Intraspecific variability of morphological characters in the species-rich deep-sea genus Acantholaimus Allgén, 1933 (Nematoda: Chromadoridae)

in Nematology
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Acantholaimus is a species-rich genus of deep-sea nematodes, often with dozens of species found at the same locality but each represented by single or few individuals. Species discrimination by morphological characters in this genus is therefore often difficult due to transitional forms that may be referred to several species because of lack of data on intraspecific variability. The aim of this study was to evaluate the intraspecific variability of morphological characters that are most often used in Acantholaimus taxonomy, in order to distinguish those which are most informative for species differentiation. A reverse taxonomic approach was applied for initial species discrimination. Two loci, one each from small and large subunits of rRNA, were sequenced for 59 Acantholaimus specimens from two deep-sea locations. Twenty-seven Molecular Operational Taxonomic Units (MOTU) were identified, of which 12 were represented by more than one individual. These were then analysed for intraspecific variability in morphological characters. Some of the examined characters showed high intraspecific variability; specifically: length of cephalic setae; distance from anterior end to amphid; shape of anterior setae; position and arrangement of cervical setae. In the absence of genetic data, these characters should be used with caution for differential diagnoses or species discrimination. Other characters were more conservative within the same MOTU: body proportions; length of outer labial setae; amphidial diam.; appearance of lateral field; general arrangement of cervical setae; and shape of tail. These characters may be successfully used for species discrimination in the absence of molecular data.

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References
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Figures
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    Illustrated explanation of ranked states of morphological characters: S1, appearance of cuticle on lateral fields in mid-body region (A-C); S2, arrangement of cervical setae (D-F); S3, position of anterior-most cervical setae (G-J); and S4, tail shape (K, L). A: Lateral differentiation of cuticle absent; B: Lateral differentiation of larger dots arranged irregularly; C: Lateral differentiation of larger dots arranged in longitudinal rows; D: Cervical setae no denser than somatic setae and not forming groups anteriorly; E: Cervical setae not denser than somatic setae but with groups of setae formed anteriorly; F: Cervical setae denser than somatic setae, with slight segregation of anterior setae groups; G: Anteriormost cervical setae located far behind amphid; H: Anteriormost cervical setae located behind amphid; I: Anterior-most cervical setae located level with posterior border of amphid; J: Anteriormost cervical setae located anterior to posterior border of amphid; K: Tail conical with distinct flagelliform part distally; L: Tail with proximal conical section elongate, smoothly transitioning to flagelliform distal section.

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    D2-D3 locus of 28S rRNA. Neighbour-joining tree of relationships of Acantholaimus specimens based on P-distances computed by the Kimura 2-parameter method. Bootstrap values (the percentage of trees (1000 replicates) in which the clusters occurred) are indicated at the branch points. Individuals marked with an asterisk were lost or in very bad condition, and therefore they were not examined morphologically.

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    V1-V2 locus of the 18S subunit of rRNA. Neighbour-joining tree of relationships of Acantholaimus individuals based on P-distances computed using the Kimura 2-parameter method. Bootstrap values (the percentage of trees (1000 replicates) in which the clusters occurred) are indicated at the branch points. The individual M137 (marked with an asterisk) was in very bad condition, and therefore was not examined morphologically.

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    Comparison of intra- and inter-specific variability of cephalic morphology in Acantholaimus specimens. MOTU-1 and MOTU-2. For MOTU-1, note different shape of amphid (round or oval), different position of anteriormost cervical setae relative to amphid. For MOTU-2, note different distance from anterior end to amphid, more numerous cervical setae for K20, and jointed cephalic setae in K27 (vs unjointed in K20). Note also, that heads of specimens from these two MOTU look quite similar.

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    Intraspecific variability of cephalic morphology in specimens of Acantholaimus MOTU-5. Note great difference in length and appearance of cephalic setae and in number and arrangement of cervical setae and cuticular pores in sublateral rows.

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    Comparison of intra- and inter-specific variability of cephalic morphology in Acantholaimus specimens. MOTU-7 and MOTU-9. Note different length of cephalic setae in MOTU-7 and different number of cervical setae in anteriormost groups (2 + 1 or 2 + 0) and the shape of amphids (approximately round or transverse oval) in MOTU-9.

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    Comparison of intraspecific variability of cephalic morphology in Acantholaimus specimens. MOTU-11 and MOTU-16. Note different length of cephalic setae in MOTU-11 and different position and arrangement of anteriormost sets of cervical setae in both MOTU.

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    Comparison of intra- and inter-specific variability of cephalic morphology in Acantholaimus specimens. MOTU-17 and MOTU-20. Note eversion of the lips in K22 (MOTU-17), and retraction of the lips and buccal cavity in K88 (MOTU-17), K1 and M135 (MOTU-20). Cephalic setae are jointed or unjointed in MOTU-17. The position and arrangement of anteriormost cervical setae vary greatly in MOTU-20.

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    Comparison of intraspecific variability of cephalic morphology in Acantholaimus. MOTU-21 and MOTU-27. Note difference in position of cervical setae in MOTU-21 and in length and shape of cephalic setae in MOTU-27.

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    Scatterplot showing correlation between body length without tail (M1) and distance from anterior end to amphid (M8) in three Acantholaimus species with greatest variation in the latter character.

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