Introduction
The acanthocephalan body is distinguished by several morphological features and is divided into two functionally different regions: the praesoma (the anterior region) and the metasoma (the trunk) (Crompton and Nickol Reference Crompton and Nickol1985). A defining feature of the acanthocephalan praesoma is the proboscis, which is equipped with hooks and serves as the attachment organ. The proboscis morphology in adult acanthocephalans has been described from a limited number of species (Dunagan and Miller Reference Dunagan and Miller1974; Hammond Reference Hammond1967; Herlyn and Ehlers Reference Herlyn and Ehlers2001; Herlyn et al. Reference Herlyn, Martini and Ehlers2001; Taraschewski Reference Taraschewski2000), and data on proboscis development in fragmentary, accompanied mainly by schematic drawings (Amin Reference Amin1982; Awachie Reference Awachie1966; Cable and Dill Reference Cable and Dill1967; DeGiusti Reference DeGiusti1949; Hynes and Nicholas Reference Hynes and Nicholas1957; Moore Reference Moore1946a, Reference Mooreb; Olson and Pratt Reference Olson and Pratt1971; Schmidt and Olsen Reference Schmidt and Olsen1964).
The acanthocephalan Arhythmorhynchus teres Van Cleave, 1920 is an Arctic parasite widely distributed in the northwestern Pacific Ocean, ranging from China to the Chukchi Peninsula, including the Sea of Japan, Sea of Okhotsk, and Bering Sea, the Kuril Islands, and Kamchatka (Atrashkevich et al. Reference Atrashkevich, Orlovskaya, Regel, Mikhailova and Pospekhov2005). In the Sea of Okhotsk basin, the definitive hosts are various species of seabirds, and the intermediate hosts are amphipods Eogammarus tiuschovi (Derzhavin 1927) (Anisogammaridae) and Bulychevia ochotensis (Brandt, 1851) (Talitridae). The latter amphipod species and two species of obligate definitive hosts (the slaty-backed gull Larus schistisagus Stejneger, 1884 and the black-headed gull Chroicocephalus ridibundus Linnaeus, 1766) provide the stability of the parasitic system in the study region (Atrashkevich et al. Reference Atrashkevich, Orlovskaya, Regel, Mikhailova and Pospekhov2005).
Preliminary studies indicate that amphipods B. ochotensis are intensely infected by A. teres, harbouring acanthocephalans at different stages of postembryonic development, from early acanthella to cystacanth (Atrashkevich et al. Reference Atrashkevich2002). Previously, the larval stages of A. teres were determined based on the system of acanthocephalan postembryogenesis proposed by Butterworth (Reference Butterworth1969). This led to the identification of two morphologically distinct stages – middle acanthella I and middle acanthella II – in addition to the early and late acanthella stages.
The praesoma morphogenesis in acanthocephalans is poorly studied, and its fine structure remains almost completely unexplored by electron microscopy. Accordingly, the present study elucidates the general organisation of the praesoma in A. teres at the middle acanthella I stage, with an emphasis on the ultrastructure of its proboscis.
Materials and methods
We collected 214 amphipods Bulychevia ochotensis at low tide in the supralittoral zone of Sikulun Island, Sea of Okhotsk, in June 2025. We selected acanthocephalans at the middle acanthella I stage. This stage corresponds to the developmental period that is followed by proboscis evagination (Butterworth Reference Butterworth1969). Acanthocephalan larvae were fixed in 2% glutaraldehyde prepared in 0.1 M phosphate buffer and subsequently post-fixed in an OsO4 solution prepared in 0.2 M phosphate buffer. After a dehydration series in ethanol and acetone, acanthellae were embedded in an Epon-Araldite mixture. At the dehydration stage, helminth specimens were stained in a saturated solution of uranyl acetate in 70% alcohol overnight. To examine the general organisation of the anterior region of acanthellae, we used semithin (1 μm thick) sections cut on an LKB4 ultramicrotome (LKB Bromma, Stockholm, Sweden). We stained the sections with a mixture of methylene blue and crystalline violet, and viewed them under an Olympus CX41 light microscope (Olympus Corporation, Tokyo, Japan). To study the ultrastructure of the acanthella praesoma, we cut serial ultrathin sections (70 nm) on a PT-PC ultramicrotome (Boeckeler Instruments, Tucson, Arizona, USA) and examined them on a JEM-1400Plus transmission electron microscope (JEOL, Tokyo, Japan).
Results
Of the 214 B. ochotensis specimens, 140 were infected with acanthocephalans A. teres, with infection intensity ranging from 1 to 76 larvae (mean 8). In the examined amphipods, acanthocephalans were found at different stages of postembryonic development (Figure 1). An approximate diagram of the praesoma structure in the acanthocephalans examined is shown in Figure 2.
Developmental stages of the acanthocephalan Arhythmorhynchus teres in the intermediate host, Bulychevia ochotensis. ( a ) Early acanthella located on the outer surface of the amphipod gut (a fragment). ( b ) Middle acanthellae I and one early acanthella (arrow). ( c ) Middle acanthellae II. ( d ) Late acanthella. ( e ) Cystacanths and one late acanthella (double arrow). Two cystacanths (asterisks) evaginated their proboscises due to mechanical damage to the cyst. Scale bar: 1 mm.
Figure 1. Long description
A series of five light micrographs labeled a through e. Each panel includes a white scale bar representing 1 millimeter in the bottom right corner.
Panel a. Top-left. A small, oval early acanthella is positioned above a larger, translucent fragment of an amphipod gut.
Panel b. Top-center. Three elongated middle acanthellae I are arranged around a much smaller early acanthella. A white arrow points directly to the small early acanthella at the center.
Panel c. Top-right. Four elongated, slightly curved middle acanthellae II are shown vertically side-by-side.
Panel d. Bottom-left. A single, large late acanthella with a more defined internal structure and a curved, elongated body.
Panel e. Bottom-right. A group of approximately ten cystacanths. A double white arrow points to one late acanthella among the group. Two specific cystacanths are marked with asterisks; these individuals have their proboscises extended outward from their protective cysts due to mechanical damage.
Schematic reconstruction of the praesoma of A. teres at the middle acanthella stage I. Letter designations: an – apical nuclei; сg – cerebral ganglion primordium; cm – circular musculature primordium; cyt – cytoplasm of tegument; gn – giant nuclei; hp – hook primordia in the form of a long cytoplasmic strand; mc – isolated muscle cell; ne – nerve; ncm – nuclei in circular musculature primordium; nhp – nuclei in hook primordia; npr – nuclei in proboscis retractor primordium; pr – proboscis retractor primordium; re – proboscis receptacle primordium; sm – subtegumental musculature; teg – tegument of acanthella.
Figure 2. Long description
A vertical cross-section diagram of a larval parasite. At the top apical tip, a n points to apical nuclei, with n c m indicating nuclei in the circular musculature primordium. Centrally, a long invaginated structure contains h p (hook primordia) as cytoplasmic strands and c y t (cytoplasm of tegument). Surrounding this central core is c m (circular musculature primordium). At the base of the invagination are n h p (nuclei in hook primordia) and additional n c m. Below the central core lies a large cluster of cells labeled c g (cerebral ganglion primordium), connected by n e (nerve) fibers. The p r (proboscis retractor primordium) and r e (proboscis receptacle primordium) form the inner sheath around the ganglion. Moving to the outer layers, s m (subtegumental musculature) and t e g (tegument) form the body wall. Large g n (giant nuclei) are embedded in the lateral tegument. Isolated m c (muscle cells) and n p r (nuclei in proboscis retractor) are scattered within the internal cavity.
Structure of acanthella tegument
Middle acanthellae I in semithin sections had an elongated shape and dimensions of 0.9–1.3 × 0.2–0.3 mm. The tegument enveloping the acanthella was ca. 30–40 μm thick over most of its surface; thickness decreased to 10–17 μm at the anterior and posterior poles of the body (Figures 3a, 4a). The superficial, striated layer (ca. 2 μm) was clearly differentiated in the tegument. It was formed by invaginates of the outer cytoplasmic membrane, with expanded terminal segments up to 1 μm in diameter (Figure 3c, d). The cytoplasm under this layer was distinguished by increased electron density and contained giant nuclei arranged in transverse rows along the longitudinal axis of the acanthella body (Figure 1). The giant nuclei (48–63 × 17–24 μm) had an uneven outline, evenly distributed heterochromatin in the nucleoplasm, and the presence of one or two large nucleoli (Figure 3c). In addition to nuclei, the tegument cytoplasm was rich in mitochondria and contained diffusely arranged fibres and lipid droplets that often formed large clusters. Three apical nuclei were located in the tegument of the anterior region of the acanthella (Figures 3a, 4a, b). These differed morphologically from the giant nuclei by their smaller sizes and the higher electron density of their nucleoplasm. These nuclei were closely adjacent to each other, making their boundaries difficult to distinguish (Figure 4a). On the acanthellae surface, a forming cyst (ca. 4.3–7.6 μm) was clearly visible, which consisted of small vesicles and elongated fragments of microvilli detaching from the tegument (Figure 3d). Two layers of muscles extended under the tegument along the entire length of the acanthocephalan body, except the apical region.
Structure of praesoma in A. teres. ( a ) Semithin section through the anterior region of the acanthocephalan praesoma. ( b ) Semithin section through the cerebral ganglion located at the base of the proboscis receptacle. ( c ) Ultrathin section through a fragment of acanthocephalan tegument with a giant cell located in its cytoplasm. ( d ) Ultrathin section through a fragment of acanthocephalan tegument with a forming cyst that was visible on the surface. Letter designations: an – apical nuclei; сg – cerebral ganglion primordium; cm – circular musculature primordium; cy – cyst; cyt – cytoplasm of tegument; hp – hook primordium; ld – lipid droplets; mi – microvilli; n – nucleus; nu – nucleolus; ncm – nuclei in circular musculature primordium; nhp – nuclei in hook primordia; pr – proboscis retractor primordium; re – proboscis receptacle primordium; sl – striated layer of tegument; teg – tegument of acanthella; ve – vesicles.
Figure 3. Long description
A four-panel figure labeled a through d.
Panel a: A semithin section of the anterior praesoma. From the outer perimeter inward, it shows the tegument (teg) containing apical nuclei (an). Moving deeper into the central cavity, there are hook primordia (hp) with associated nuclei (nhp), surrounded by the cytoplasm of the tegument (cyt). The circular musculature primordium (cm) and its nuclei (ncm) are visible alongside the proboscis retractor primordium (pr). Scale bar is 80 micrometers.
Panel b: A semithin section of the cerebral ganglion (cg) located between the proboscis receptacle primordium (re). The ganglion appears as a cluster of large cells with distinct nuclei. Scale bar is 45 micrometers.
Panel c: An ultrathin section of the tegument. At the top is a cyst (cy) above a striated layer (sl). Below this, a large nucleus (n) containing a dark nucleolus (nu) is embedded in the cytoplasm. Dark, circular lipid droplets (ld) are scattered in the lower left. Scale bar is 20 micrometers.
Panel d: A high-magnification ultrathin section. The top half shows a cyst (cy) containing small vesicles (ve). A boundary of microvilli (mi) separates the cyst from the lower half, which contains the striated layer (sl) and the dense tegument (teg) of the acanthella. Scale bar is 2 micrometers.
Ultrastructure of praesoma in A. teres. ( a ) Anterior region of the forming proboscis; see the tegumental cytoplasm, where the hook primordia are embedded, forming an outgrowth protruding into the acanthocephalan’s body cavity. ( b ) Anterior part of acanthocephalan proboscis at a higher magnification. ( c ) Morphology of the two zones identified in the structure of hook primordia. ( d ) Contacts between the surface of the hook primordium and adjacent tegumental cytoplasm. Letter designations: an – apical nuclei; cm – circular musculature primordium; cp – central part of hook; cy – cyst; cyt – cytoplasm of tegument; ger – granular endoplasmic reticulum; hp – hook primordia in the form of a long cytoplasmic strand; ld – lipid droplets; m – mitochondria; pp – peripheral part of hook; pr – proboscis retractor primordium; teg – tegument of acanthella.
Figure 4. Long description
A four-panel T E M micrograph series labeled a through d.
Panel a, top-left, shows the anterior region with a 20 micrometer scale bar. From the top-left corner moving inward, the layers include the cyst cy, apical nuclei an, and the tegument of acanthella teg. Embedded within the tegumental cytoplasm cyt are multiple elongated hook primordia h p. Below these are the circular musculature primordium c m and proboscis retractor primordium p r.
Panel b, bottom-left, provides a closer view with a 10 micrometer scale bar. It shows apical nuclei an and lipid droplets l d on the left. The center contains several h p structures surrounded by tegumental cytoplasm cyt, with c m and p r visible on the right.
Panel c, top-right, shows hook primordia morphology with a 5 micrometer scale bar. It identifies the central part of the hook c p and the peripheral part p p. Mitochondria m are visible within the structure. The surrounding area includes c m, p r, and cyt.
Panel d, bottom-right, is a high-magnification view with a 2 micrometer scale bar. White arrows point to the contact points between the hook primordium surface and the adjacent tegumental cytoplasm cyt. The internal structure shows granular endoplasmic reticulum g e r and mitochondria m.
Organisation of acanthocephalan praesoma
The proboscis receptacle was located in the praesoma in the form of a bulky sac formed by two layers of muscles (Figure 1). An elongated primordium of the proboscis was found in the anterior region of the proboscis receptacle, and a cerebral ganglion was located in its posterior section (Figures 1, 3a, b). The base of the proboscis was formed by the tegument cytoplasm located at the apical end of the acanthella (Figures 1, 3a, 4a, b). This cytoplasm formed an elongated outgrowth protruding into the body cavity (Figures 1, 3a, 4a, b, 5a, b). The contents of this outgrowth included hook primordia that appeared as long cytoplasmic strands forming vertical rows (Figures 1, 4a). Fragments of hooks were also visible at the base of the proboscis under the tegument, where they were slightly embedded within it (Figures 3a, 4a, b). In ultrathin sections, hook primordia appeared as electron-lucent, dumbbell-shaped structures adjacent to the more electron-dense cytoplasm of the tegument (Figure 4c). The primordial hook cytoplasm was unevenly distributed and had a rich set of organelles. In ultrathin sections, two zones were distinguished in the hook primordia (Figure 4c). The central zone was lighter and devoid of organelles, while the peripheral area surrounded the central zone and included focal aggregations of mitochondria, tubules of the granular endoplasmic reticulum that were often oriented parallel to each other, lipid droplets, and vesicles of different sizes. In the hooks, single fibrils were also found that were diffusely distributed and did not form aggregations. In the zone of contact between the hook surface and the tegument cytoplasm, short contacts were consistently found that were morphologically similar to ‘tight junctions’ (Figure 4d). It should be noted that in all the acanthellae examined, four nuclei were localised in the hook primordia located at the apex of the proboscis (Figures 1, 5b).
Ultrastructure of praesoma in acanthella of A. teres. ( a ) Posterior part of forming proboscis. ( b ) Apex of proboscis at a higher magnification, with four nuclei clearly visible in the cytoplasm of hook primordia and two nuclei in the cytoplasm of circular musculature primordium. ( c ) Presumptive muscle cell in the anterior part of the acanthocephalan’s body. ( d ) Cerebral ganglion. Letter designations: cm – circular musculature primordium; cyt – cytoplasm of tegument; ger – granular endoplasmic reticulum; hp – hook primordium; ld – lipid droplets; m – mitochondria; n – nuclei; ne – nerve; nu – nucleolus; ncm – nuclei in circular musculature primordium; nhp – nuclei in hook primordia; pr – proboscis retractor primordium; re – receptacle of proboscis primordium; teg – tegument of acanthella.
Figure 5. Long description
A four-panel T E M micrograph series labeled a through d.
* Panel a: Shows the posterior part of the forming proboscis. Labels include c y t (cytoplasm of tegument) at the top, h p (hook primordia) in the center, and c m (circular musculature primordium) surrounding them. Below these are n h p (nuclei in hook primordia) and n c m (nuclei in circular musculature primordium). The outer layers are marked r e (receptacle of proboscis primordium) and n e (nerve). Scale bar is 20 micrometers.
* Panel b: A higher magnification of the proboscis apex. It highlights several n h p (nuclei in hook primordia) clustered in the center, surrounded by c y t. The outer boundary shows c m (circular musculature primordium) and p r (proboscis retractor primordium). Scale bar is 10 micrometers.
* Panel c: Displays a presumptive muscle cell. At the top are m (mitochondria) and l d (lipid droplets). The center contains a large n (nucleus) with a dark n u (nucleolus). The surrounding area includes g e r (granular endoplasmic reticulum) and t e g (tegument of acanthella). Scale bar is 10 micrometers.
* Panel d: Shows the cerebral ganglion, a large cluster of cells with prominent n (nuclei) containing dark nucleoli, organized in a dense mass. Scale bar is 10 micrometers.
A muscle layer, which represented the primordium of circular musculature of the praesoma, was closely adjacent to the lateral surface of the proboscis primordium (Figures 1, 3a, 4a, b, 5a, b). An analysis of serial sections showed that this primordium originated in the subterminal region of the parasite’s anterior end, beneath the tegument, and formed a sheath that completely encased the developing proboscis (Figure 1). In the electron-dense cytoplasm of these muscles, six nuclei were observed, characterised by their constant localisation. Of these, two nuclei were invariably found at the apex of the proboscis, and four were located at the base of the proboscis in all the studied acanthellae (Figures 1, 5a, b). These four nuclei were localised in pairs at different depths within the parasite’s body.
An additional muscle layer, a proboscis retractor, lay external to the circular musculature surrounding the proboscis (Figures 1, 3a, 4a, b, 5a, b). This retractor formed a hollow tube that extended from the base of the proboscis along the surface of the circular musculature, curved around the cerebral ganglion, and attached to the muscles in the lower part of the proboscis receptacle. Unlike the circular muscles, the proboscis retractor was characterised by electron-lucent cytoplasm and contained four large nuclei, along with numerous mitochondria and myofilaments.
Below the tegument, a single cell measuring 24–25 × 8–10 μm was found between the hook primordia and the subtegumental muscles (Figure 5c). The nucleus of this cell was characterised by a uniformly distributed and finely dispersed nucleoplasm and a large, rounded nucleolus. A synthetic apparatus, represented by tubules of granular endoplasmic reticulum (GER) that were often in a dilated state and contained a material of moderate electron density, was developed in the cytoplasm. Presumably, this cell was functionally associated with the subtegumental muscle system. This was evidenced by its localisation and the composition of organoids, such as the GER cisternae.
The cerebral ganglion had a typical structure and consisted of peripherally located nerve cells and a neuropil formed by the processes of these cells (Figure 5d). The nerve cell processes extended from the cerebral ganglion in two directions. One nerve ran anteriorly towards the proboscis and adjoined the inner surface of the proboscis receptacle approximately at the level of the proboscis apex (Figures 1, 5a). The other nerve was directed posteriorly to the base of the proboscis receptacle (Figure 1). The nerve cells were rounded in shape and up to 10–12 μm in diameter. Cytoplasm in most of these cells contained numerous mitochondria, vesicles with electron-lucent contents, and lipid droplets. Aggregations of electron-dense secretions and neurofibrils were observed in the processes of the cells. Most nerve cells of the ganglion were mononuclear, and only two, which were facing the wall of the posterior region of the proboscis receptacle, contained two nuclei each.
Discussion
Our results revealed that the A. teres specimens retrieved from naturally infected intermediate hosts at the middle acanthella I stage already possessed a well-defined proboscis primordium within the forming proboscis receptacle. In addition, the primordia of the praesomal circular muscles, the proboscis retractor, and the cerebral ganglion localised at the base of the proboscis receptacle were differentiated at the same stage of development.
According to existing evidence, the initial stage of proboscis formation in most representatives of the class Palaeacanthocephala is characterised by its initially inverted position (Awachie Reference Awachie1966; DeGiusti Reference DeGiusti1949; Hynes and Nicholas Reference Hynes and Nicholas1957). In some acanthocephalan species, proboscis development is associated with the emergence of vertical cytoplasmic rows at the anterior end of the body, referred to as uncinogenous bands (Amin Reference Amin1982; Hehn et al. Reference Hehn, Ehlers and Herlyn2001; Olson and Pratt Reference Olson and Pratt1971). These serve as a basis for the subsequent development of hook primordia. These strands are clearly shown in the drawings of 18-day-old acanthella of Echinorhynchus lageniformis (Olson and Pratt Reference Olson and Pratt1971), 22-day-old Leptorhynchoides thecatus (DeGiusti Reference DeGiusti1949), and 84-day-old acanthella of Acanthocephalus dirus (A. parksidei sensu Amin, Reference Amin1982). Apparently, the anterior region of the body of a developing acanthocephalan, Polymorphus minutus, also has a group of uncinogenous bands that form a ‘balloon-like’ ridged structure (Hynes and Nicholas Reference Hynes and Nicholas1957). This assumption is confirmed by the structure of an 84-day-old acanthella of P. minutus (Butterworth Reference Butterworth1969), in which a drawing depicts in detail the strands with alternating thicker and narrower segments. In A. teres, as in other palaeacanthocephalan species, the forming proboscis has the shape of an elongated cylinder formed by vertically oriented cytoplasmic strands. These strands, like those in P. minutus, have a corrugated structure and are embedded in the tegument cytoplasm, part of which extends into the body cavity.
The subsequent proboscis development in palaeacanthocephalans has been described for several species (Awachie Reference Awachie1966; DeGiusti Reference DeGiusti1949; Olson and Pratt Reference Olson and Pratt1971). It has been shown that the position of the proboscis changes in two stages during acanthellae development (Amin Reference Amin1982). During the first stage, the initially inverted proboscis begins to evaginate. The stages of the proboscis evagination process in acanthocephalans are well illustrated in the diagram published by Hamann (Reference Hamann1891). During evagination, the uncinogenous bands turn outward and form tubercles under the tegument, which are subsequently transformed into proboscis hooks (Olson and Pratt Reference Olson and Pratt1971). They grow towards the tegument surface, gradually penetrating it (DeGiusti Reference DeGiusti1949). The second stage is characterised by invagination of the proboscis. According to DeGiusti (Reference DeGiusti1949), the proboscis in Leptorhynchoides thecatus is retracted after its hooks are completely formed. However, Hynes and Nicholas (Reference Hynes and Nicholas1957) note that hook morphogenesis can continue even after proboscis inversion.
According to published data, the circular musculature in the praesoma of adult acanthocephalans is mainly located beneath the tegument, whereas the metasoma contains two layers of subtegumentary musculature: circular and longitudinal (Herlyn and Ehlers Reference Herlyn and Ehlers2001; Herlyn and Taraschewski Reference Herlyn and Taraschewski2017; Miller and Dunagan Reference Miller, Dunagan, Crompton and Nickol1985; Nikishin Reference Nikishin2005). Praesoma of acanthocephalans from different classes typically show similarity both in the number of nuclei of the circular musculature and in their localisation (Herlyn Reference Herlyn2002). It has been found that in the acanthocephalans Paratenuisentis ambiguous (Eoacanthocephala), Acanthocephalus anguillae (Palaeacanthocephala), and Macracanthorhynchus hirudinaceus (Archiacanthocephala), the circular musculature of the praesoma usually contains six nuclei (Herlyn Reference Herlyn2002; Herlyn and Ehlers Reference Herlyn and Ehlers2001; Herlyn et al. Reference Herlyn, Martini and Ehlers2001). Of these, two are located in the apical part of the proboscis and four on the dorsal and ventral surfaces of the posterior half. An exception has been recorded in two of the four examined A. anguillae males, in which four nuclei were found in the circular musculature (Herlyn and Ehlers Reference Herlyn and Ehlers2001).
In this study, we have found that the developing proboscis of Arhythmorhynchus teres is surrounded by a dense muscular layer that forms a sheath. We assume this layer to be a primordium of the praesomal circular musculature, supported by the following facts: first, as noted above, during the period of middle acanthella development, the initially inverted proboscis begins to gradually evaginate. During this process, the muscle layer appears to move together with the proboscis and lies beneath the tegument in the area where the hook primordia are located. Second, three pairs of nuclei have been found in the cytoplasm layer of the circular musculature in A. teres, as well as in the circular musculature of the praesoma in other acanthocephalan species (Herlyn Reference Herlyn2002). These nuclei are located in strictly defined areas of the circular musculature: one pair of nuclei at the apex of the proboscis, and the other two pairs at its base.
As is well known, the proboscis retractor in adult acanthocephalans is an unpaired structure. In most species, it is formed by two muscular longitudinal tubes that merge inside the proboscis apex (Herlyn et al. Reference Herlyn, Martini and Ehlers2001; Herlyn and Taraschewski Reference Herlyn and Taraschewski2017). The outer tube of the retractor is characterised by a subtegumental localisation and extends posteriorly from the proboscis apex to the proboscis receptacle. The muscle fibres of the inner tube extend posteriorly from the apical end of the proboscis, cross the entire cavity of the praesoma body, and then enter the receptacle posteriorly. In the A. teres specimens examined here, the proboscis retractor formed a hollow tube that originated at the base of the inverted proboscis, surrounded it from the outside, passed through the cavity of the proboscis receptacle, and attached to the muscles at the receptacle base. Unlike the proboscis retractor in adult acanthocephalans, the proboscis retractor tube in A. teres is formed by a single wall. It can be assumed that, during the further post-embryonic development of acanthocephalans, the proboscis retractor splits into two tubes that connect at the proboscis apex. However, this assumption needs to be verified at subsequent developmental stages.
In several acanthocephalan species, a noteworthy feature of the structure of the developing proboscis is a group of nuclei at its apex. In particular, the acanthocephalan Echinorhynchus lageniformis has eight nuclei located at the apical end of the proboscis (Olson and Pratt Reference Olson and Pratt1971). These nuclei apparently correspond to the nuclear ring described from the acanthocephalan Paulisentis fractus (Cable and Dill Reference Cable and Dill1967). As is known, during proboscis eversion, these nuclei move anteriorly towards the apical end of the acanthella. However, data on their subsequent changes at later stages of proboscis development are scarce (Hynes and Nicholas, Reference Hynes and Nicholas1957; Olson and Pratt, Reference Olson and Pratt1971). According to Hamann (Reference Hamann1891), a group of cells rather than nuclei are located near the tip of the inverted proboscis. Some of these cells give rise to the proboscis retractors, while others remain at the proboscis apex and shift towards the anterior end of the body during evagination. Similar results were obtained for the archiacanthocephalan Mediorhynchus grandis (Moore Reference Moore1962). Of the group of nuclei localised at the proboscis apex in this acanthocephalan, some are also involved in the formation of the proboscis retractors, while the remaining six nuclei, whose function has not been identified, are included in the so-called ‘apical ring’. Our electron microscopic examinations have revealed six nuclei located at the proboscis apex in the acanthocephalan A. teres. As noted above, two of these nuclei are involved in the formation of the praesomal circular musculature. The function of the other four nuclei remains unknown, as they are located in the cytoplasm of the hook primordia.
Most studies of the fine structure and chemical composition of acanthocephalan hooks were conducted on juvenile or adult specimens. In some species, the distribution patterns of chemical elements in different parts of the hooks were determined by energy-dispersive X-ray analysis (EDXA) (Amin et al. Reference Amin, Caracciolo, Wendt, Rubtsova and de Souza2025a; Heckmann et al. Reference Heckmann, Amin, Radwan, Standing and Eggett2012a, Reference Heckmann, Amin, Radwan, Standing, Eggett and El Naggarb). For example, the dynamics of calcium, phosphorus, and sulphur content in hooks throughout development from the cystacanth to the juvenile and adult stages were assessed in Moniliformis kalahariensis (Amin et al. Reference Amin, Heckmann and Rubtsova2025b). Several studies also conducted ultrastructural examination of hooks in acanthocephalans, including members of the class Palaeacanthocephala (Brázová et al. Reference Brázová, Poddubnaya, Miss and Hanzelová2014; Heckmann et al. Reference Heckmann, Amin, Radwan, Standing, Eggett and El Naggar2012b; Taraschewski Reference Taraschewski2000). In A. anguillae, longitudinal sections through hooks have distinguished three zones: fibroretinacular, lamina densa, and lamina lucida (Herlyn and Ehlers Reference Herlyn and Ehlers2001). The fibroretinacular zone, composed of fibrous elements, constitutes the major part of the hook. Its peripheral part is bound by the thin, electron-dense lamina densa zone. Outside this, the faint, electron-lucent lamina lucida zone is found. All three layers are covered with a modified thin layer of the acanthocephalan’s tegument. The hooks in Acanthocephalus lucii – another species of palaeacanthocephalan – are arranged in a similar pattern (Brázová et al. Reference Brázová, Poddubnaya, Miss and Hanzelová2014). Their inner layer likely corresponds to the fibroretinacular zone in the hooks of Acanthocephalus anguillae, and the outer layer corresponds to the thin layer of modified tegument. It should be noted that the tegument layer (‘epidermal covering’ or ‘cortical layer’ according to the terminology by other researchers), covering the central part of the hooks, is also characteristic of the class Eoacanthocephala, whereas members of the class Archiacanthocephala lack such a layer on the surface of their hooks (Taraschewski Reference Taraschewski2000; Herlyn and Ehlers Reference Herlyn and Ehlers2001; Herlyn et al. Reference Herlyn, Martini and Ehlers2001; Amin and Heckmann Reference Amin and Heckmann2022).
To date, there is only limited information on the morphogenesis of acanthocephalan hooks (Nikishin Reference Nikishin2004; Hutton and Oetinger Reference Hutton and Oetinger1980). Complete data on hook formation are available only for the archiacanthocephalan Moniliformis moniliformis, whose developmental stages were documented by experimental infection of cockroaches, Periplaneta americana (Hutton and Oetinger Reference Hutton and Oetinger1980). As the authors noted, the hook primordium in three of the four identified acanthella stages was characterised by a rich set of organelles evenly distributed in the cytoplasm, including mitochondria, GER, and vesicles. The authors emphasised that these organelles provide the synthesis of fibrillar material for the future hook. Similar to the above data, the forming hooks in the acanthellae of A. teres exhibited a substantial number of organelles; however, they were unevenly distributed in the cytoplasm. We identified two zones in their structure: the central zone, which lacked organelles, and the peripheral zone, which contained mitochondria, GER, vesicles of various sizes, and lipid droplets.
Overall, the results of this study shed light on the morphology of the praesoma in developing acanthocephalans at the middle acanthella I stage. Our future research is aimed at studying praesoma morphology in acanthocephalans at subsequent developmental stages, up to the cystacanth.
Acknowledgements
The author expresses sincere gratitude to G.I. Atrashkevich (Institute of Biological Problems of the North FEB RAS) for his assistance in identifying the larval stages of the acanthocephalans examined. Special thanks are due to the reviewers for their detailed analysis and constructive comments.
Financial support
Not applicable.
Competing interests
The author declares no competing interests.
Ethical standards
Not applicable.
