Haplotaxoides, Fend, 2024

Martin, Patrick, Fend, Steven, Martinsson, Svante, Klinth, Mårten, Torii, Takaaki & Erséus, Christer, 2024, Towards an integrative revision of Haplotaxidae (Annelida: Clitellata), Zoological Journal of the Linnean Society (zlae 141) 202 (4), pp. 1-38 : 20-22

General remarks on Haplotaxoides material

Somatic characters

The apparently convergent morphology (chaetae single, hooked, and strongly developed ventrally; peristomium extending well anterior of the mouth; narrow-elongate body form with thick cuticular layer; midventral glands in most segments; massive nephridia; pharyngeal glands not developed) of Haplotaxoides (Clade 3) and Haplotaxis (Clade 1) is indeed remarkable and may be partly related to predaceous habits. However, the narrow body form and characters related to chaetae and ventral glands may be more general adaptations for medium-size worms living in groundwater. As in typical Haplotaxis species, Haplotaxoides chaetae are single, with dorsals much smaller than ventrals, and ventral chaetae are distally hooked. Despite these general similarities, other somatic characters easily distinguish known populations of Haplotaxoididae from typical haplotaxids.

The strongly developed musculature surrounding the buccal area in Haplotaxoides resembles that of Haplotaxis , but in contrast to a well-developed muscular pharynx ( Figs 2A–C , 3A–C ), Haplotaxoides specimens have a thin, short, circular muscle layer surrounding a section of the simple pharynx at about 2/3 ( Figs 2D , 4I , 5D ). The musculature surrounding the pharynx may have a similar function to that of the haplotaxid pharynx, in which more extensive musculature is incorporated within the gut wall. Haplotaxoides also lacks the capacious oral cavity of Haplotaxi s. Pharyngeal glands, which are well developed in many taxa in the present analysis, including those assigned here to Pelodrilidae ( Fig. 2F ), have not been seen in either Haplotaxis or Haplotaxoides . Unlike in most other microdrile taxa, the brain of both Haplotaxoides and Haplotaxis appears to be located well anterior to the mouth, within the fore part of the elongated peristomium—probably a consequence of the highly developed buccal musculature.

Other characters related to gut histology, including the chloragogen layer, should be further studied and compared among haplotaxid-like taxa. A well-developed surface layer of chloragogen tissue was never observed on the Haplotaxoides gut wall ( Fig. 5N, O ). In comparison, available Haplotaxis preparations show a distinct chloragogen layer on the gut, beginning in middle segments ( Fig. 3E, F ). The monotypic, Japanese Heterochaetella glandularis also lacks chloragogen on the gut, but has a dense such layer on the dorsal blood vessel ( Yamaguchi 1953).

Abundant coelomocytes have not been described for any Haplotaxis species, nor have we seen them in Haplotaxis specimens used in the present study, but they are common in most observed Haplotaxoides populations (the only exception being ‘species C’), and they are at least superficially similar to the flat, oval discs described in many Enchytraeidae (reviewed by Schmelz and Collado 2010).

The body wall of Haplotaxoides has a distinct cuticular layer, about 5 μm thick in Haplotaxoides decipiens and the Stony Creek population, although it is somewhat thinner (2–3 μm) in other populations. This differs from the very thin layer seen in most microdriles, but resembles the Haplotaxis specimens in the present analysis, as well as published descriptions of Haplotaxis s.s. The diagnosis of Haplotaxidae (s.l.) by ( Brinkhurst 1966) stated ‘Body-wall with cuticle thick in at least some species’. The description of Haplotaxis gastrochaetus states that body wall cuticle is 10 μm thick ( Yamaguchi 1953), and de Eguileor et al. (1990) note 4.5 μm in Haplotaxis gordioides from Europe. A review of about 200 slide-mounted, North American haplotaxid specimens (S. Fend, unpublished data) indicates a wide range in cuticle thickness among populations, from less than 1 to 7 μm, although the median was only 1.5 μm.

The large ventral glands are usually 3 per segment in all Haplotaxoides populations, and although they are midventral, with external secretory areas, the elongate-tubular form differs from the small, compact midventral ‘Timm’s glands’ ( Figs 2A– C , 3H ) seen and described in Haplotaxis species ( Timm 1883: fig. 12, Forbes 1890: figs 5–7, Hrabě 1931: fig. 9a, Yamaguchi 1937, Brinkhurst 1966). Other body-wall glands are diverse and species-specific in taxa previously assigned to Haplotaxidae (e.g. Heterochaetella glandularis ; Adenodrilus denticulatus ), but we are not aware of any that closely resemble those of Haplotaxoides . For instance, A. denticulatus has long, tubular glands—but these are restricted to reproductive segments (X– XIII), and located on the chaetal lines ( Čekanovskaja1959). Although the gland tissue appears similar to that of nephridia (transparent, granular) in Haplotaxoides , we did not observe any connections to nephridial ducts, and the glands also occur in pre-nephridial segments.

Nephridia ( Fig. 4H , 5G ) are difficult to see in most available Haplotaxoides preparations, but appear similar to those of Haplotaxis ( Fig. 3G ): the indistinct ducts are difficult to follow through a large mass of slightly granular tissue, which fills much of the dorsolateral coelom. However, they are generally more transparent than those of Haplotaxis , and nephridial glands (as described by Brinkhurst 1966 in Haplotaxis forbesi, Smith, 1918 ) were not seen in our material. The Haplotaxoides nephridia bear no resemblance to the more simple, tubular ones in the Pelodrilus , Hologynus , Limpluvia setoensis sp. nov., or Ohtakiana kakidaensis sp. nov. specimens used in the present analysis, nor to those of lumbriculids and enchytraeids. Delaya also has tubular nephridia, as described by Delay (1970: fig. 9).

Reproductive characters

Testes are located in XII – XIII in partially mature specimens of all Haplotaxoides populations investigated, distinguishing these worms from known Haplotaxis species, where they are typically in X– XI (less commonly beginning in IX). Partially developed ovaries were seen in XV in some specimens from most sampled populations, but always appeared absent in XIV, suggesting that mature individuals are metagynous—and this is confirmed by the two nearly-mature specimens from the Sacramento River. Again, this differs from most other taxa that have been associated with Haplotaxidae . The family diagnosis by Brinkhurst (1988) specifies that haplotaxids are hologynous or progynous, whereas metagynous taxa with haplotaxid-like male ducts were attributed to Metataxis Righi, 1985 (Metagynophora sensu Jamieson, 1988 ). Metataxis (sensu Omodeo, 1987) includes a diverse set of species, differing from both Haplotaxis and Haplotaxoides in having paired chaetae and a typical (eversible) pharynx with pharyngeal glands. Unfortunately, no Metataxis species were available in the present study, which leaves their family classification, as well as their relationship to Haplotaxoides unknown. However, other Metagynophora, including a specimen tentatively identified as ‘ Alluroididae sp’. and several earthworms ( Crassiclitellata ) did not appear closely related to Haplotaxoides in the present DNA analysis.

No fully mature worms of Haplotaxoides decipiens were observed, so morphology of genital ducts in this species is unknown. Small organs that may represent developing spermathecae were seen in segments V – VIII in several partially mature specimens from the Haplotaxoides decipiens type locality, and their dorsolateral location, anterior to the dorsal chaetal bundles of preclitellar segments, is reasonably consistent with positions of spermathecae described in some Haplotaxis ( Benham 1903, Michaelsen 1905, Hrabě 1931, Brinkhurst & Marchese 1987), and in other ‘ Haplotaxidae s.l. ’. ( Beddard 1891, Hrabě 1958), and Metataxis ( Cook 1975, Omodeo 1987).

The simple, prosoporous male ducts observed in the two nearly mature (but unmated) Haplotaxoides tehama resemble those described for Haplotaxis , as well as those of Pelodrilidae and other taxa previously associated with the Haplotaxidae . However, spermathecae were not visible in those specimens, and there was no evidence of the dorsolateral cell masses seen in anterior segments of Haplotaxoides decipiens . This may relate to poor preservation, and small spermathecae may simply have been obscured by contents of the sperm sacs. The paired gland masses in XIII of Haplotaxoides tehama resemble ‘copulatory glands’ described for some Haplotaxis species (e.g. Haplotaxis heterogyne , Haplotaxis vermivorus ), as well as Pelodrilus , and Hologynus species (e.g. Brinkhurst 1966).

Both the basally hooked, elongate chaetae at the male pores, and greatly thickened chaetae associated with female pores of mature Haplotaxoides tehama differ from genital chaetae of other taxa associated with the former concept of Haplotaxidae s.l. Genital chaetae are infrequently reported in descriptions of Haplotaxidae s.s., although they have been described for Haplotaxis aedeochaeta and Haplotaxis dubius . Those in Haplotaxis aedeochaeta are long and hair like, and associated with male pores in XII ( Brinkhurst and Marchese 1987); genital chaetae in Haplotaxis dubius are associated with large chaetal glands in XII – XIV and have thin, hair-like tips ( Hrabě 1931: figs 8–9). Genital chaetae have also been reported in other taxa formerly associated with Haplotaxidae s.l. Those of Omodeodrilus guanivorus bear some resemblance to the elongate, basally hooked chaetae at male pores of the mature Sacramento River specimens. However, although O. guanivorus has been associated with Haplotaxidae s.l., it differs from both Haplotaxidae and Haplotaxoididae (as defined here) in terms of body form, chaetae, nephridia and arrangement of gonads ( Omodeo 1987: fig. 3). Genital chaetae are mentioned, but not well described for Omodeodrilus kraepelini (Michaelsen, 1914) and the metagynophoran Metataxis eliae Righi, 1985 .

Gut contents

Some Haplotaxoides specimens had guts partially filled with very fine organic matter that appeared detrital; although gut contents sometimes included diatom frustules, large amounts of mineral particles and obvious plant detritus (filamentous algae, macrophyte fragments) were never seen. At the type locality and nearby sites, 15 specimens of Haplotaxoides decipiens contained small amounts of amorphous, often transparent organic matter; seven had small amounts of detritus, and five contained chaetae from other oligochaete taxa, including Haplotaxis sp. , Rhyacodrilus sp. , and Eremidrilus sp. One specimen contained a chironomid larva (cf. Parametriocnemus sp. ), and 24 had empty guts. Across all other sites, guts of 10 Haplotaxoides spp. individuals contained small amounts of fine detritus, 24 had amorphous organic matter, 10 had oligochaete chaetae, two had dipteran ( Chironomidae ) fragments, and 29 had empty guts. The unidentified organic matter sometimes consisted of small globules (probably lipid), and in other cases appeared striated (possibly animal tissue).

Figures

Figure 2. Comparison of anterior ends of haplotaxid-like worms analysed here, schematic figures with emphasis on somatic characters. C, F, and H are from the genetically analysed specimens, the other figures are based on individuals from the same site (and usually same date) as the analysed worms, but better fixed for morphology. A, Haplotaxis spUS2 from Corral Creek, Idaho, USA.Oral cavity of gut is contracted, muscular pharynx extends through two segments.B, Haplotaxis spJP2 from Gudari-numa, Aomori, Japan.Oral cavity of gut is expanded, muscular pharynx is shorter than in the Idaho specimen. C, Haplotaxis spJP1 from Kakida River, Shizuoka, Japan.Timm’s glands are larger than in (A) or (C), muscular pharynx is short.D, Haplotaxoides decipiens from Guadalupe Creek, California, USA.E, Ohtakiana kakidaensis from Kakida River, Shizuoka, Japan. F, Pelodrilus cf. darlingensis from Western Australia. G, Limpluvia setoensis from Asahina River, Shizuoka, Japan. H, undetermined Lumbriculidae, specimen CE32192, from Euchre Creek, Oregon, USA.

Figure 4. Haplotaxoides spp. from various sites. A, chaetae, Haplotaxoides decipiens from Guadalupe Creek, California, USA (non-type specimen): from left to right, dorsal and ventral chaetae in X, a mid-body segment, and a posterior segment. B, chaetae, Haplotaxoides spA from Smith River, California (specimen CE32193):dorsal and ventral chaetae from XX, and from a posterior segment (the ventral chaeta with a partially formed replacement). C–D, chaetae, Haplotaxoides tehama from Sacramento River, California:(C) from left, dorsal and ventral chaetae from segments XX and C (large, immature non-type worm); (D) dorsal and ventral chaetae from XXV, from a posterior segment, and genital chaetae from XIV and XVI (paratype). E, chaetae, Haplotaxoides spC from the Metolius River, Oregon, USA (not genetically analysed): dorsal and ventral chaetae from a mid-body segment and a posterior segment (ventrals have developing replacements). F, possible spermathecae in VI–VII, a partially mature Haplotaxoides decipiens from Los Gatos Creek, California (paratype). G, midventral glands and ventral nerve cord in VII–VIII, an immature Haplotaxoides decipiens from Guadalupe Creek (non-type). H, nephridium in XLIV, Haplotaxoides decipiens from Los Gatos Creek. I–K, Haplotaxoides tehama, from Sacramento River (paratype): (I) anterior end; (J) reproductive segments (XI–XVIII), ventrolateral view, ventral glands and chaeta greatly enlarged in XVI; (K) a lateral view of XIII.L, reproductive segments (XII– XVI) from a nearly mature Haplotaxoides tehama, Sacramento River (paratype), ventral view, ovaries and female funnels obscured by gut contents, ventral glands enlarged in XVI.

Figure 5.Haplotaxoides decipiens from Los Gatos (A, H, L) and Guadalupe (B–G, I–K,M–O) creeks.A, a whole,slide-mounted Haplotaxoides decipiens (non-type worm) showingelongate habitus.B,head andpharynx of whole mounted worm (paratype).C–D, sagittal section showinghead and pharynx,and (D) shows detail of thickenedcircular musculature (non-type).E, pharyngeal musculature,from a whole mount (paratype); circular musclevisible in cross section,between the radial muscle strands in upper right.F,ventral chaeta,a whole mounted worm(non-type).G, sagittal section showing nephridial tissue in a posterior segment(non-type).H, ventral chaeta and ventral glands leading to midventral secretory surface;whole mount (ventrolateral view;non-type).I, ventral glands and coelomocytes;whole mounted worm (paratype).J–K, ventral view of whole mounted worm showingmidventral secretory surfaces of ventral glands in twosegments (non-type specimens).L, reproductive segmentsof a partially mature worm stained withhematoxylin,showing spermsacs in XII–XIII andsmall ovary in XV.M,gut in about segment VI,from a sagittal section (non-type).N–O, gut in posterior segments,showing granular tissue;(N) is from a dissection,(O) from a sagittal section (both non-types).

Figure3.Haplotaxis species,somaticcharacters.A,Haplotaxis spUS2 fromCorral Creek,Idaho,USA.B,D–H,unidentifiedHaplotaxis sp.(population notanalysed genetically) fromsprings near Brewer,Montana,USA.C, I, Haplotaxis spUS3from Guadalupe Creek,California,USA.J,unidentified Haplotaxis sp.(population not analysedgenetically) from StonyCreek,California,USA.A, anteriorend ofdissected worm,showing extensive musculaturebetween gut and bodywall in I–III,and thick-walledmuscular pharynx in IV–V.B,sagittal section,showingthin-walled oral cavity (right, anterior) and contracted,muscular pharynx (left,posterior).C, wholemounted worm, showingpartially contracted oral cavity and muscular pharynx, and secondary annulationbeginning in V.D,sagittal section ofa contracted muscular pharynx,with central band ofcircularmuscles,surrounded by region with more radial muscles.E, F,sagittal sections ofmiddle and posterior regions showing gut wallwith extensive chloragogen layer.G, sagittal section oftwo middle segments showingextensive nephridia.H, sagittal section,showing Timm’sgland beneath ventral nerve cord.I, smalldorsal and large ventral chaeta (with a partially developedventral chaeta) in mid-bodysegment.J,ventral chaeta frommid-body segment.