Ornamentula Kisielewski, 1991 is a monospecific genus in Order Chaetonotida. The sole species, O. paraensis Kisielewsk, 1991, is a semiplanktonic gastrotrich found in a single pond in the Amazon region of Brazil. Herein we describe a new species of the genus Ornamentula, collected in a small urban lagoon in the Atlantic Forest of southeast Brazil. Ornamentula miyazakii sp. nov. resembles O. paraensis, but it shows differences in the ornamented trunk scales and spinal spines distribution, sufficient to proposte it as it’s a new species.

Atlantic rainforest, semiplanktonic gastrotrich, taxonomy, urban lagoon

Gastrotricha are free-living microinvertebrates found in benthic and epiphytic as well as periphytic biotopes in marine and freshwater habitats around the world (Balsamo et al. 2008, 2014, Kieneke and Schmidt-Rhaesa 2015, Todaro et al. 2019). The phylum is divided in two orders (Macrodasyida Remane, 1925, Chaetonotida Remane 1925), 18 families, 69 genera and more than 860 species (Balsamo et al. 2014, 2015, Garraffoni et al. 2017, Todaro et al. 2019).

Although genera such as Chaetonotus Ehrenberg, 1830, Turbanella Schultze, 1853, Aspidiophorus Voigt, 1903, Tetranchyroderma Remane, 1926, and Macrodasys Remane, 1924 are considered highly specious with dozens of species, other genera, e.g. Dendrodasys Wilke, 1954, Crasiella Clausen, 1968, Diuronotus Todaro et al., 2005, Bifidochaetus Kolicka & Kisielewski, 2016, have very few species and some even consist of a single species, e.g. Ornamentula Kisielewski, 1991, Hummondasys Todaro et al., 2014, Thaidasys Todaro et al., 2015 and Cephalionotus Garraffoni et al., 2017.

Ornamentula is a monotypic genus belonging to Dasydytidae Daday, 1905 (Chaetonotida). It was originally described by Kisielewski (1991) to accommodate the new species Ornamentula paraensis Kisielewsk, 1991, collected in a small pond from the Brazilian Amazon rainforest. Originally, Kisielewski (1991) spelled the species name as Ornamentula paraënsis, with a diacritic. However, the ICZN – Art. 32.5.2. states that: “A name published with a diacritic or other mark, ligature, apostrophe, or hyphen, or a species-group name published as separate words of which any is an abbreviation, is to be corrected.

Although most gastrotrich species are meiobenthic, O. paraensis has a semiplanktonic lifestyle, like few other gastrotrichs (Balsamo et al. 2014, Kieneke and Schmidt-Rhaesa 2015, Todaro et al. 2019, Minowa and Garraffoni 2017, 2020). These species show various morphological adaptations such as the complete loss of posterior adhesive tubes, a system of ciliary bands in the head region and short diagonal ciliary bands or tufts on the ventral trunk, and lateral motile spines to perform saltatory movements in the water column (Kisielewski 1991, Kieneke et al. 2008, Kieneke and Ostmann 2012).

Species of Ornamentula are characterized by a type of well-developed cuticular armature, unique among all Gastrotricha, that consists of a complex reticular ornamentation on the scales associated with thick and long spines (Kisielewski 1991). Kisielewski (1991) highlighted a possible proximity of Ornamentula’s lineage with those of Setopus and Haltidytes based on the presence of scales and bifurcated spines. This hypothesis was also accepted by Kånneby et al. (2013) and Kånneby and Todaro (2015) based on a molecular analysis based on 3 genes, but it was not confirmed by studies based on morphological data (Kieneke et al. 2008, Kieneke and Ostmann 2012).

So far, our knowledge on the biology, life cycle and distribution of many semiplanktonic gastrotrichs is very limited, probably due to a lack of specialists, but especially to the rareness of Dasydytidae specimens and their difficult preparation and handling (Kieneke et al. 2008, Kieneke and Ostmann 2012). In order to contribute to the poor knowledge on semiplanktonic gastrotrichs in the Neotropical region (Kisielewski 1991, Kånneby et al. 2013, Kånneby and Todaro 2015, Minowa and Garraffoni 2017, 2020), herein we describe the second species of the genus Ornamentula found in an urban lagoon surrounded by Atlantic rainforest, in Southeast Brazil.

The material for the research was collected in a small urban lagoon, São Paulo, Brazil, with an area of 0.05 km² (22°79’S; 47°14’W), surrounded by fragments of Atlantic rainforest. The collection was carried out during an ongoing freshwater Gastrotricha sampling project, in which periodic biweekly collections have been carried out since 2017. The organisms were found in the samples from October 2017, from the upper 30 cm of the water surface, among floating vegetation roots, using 5 L buckets. The material was stored with constant aeration, with a temperature around 20 °C, and processed within seven days.

The sorting was carried out according to the protocol reported by Minowa and Garraffoni (2020) using 500 ml water samples sieved through a 30 µm mesh, poured into Petri dishes, and observed under a Zeiss Stemi 2000 stereomicroscope, focusing on the water column searching for of semiplanktonic gastrotrichs.

Each individual was isolated, anesthetized with 2% MgCl₂, mounted individually and digitally documented under a Zeiss Axio Imager M2 light microscope equipped with DIC and AxioCam MRC5 digital video camera. The images were recorded and measurements were taken using the ZEN lite 2.5 2018 image software.

It is well known in the literature how difficult the identification process is for meiofauna species (Fonseca et al. 2018, Garraffoni et al. 2019) and how remarkably poor the biodiversity knowledge is of these microscopic organisms compared to other groups of animals (Appeltans et al. 2012, Fonseca et al. 2018). Consequently, accurate diagnosis and meiofaunal species delimitation are difficult to access, leading to shortfalls (sensu Hortal et al. 2015) regarding identity, distribution and evolution of meiofaunal taxa.

As pointed out by Fonseca et al. (2018), to overcome these shortfalls it is highly recommended that meiofaunologists use the available microscopical techniques (optical, scanning, transmission, and confocal microscopy) as much as possible to better describe the external morphology and internal anatomy of new species. Thus, we are aware of the risk of conducting a nomenclatural act based on morphological information obtained only with the use of light microscopy with DIC gathered from two adult specimens. However, our decision to designate a new species is grounded in practical and philosophical standpoints that supported us to consider O. paraensis and O. miyazakii sp. nov. as two distinct evolutionary entities, or two distinct explanatory hypotheses for the intrinsic or relational properties of organisms that can be accounted for reproductive events (Fitzhugh 2005, 2009).

In some cases due to environmental remoteness, sampling depth (e.g. deep sea), or rarity of some taxa, only one or very few meiofaunal specimens are collected and can be used to observe a sufficient number of morphological features to identify and delimit the species. In the last five years, we have been continuously sampling the lagoon where O. miyazakii sp. nov. was found and we were able to find many specimens of three unknown species of semiplanktonic gastrotrich (two of them were described by Minowa and Garraffoni 2017, 2020) and at least five unknown benthic gastrotrich (one of them described by Garraffoni and Melchior 2015). However, until October 2019, this was the only sampling moment we found specimens belonging to O. miyazakii sp. nov. Thus, taking into account these specificities and the ontological status of species taxa as a species-as-individuals hypothesis (e.g. Coleman and Wiley 2001, Fitzhugh 2009), we agree with Kieneke and Nikoukar (2017) and interpret the description of new species based on very few specimens as “primary species hypothesis”. In this case, the use of “primary” is only an indication of the initial essay for species delimitation, because O. miyazakii sp. nov. or Thaumastoderma antarctica Kieneke, 2010 (among many other examples in gastrotrichs) were hypothesized as entities that have unique common evolutionary origins and are spatiotemporally located (Rieppel 2009), in the same way that any taxonomist would do if they had many specimens to analyze.

Besides, hypotheses of homology between characteristics of species of distinct lineages are as important as the hypotheses of species themselves. Highlighting the existence of the pairs of granular transverse scales on both species of Ornamentula, inserted on both sides of the head, arranged in such a way that the transverse ciliary band lies between them. However, these granular scales, despite the similar relative positions on the head, are not homologous to the cephalic pleura present in other Paucitubulatina (Kieneke and Schmidt-Rhaesa 2015). This statement is supported by the position relative to the ciliary arrangement, as such structures are arranged adjacent to the cephalion, and each ciliary cephalic tuft is inserted ventrally (Minowa and Garraffoni 2020) on benthic representatives. Even compared with the semiplanktonic species, the cephalic reinforcement (pleura) is inserted anteriorly to the ciliary band, in contrast with the posterior insertion of the second scale in the Ornamentula species.

Garraffoni and Araújo (2020) provided a taxonomic key for Neotropical Gastrotricha species, which we updated here with the Ornamentula identification and included the new species.

We are grateful to the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 2014/23856-0, 2017/20312-8, and 2019/01955-0) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes Finance Code 001) for the financial support. We are grateful to Rick Hochberg and an anonymous reviewer for their constructive criticism that greatly improved the first version of the manuscript. Additional thanks to Yasmina Shah Esmaeili for editing the English text.