurum vejdovskyanum Štolc, 1886 , Peristodrilus montanus , and Protuberodrilus tourenqui . Stephenson (1930) described their function in earthworms as related to the protection against desiccation, keeping the body wall moist for gas exchange, and providing protection against bacteria and parasites. The first pore is in the secondary annulation, at the front of segment III, while those reported in other rhyacodrilines were located in the intersegments 3/4 or 5/6 (Caramelo & Martínez- Ansemil 2012: Fig. 5 ).

Figures

FIGURE 5. Rhyacodrilus quileuticus sp. n. A. Anterior part of the body. B. Reproductive organs. C. Spermathecal chaetae. D. Penial chaetae.

FIGURE 12. Rhyacodrilus alcyoneus sp. n. A. Anterior part of the body showing the position of clitellum, chaetal tubercles and male pores. B. Dorsal pores. C. Coelomocytes. D. chloragenous cells with granules. E. Dorsal pore in the secondary annulation of segment V. F – H. Dorsal chaetae in segments VI, XIII and posterior. I – J. Ventral chaetae in segment VII and posterior, respectively. K. Atrium and penial chaetae arranged fan-wise, with numerous dorso-ventral muscular strands. L. Detail of penial chaetae and penial sac surrounded by musculature. M. Ventral chaetae in segments VII and X (spermathecal segment), for comparison.

FIGURE 13. Rhyacodrilus alcyoneus sp. n. Schematic drawings of the reproductive system and penial chaetae in XI. A. View from median body line, B. Lateral view of atrium and chaetal tubercula in segments IX and X.