Redescription of the dasydytid gastrotrich Haltidytesooëides (Brunson, 1950) based on type material

Abstract The semi-pelagic gastrotrich species Haltidytesooëides (Brunson, 1950) is redescribed based on original type material deposited at the Smithsonian National Museum of Natural History. Herein we present a new diagnosis and figures of the species, detailing the insertion position of the lateral spines, misinterpreted in the original description. Furthermore, we reassess the taxonomic key for the genus Haltidytes Remane, 1936 based on our new findings.


Introduction
While most gastrotrichs are epibenthic, periphytic, or interstitial, some species belonging to the family Dasydytidae Daday, 1905 present a semi-pelagic lifestyle (Kieneke et al. 2008, Balsamo et al. 2014, Kånneby and Todaro 2015. Seven genera are currently assigned to Dasydytidae, including the genus Haltidytes Remane, 1936 recently found as monophyletic (Minowa and Garraffoni 2017). Haltidytes was originally established as a subgenus of Dasydytes Gosse, 1851 by Remane (1936), who then elevated it to a genus rank (Remane, 1967). Currently, the genus Haltidytes contains six valid species (Minowa and Garraffoni 2017): H. festinans (Voigt, 1909) (type species), H. crassus (Greuter, 1917), H. ooëides (Brunson, 1950), H. saltitans (Stokes, 1887), H. squamosus Kisielewski, 1991, andH. pseudosquamosus Minowa & While preparing a forthcoming study incorporating phylogenetic analyses of all valid Dasydytidae species based on morphology (Minowa and Garraffoni, in preparation), we came across the possible type specimen of Haltidytes ooëides (Brunson, 1950), originally described as Dasydytes ooëides (USNM W 26869S). Although Brunson (1950) had not designated any type specimen, the locality and sampling date (Michigan State, Washtenaw County, Half-Moon Lake;May, 30, 1944) registered in the Smithsonian Data Base are the same as those reported in the Brunson's study. It came as a surprise to us that after more than 75 years the specimen is still preserved. Due to small size and fragile bodies, fixed specimens of gastrotrichs usually have their diagnostic morphological characters deteriorated after fixation (e.g. Balsamo et al. 2014, Kånneby 2016, Garraffoni and Freitas 2017. It is interesting to highlight that we also found the possible type specimen of Stylochaeta scirtetica Brunson, 1950 (USNM W 26870), but in this case, the material is in a poor condition and could not be used for a reanalysis.

Materials and methods
Herein we redescribe Haltidytes ooëides based upon a single type material deposited at the Smithsonian National Museum of Natural History. External morphology was observed using an Olympus BX63F compound fluorescence microscope with a digital DP80 camera and cellSens software (Olympus, Philadelphia, USA). Videos were prepared using the open-source platform Fiji (Schindelin et al. 2012). The necessity for a re-examination was caused by the shallow description by Brunson (1950), who only briefly reported and illustrated a few morphological features of the new species. This need was further noted by Balsamo et al. (2014), suggesting a misinterpretation of the insertion positions of the lateral spines. In the redescription of the species, the groups of spines are coded according to Kisielewski (1991).
Trunk locomotory ciliation divided into 2-paired ventral tufts at 15U and 93U on the ventral side of at the neck and posterior trunk, respectively ( Figure 2C, D). No dorsal sensory bristles were observed.
Remarks. Usually, the trunk width is given as the maximum trunk widthwhich is at the midgut level. In this case, the type specimen H. ooëides is 42 μm wide. However, Brunson (1950) measured the trunk width posterior to the midgut level (close to the posterior end of the body) and found it to be 36 μm wide.
Differences in spines length between the original description and the present one (Table 1) are due to different measurement methods. We chose to measure each spine length outlining its curvature (100,90,75,90,82,80 μm respectively) instead of  (Brunson, 1950): measures are expressed in μm; the relative positions of morphological structures along the body are expressed in percentage unities (U) in relation to the total body length. measuring the distance between the spine base insertion and apex as a straight line, as Brunson (1950) did (86, 86, 67, 82, 82 and 58 μm, respectively). Additionally, the original description mentions a pair of caudal bristles ( Figure 2C) that originate 10 μm from the posterior end of the trunk. After reexamination of the type specimen (Figure 2A, B-D) we conclude that Brunson (1950) may have misinterpreted these structures. In fact, our observations revealed that the caudal bristles described by Brunson (1950) actually are the ta1 spines, due to their similar position relative to the posterior trunk, size and shape (Figure 2A, B).
As previously mentioned, the description of some morphological characters of H. ooëides were misinterpreted by Brunson (1950) and incorrectly replicated by Balsamo et al. (2014) and Minowa and Garraffoni (2017). We address this issue by correcting the taxonomic key Haltidytes in order to correct previous misinterpretations.