Oithona flemingeri (Ferrari & Bowman 1980)

Paroithona flemingeri

MATERIAL.—38 from PN-13-60.

FEMALE.—Length range 0.40–0.45 mm; Pr/Ur–1.2. Head rounded in lateral view, rostrum absent (Figure 14a). CR (Figure 14d) slightly longer than wide; dorsal and 1 apical seta thick and elongate, bearing lateral hyline membrane similar to StP1–4 of most oithonids; 2 apical setae absent; lateral seta reduced. A1 (Figure 14e) with 1 free segments. A2 with 1st segment elongate (Figure 14f). B2Md (Figure 14g) with 1 large, thick, slightly curved spine, bearing spinules and 1 small, thin seta in inferior position; ReMd 4 segments, terminal seta reduced; RiMd a bump with 4 setae. RiMx1 (Figure 14h) naked. Mx2 and Mxp as illustrated (Figure 14i, j). ReP1 2-segmented; Re2 and 3 fused; ReP2–4 3-segmented, ReP1–4 2-segmented. ReP1–4 Se 1–3, 1–1–2, 1–0–2, 1–0–1; ReP1–4 Si 0–4, 0–1–5, 0–1–5, 0–0–5; RiP1–4 Se 0–1, 0–1, 0–1, 0–1; RiP1–4 Si 1–6, 0–3, 0–3, 1–3. All Si RiP4 unmodified. P5 with 1 seta. Knob near genital opening with 1 spine pointing posterodorsally and below this 1 thinner spine (Figure 14b).

MATERIAL.—Female holotype (USNM 172189), 34 paratypes (USNM 172190) from PN-13-60, 10°41.2′N 63°14.2′N, 800 m from jetty at Carupano, Venezuela; 22 Jun 1977.

Commentary

About 60 species have been placed in the family Oithonidae since Baird's (1843) initial description of Oithona plumifera. Despite the amount of information that has accumulated about oithonids over the past century, there are still difficulties in assigning values to characters studied in standard systematic accounts. Aside from denoting characters that have intrinsic value in classification, simply because they do vary, knowledge of the biology of oithonids is of such a poor state that there has been no attempt to order these characters in a system reflecting the evolutionary trends of the animals. Over 50 years have passed since the only attempt was made to relate several species within the genus Oithona (Burckhardt, 1913).

As an example of the lack of basic biological information, we know of no direct observations of oithonid mating behavior. However, this behavior in other gnathostome cyclopoids of the family Cyclopidae has received attention from a number of authors, e.g., Hill and Coker (1930) and Rylov (1948). These observations indicate that males use their digeniculate first antenna to grasp the ReP4 of the female. Their ventral surfaces are facing. There is some question as to whether this is the initial contact position or whether the male moves to this position from a previous one, grasping the Pr/Ur articulation of the female dorsally (Hill and Coker, 1930). Nonetheless the male must reach the A1/P4 position in order to attach the spermatophores ventrally on the female genital segment over the ventrally placed female genital openings. Spermatophore transfer is affected with the tips of the male swimming legs. Thus spermatophores can be transferred by use of virtually unspecialized swimming legs; specialized appendages or seta need not have evolved for this purpose as Ferrari (1977) has suggested.

Although no observations have been published on mating behavior of oithonids, certain observations suggest a similar pattern of events. Female genital openings in the Oithonidae are dorsal or dorsolateral in position but spermatophores are attached to females of O. plumífera and O. oculata ventrally as in the Cyclopidae. It is not known whether the male grasps the female as in Cyclops, at the ReP4. Assuming that he does, however, one might expect to find a tactile recognition system during this positioning and the system may be reflected in the morphology of the appendages involved. No study of the complex male digeniculate Al has been undertaken to enumerate differences between species. However, distinct differences in numbers and shapes of spines on ReP4 have been used quite commonly in separating species of oithonids, and the taxonomic value of these structures has been emphasized by Kiefer (1956).

If the position of male/female at the time of spermatophore transfer is similar to the final mating position of, e.g., Cyclops amencanus, as illustrated by Hill and Coker (1930, fig. 2), the female could use the specialized setae of RiP4 for tactile interrogation of the male pore signature prior to spermatophore transfer. All females of the genus Oithona have modified setae on RiP4, although in O. simplex these setae are reduced rather than bearing a flange. Not all males possess a well organized pore signature, however. Oithona oculata and O. bjornbergae lack this pore signature, although in SEM micrographs, the lateral surface of Cph of O. oculata is densely pitted.

Observations of living oithonids will eventually elucidate the functions of modified setae on RiP4 of females and pore signature of males. Because of their value in systematic studies, we feel it would be beneficial to know the role these structures play in the biology of oithonids. If it is shown that these integumental organs function in some interrogating system during reproduction, this would suggest their importance in a speciation process emphasizing the development of prezygotic isolating mechanisms. Further, an effort should be made to collect males of closely related species over their entire zoogeographic range, so that the general organization and intraspecific variation of the pore signature can be characterized and compared to the variation between species. Once the extent of intraspecific variation is understood, more difficult problems can be addressed, such as characterization of populations, definition of population boundaries, and extent of interpopulation movements.

Regardless of their denouement in the evolution of the genus Oithona, several types of pore signatures can be recognized in species studied. All species possess vertical columns of organs, which are interconnected ventrally. This pattern is present even in O. simplex, which possesses the fewest number of organs and no Cph flap. Except for O. simplex, all species possess a distinct anterodorsal cluster of organs. Two males (Figure 13f, g) from Melville Stations 359 and 353, respectively, although geographically beyond the scope of this study, serve to emphasize the importance of this anterodorsal cluster. Oithona sp. 3 (male) is 0.80 mm long with the same spine count as Oithona plumifera (1–1–2, 1–1–3, 1–1–3, 1–1–2). The pore signature and flap development (Figure 13f) is similar to O. plumifera. However, the anterodorsal cluster has more organs than O. plumifera and these seem more densely spaced. In O. plumifera (Figure 10h) there is a rather abrupt transition between the anterodorsal cluster and horizontal row. On Oithona sp. 3 the transition is immediate but the horizontal row begins as a jumbled group of organs before resolving into the single row.

Oithona sp. 4 (male) is 0.76 mm long; the spine count on ReP1–4 is 1–1–3, 1–1–3, 1–1–3, 1–1–2. The pore signature is also similar to O. plumifera. The anterodorsal cluster (Figure 13g) is composed of more organs than O. plumifera or the previous male. It merges rather indistinctly, again via a jumble of organs, into the horizontal row, which eventually resolves into a single row of organs.

The anterodorsal cluster can be considered the anterior end of the horizontal row of organs, the third general feature of the pore signature. This row is either set dorsally to the columns, in effect confining them, e.g., O. amazonica, O. hebes, O. plumifera, or appears less distinctly as a forward extension of the ventral organs in the columns as in O. nana and Oithona sp. 2. These latter two males also possess numerous scattered organs ventral to the columns and horizontal row. The organs are separated from columns and row in the former species by an area devoid of organs and in the latter by a ridge of the integument.

All females show modifications of one or more internal setae on the RiP4. These modifications may simply be a reduction in the size of the setae compared to other internal plumose ones as in O. simplex but more often involves development of a medial flange on the tip of a thick seta. One, (O. amazonica), two (O. fonsecae and O. hebes), or three (O. plumifera, O. decipiens, O. oculata, and O. bjornbergae) setae may be modified with varying degrees of curvature; the flange length also varies in relation to the length of seta. These female characters may contain as much taxonomic information as the pore signature. The latter are more easily studied because slight variations in the setae may be reflected in the degree of curvature and length of flange. These are more difficult to analyze than the discrete male organs.