Rediscovered after 77 years: Odontodiaptomus thomseni – a rare species of calanoid (Crustacea: Copepoda) from South America

The freshwater copepod Odontodiaptomus thomseni (Brehm, 1933) (Calanoida: Diaptomidae) is a rare species that has been reported only once - in its original description (BREHM 1933). The lack of subsequent records led to its inclusion in the Red List of threatened species (IUCN). Here we present a new record for O.thomseni. It was discovered in Salto Grande reservoir, which is located in the lower stretches of the Uruguay River, between Uruguay and Argentina, at the River Plate basin. In January 2010, three specimens (two males and one female) were found, and these were studied in detail using scanning electron microscopy (SEM). We only had material of Odontodiaptomus paulistanus (Wright, 1936) for comparison, but the position of the lateral spine in right P5 of the male, and the shape and size of lateral wings of the female are especially distinctive. Odontodiaptomus thomseni remains a rare species and we recommend keeping it on the IUCN Red List.

. Records of O. thomseni. The square shows the record of BREHM (1933) and the circles mark the present new records from Salto Grande Reservoir.
Limnological variables were measured during sampling (Tab. I). Plankton samples were collected by vertical hauls using a conical plankton net of 68 µm mesh size, equipped with an antirefluxing device. The samples were preserved using Karnovski's solution (glutaraldehyde 2.5%, paraformaldehyde 20%).

TAXONOMY
All observations were compared with a second species of the genus, O. paulistanus, using material collected by Carlos E.F. da Rocha in the Ecological Station of Boracéia, São Paulo State, Brazil.
Diagnosis. Male: prosome with patches of setules on dorsal surface (Fig. 3); with sclerotized tooth-like process present on left caudal ramus, and other similar processes on surface of caudal ramus and urosome (Figs 4 and 5); two vestigial setae present on segment 2 of right geniculate antennule; well-developed modified setae present on segments 10, 11 and 13 of right antennule (Fig. 10); hyaline lamella on segment 20 of right antennule; endopod of right leg 5 well developed, 2-segmented (Figs 7 and 8); lateral spine of second exopod segment of right leg 5 positioned basally on segment and reaching beyond middle of segment (Fig. 7); terminal claw longer than segment (Fig. 7). Female: patches of setules present on dorsal surface of prosome; posterolateral wings of prosome strongly asymmetrical (Fig. 25); left lateral wing of genital double-somite with tapering process bearing spine at apex (Fig. 25); second urosomite with wing-like expansion on right side (Fig. 25); sclerotized tooth-like process on left side of third urosomite (Fig. 26); endopod of leg 5 well developed (Fig. 29).
The preparation of specimens for SEM investigation followed the protocols of FELGENHAUER (1987) and HUYS & BOXSHALL (1991). The material was washed in 0.1M phosphate buffer solution at pH 7.3 (3 changes, five minutes each), then preserved in Osmium tetroxide (0.5% in water) for 20 minutes. The material was dehydrated through graded ethanol as follows: 7.5%, 15%, 30% and 50% (two changes in each concentration, five minutes); after 70% (three changes, 10 minutes); and to 90% and 100% (two changes in each concentration, five minutes). Then the material was stored in permeable plastic capsules and critical-point-dried in a BALZERD UNION -CTD-020, using Carbon dioxide as the exchange medium). Finally the copepods were mounted on stubs. At this point some limbs were dissected off the female using fine insect pins to provide the best view of particular limbs; the male was kept intact. The material was sputter-coated with 15 nm of gold (BALZERS UNION -MED-10) and analyzed using a Quanta 200 FEI scanning electronic microscope.
The taxonomic diagnosis presented here supplements and updates that of BREHM (1933), using SEM images as well as light microscope observations. Morphological terminology follows SANTOS-SILVA et al. (1999) and PAGGI (2006).

DISCUSSION
In his original work BREHM (1933) mentioned that he had found a "strange" copepod that was very different in morphology when compared with the known species. In fact, the species included in Odontodiaptomus share the possession of chitinous nodules on various parts of the body surface, including a toothlike process on the caudal rami and another on the urosome, from which the genus takes its name. Odontodiaptomus thomseni differs from the other two species included in Odontodiaptomus especially in the shape of the fifth leg of the male, which has the lateral spine on the second exopod segment inserted near the base of the segment, a well-developed endopod, and a rounded process on the first exopod segment. The females also differ markedly from the two other Odontodiaptomus species, particularly in the asymmetry of the genital receptacle. Consideration of these features makes it is easy to understand why BREHM (1933) regarded this species as "strange". This is only the second confirmed record of O. thomseni. The discovery of only three individuals suggests the species is rare and provides support for maintaining it on the Red List of threatened species. REID et al. (2002) pointed to possible factors that currently affect the Brazilian copepod fauna and impact species' abundance. These factors include the damming of rivers and disappearance of floodplain lakes; the introduction of alien species, and the increasing pollution of the rivers by domestic and industrial waste. Such factors may impact species abundances across the entire South America. REID (1996) noted the existence of another record of O. thomseni from pools in Venezuela. We consider this record, so remote from the type locality (approximately 4,600 km), as not valid, without reexamination. BREHM (1933), WRIGHT (1938), MATSUMURA-TUNDISI (1986) and SANTOS-SILVA (2008) all emphasized that the three species of Odontodiaptomus are endemic to particular regions or latitudinal ranges, further casting doubt on the record from Venezuela. It is possible that the material reported from Venezuela represents a new species of the genus Odontodiaptomus. According to J.C. Paggi (pers. comm. 2011) there is another undescribed species of Odontodiaptomus inhabiting the Salto Grande reservoir (Uruguay/Argentina), although only one male individual has ever been found.
There are only three species of Odontodiaptomus and, as WRIGHT (1937) commented, O. thomseni is very different from O. paulistanus and O. michaelseni. These last two are very similar morphologically. In the absence of type material, is difficult to construct a dichotomous identification key due the paucity of knowledge of O. michaelseni. New material needs to be found -near Buenos Aires. Further study of this genus is necessary in order to fully understand its geographic distribution and taxonomic complexity. SUÁREZ-MORALES et al. (2005) point to the lack of information from some areas in the Neotropical region, and they show the distribution of Odontodiaptomus in South America as basically restricted to the La Plata River Basin.
More sampling should be concentrated in regions where O. thomseni has been found, especially in small water bodies, such pools and reservoirs. Although the nature of these small water bodies contrasts with the new locality reported here, this is the kind of environment sampled by BREHM (1933). Sampling such small habitats is important as PERBICHE-NEVES et al. (2011) demonstrated when they reported high abundance of Argyrodiaptomus bergi (Richard, 1898) in a pool at high altitude in the state of Santa Catarina, southern Brazil. Prior to this record, this copepod had not been found for 36 years.