Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts

Three new species of Mesacanthion Filipjev, 1927 were found along Patagonian coasts (Argentina). Mesacanthion bifidum sp. nov. is characterized by short labial and cephalic setae, onchia of equal size, spicule arcuate, and gubernaculum with caudal apophysis, ending in two teeth. The species is related to M. virile (Ditlevsen, 1930) De Coninck & Schuurmans Stekhoven, 1933. However, the spicules and gubernaculum of both species are different in shape. Mesacanthion longigubernaculum sp. nov. is characterized by its long and slender body, striated cuticle, relatively long cephalic and cervical setae, onchia of different sizes, amphidial fovea lentil-shaped, spicule arcuate, gubernaculum surrounding the spicule, and tail conical-cylindrical with terminal setae. Mesacanthion sanantoniensis sp. nov. is characterized by its long and stout body, striated cuticle, long cephalic setae, onchia of different sizes, amphidial fovea pouch-shaped, spicule arcuate, gubernaculum with dorsal apophysis, and tail conical without terminal setae. Following the key of Jeong et al. (2019), the last two species are related to M. pali Wieser, 1959 and M. longissimesetosum Wieser, 1953, so we provide a key to differentiate the four species.

Regularized Stokeslets Lines Suitable for Slender Bodies in Viscous Flow

Slender-body approximations have been successfully used to explain many phenomena in low-Reynolds number fluid mechanics. These approximations typically use a line of singularity solutions to represent flow. These singularities can be difficult to implement numerically because they diverge at their origin. Hence, people have regularized these singularities to overcome this issue. This regularization blurs the force over a small blob and thereby removing divergent behaviour. However, it is unclear how best to regularize the singularities to minimize errors. In this paper, we investigate if a line of regularized Stokeslets can describe the flow around a slender body. This is achieved by comparing the asymptotic behaviour of the flow from the line of regularized Stokeslets with the results from slender-body theory. We find that the flow far from the body can be captured if the regularization parameter is proportional to the radius of the slender body. This is consistent with what is assumed in numerical simulations and provides a choice for the proportionality constant. However, more stringent requirements must be placed on the regularization blob to capture the near field flow outside a slender body. This inability to replicate the local behaviour indicates that many regularizations cannot satisfy the no-slip boundary conditions on the body’s surface to leading order, with one of the most commonly used blobs showing an angular dependency of velocity along any cross section. This problem can be overcome with compactly supported blobs, and we construct one such example blob, which can be effectively used to simulate the flow around a slender body.

Remarks on Regularized Stokeslets in Slender Body Theory

We remark on the use of regularized Stokeslets in the slender body theory (SBT) approximation of Stokes flow about a thin fiber of radius ϵ>0. Denoting the regularization parameter by δ, we consider regularized SBT based on the most common regularized Stokeslet plus a regularized doublet correction. Given sufficiently smooth force data along the filament, we derive L∞ bounds for the difference between regularized SBT and its classical counterpart in terms of δ, ϵ, and the force data. We show that the regularized and classical expressions for the velocity of the filament itself differ by a term proportional to log(δ/ϵ); in particular, δ=ϵ is necessary to avoid an O(1) discrepancy between the theories. However, the flow at the surface of the fiber differs by an expression proportional to log(1+δ2/ϵ2), and any choice of δ∝ϵ will result in an O(1) discrepancy as ϵ→0. Consequently, the flow around a slender fiber due to regularized SBT does not converge to the solution of the well-posed slender body PDE which classical SBT is known to approximate. Numerics verify this O(1) discrepancy but also indicate that the difference may have little impact in practice.

A new species of Thoropa Cope, 1865 (Anura, Cycloramphidae) from the Serra da Mantiqueira, Southeast Brazil

We describe a new species of Thoropa, previously identified as T. lutzi, from the northern region of the Serra da Mantiqueira in the Atlantic Forest domain in Southeast Brazil. The new species is diagnosed by the following combination of characters: small size; slender body; head longer than wide; dark colored nuptial pads on the inner side of the Finger I and on the internal carpal tubercle; nuptial pads with epidermic cone-shaped papillae measuring of 53.1–91.6 μm in diameter, and at a density of 14–32 papillary epidermic projections/mm2; presence of vocal sac and vocal slits; and advertisement call with 5–10 harmonics, duration of 0.23–0.42 s, and peak of frequency of 2060–4470 Hz. With the description of the new species, T. lutzi is now only known for the state of Rio de Janeiro.