Odontodiaptomus thomseni (Brehm, 1933)

Perbiche-Neves, Gilmar, Boxshall, Geoffrey Allan, Previattelli, Daniel, Nogueira, Marcos Gomes & da Rocha, Carlos Eduardo Falavigna, 2015, Identification guide to some Diaptomid species (Crustacea, Copepoda, Calanoida, Diaptomidae) of " de la Plata " River Basin (South America), ZooKeys 497, pp. 1-111 : 60-62

Taxon classification Animalia Calanoida Diaptomidae

Odontodiaptomus thomseni (Brehm, 1933) Figs 79, 80, 81, 82, 83

Diaptomus thomseni Brehm, 1933

Diagnosis.

Adult male, body length 1088 µm. Small chitinous knob present distally on internal margin of left CR, other small chitinous processes present on right CR (Fig. 80B, E). Modified setae on segments 10, 11 and 13 of A1R large (Figs 79C, 80A); segment 14 lacking spinous process; spinous processes on segments 15 and 16 well developed; segment 20 of A1R bearing dentate hyaline membrane (Fig. 79B). Right CxP5 with small chitinous knob on surface in middle of segment (Fig. 79A). Right BspP5 bearing three rounded chitinous projections on internal margin. Right EnpP5 2-segmented (Figs 79A, 80D). Right Exp1P5 with semi-circular chitinous knob on distal margin, visible in anterior view; right Exp2P5 with lateral spine inserted proximally (Figs 79A, 80C), about 6 to 7 times longer than wide. Terminal claw of right Exp2P5 about 2.3 times longer than lateral spine. CxP5L with distal outer projection bearing sensilla about 3 times longer than wide (Fig. 79A); well-developed outer seta present on BspP5L.

Adult female, body length 1245 µm. Incomplete suture present between Ped4 and Ped5; surface of Ped4 and Ped5 smooth, lacking rows of spinules dorsally along posterior margin (Fig. 81A); lateral wings slightly asymmetrical, similar in size; left sensilla directed posteriorly, right sensilla perpendicular to body axis (Fig. 82A). GS asymmetrical, about 1.6 times longer than wide; anterior part slightly dilated, with swelling on right side anterior to that on left; right side margin with large rounded process at posterior end (Fig. 81A). Ur2 with small chitinous process on outer margin on left side (Fig. 82B). P5 symmetrical (Fig. 81B) with small conical process at outer distal corner of CxP5, bearing short robust sensilla; BspP5 with short outer seta, about as long as EnpP5. EnpP5 one-segmented. ExpP5 3-segmented; lateral spine of Exp2P5 exceeding external margin of Exp3P5; external seta of Exp3P5 nearly 2/3 length of internal seta; internal seta robust, only about 1/5 length of terminal claw (Fig. 82C, D).

Remarks.

The specimens were collected in the low stretch of the Uruguay River at Salto Grande Reservoir (Fig. 83, SGRA-D). Only three individuals were found and a full re-description was published by Perbiche-Neves et al. (2012). This species is on the red list of endangered species ( IUCN–Red List of Threatened Species, 2010- Reid 1996), due to lack of recent records. The only previous confirmed record of this species was its original description ( Brehm 1933). There is a subsequent record of this species from pools in Venezuela, but it is doubtful and requires confirmation ( Perbiche-Neves et al. 2012). Ringuelet (1958) did not find this species in his work on Argentinian co pepods, nor did Paggi and José de Paggi (1990). This was only the second confirmed record of this rare species in 77 years.

The genus Odontodiaptomus comprises three species (equivalent to the thomseni group of Wright 1936), and there is a great interest in the genus because of its apparent isolation from other Neotropical Diaptomidae ( Santos-Silva 2008). A second species of this genus, Odontodiaptomus paulistanus (Wright, 1936) is relatively common in Brazil, but was not found in the present study. The third species, Odontodiaptomus michaelseni ( Mrázek, 1901), was not found in our study.