Bythocypris Brady 1880

Genus Bythocypris Brady, 1880

Type species. Bythocypris reniformis Brady, 1880.

(1) Remarks on subgenera of Bythocypris

Based on carapace characters, Warne (1990) described a new subgenus of Bythocypris –Bythotriangularia Warne, 1990 —to include the species with “subtriangular to oval” lateral valve outline, “inner lamella moderately broad”, and “adductor muscle scar pattern typically bythocypridid except that individual scars are commonly subdivided”. The following species were included in the subgenus Bythotriangularia: the type species B. spiriscutica Maddocks, 1969; plus B. eltanina Maddocks, 1969; B. promoza Maddocks, 1973; and B. mozambiquensis Maddocks, 1969.

Warne (1990) diagnosed the subgenus Bythocypris Brady, 1880, as follows: valve lateral outline “smooth, reniform to sub­reniform or sub­rectangular”, “inner lamella of moderate size, and adductor muscle scars that are usually undivided”.

Warne (1990) completely ignored the several anchistrocheline soft part characters (elongated hypostome and labrum; fused podomeres of antenna I and appendages V to VII; hemipenis with short copulatory process and one also short accessory process) of Anchistrocheles antemacella Maddocks, 1969, A. barnharti Maddocks, 1976, A. hartmanni Maddocks, 1976 and A. mcquadei Maddocks, 1976 and transferred these four species to the subgenus Bythocypris. These assignments are herein considered invalid, and these four species are considered to belong to the genus Anchistrocheles Brady & Norman, 1889 (sensu Maddocks 1976).

The author himself (Warne, 1990: 106) noticed that intermediate carapace lateral outlines exist between the two “diagnostic” sub­reniform and subtriangular forms, and even further, that the type species of the subgenus Bythocypris—B. (B.) reniformis Brady, 1880 —is one of these intermediate forms: “the sub­reniform carapaces of the type species is intermediate in shape between the sub­rectangular carapace of B. (B.) subrectangularia sp. nov. and the subtriangular carapace of B. (Bythotriangularia), (…) but is closer to the sub­rectangular forms than to the subtriangular forms”. Another example of intermediate valve outline was cited by Warne (1990: 111): “ B.elongata Brady, 1880 may also belong to B. (Bythotriangularia) but it is more elongated than other species included in the new subgenus and has a very narrow inner margin”.

Ostracod valve outline, especially in smooth forms, is known to be a very plastic character, with similar outlines being convergently present in different families (for example: Bythocyprididae, Cyprididae, Krithidae, Macrocyprididae, and Pontocyprididae). In addition, the width of the zone of concrescence may also vary, since it changes according to the time of the last moult, with newly moulted individuals presenting a thinner zone of concrescence than those which moulted longer before (Keyser, pers. comm.). Moreover, the subdivision of adductor muscle scars varies according to “preservation, individual variation, method of illustration, and interpretation by an individual observer” (Maddocks, 1995: 207). Unfortunately, no consistent character is left for the recognition of both subgenera of Bythocypris as proposed by Warne (1990), making it impossible to recognise these taxa. Consequently, no subgeneric assignment is attempted herein.

(2) Remarks on Bythocypris reniformis

There is considerable taxonomic confusion concerning the name Bythocypris reniformis, which was reported from the Western Atlantic (North and South), Southwestern Pacific, Southwestern Indic and Southern oceans, and from the Pleistocene to recent sediments (Fig. 22, Tab. 4).

Brady (1880: 46) described Bythocypris reniformis based on a “considerable number of detached valves, together with a few entire specimens,” collected through dredgings off Culebra Islands, 713m (# 24), off North Brazil, 340 and 1234m (# 120 and # 122), off Prince Edward’s Island (subantarctic region), 91 to 274m, and off Moncoeur Island, Bass Strait, 70 to 73m. Chapman (1941) recorded B. reniformis from southeast Australia (860 and 924m). Maddocks (1969) assigned to B. reniformis subfossil specimens with very different valve morphology collected “near Tulear, Madagascar, and elsewhere on the western Madagascar coast to depths of” 3530m (75 specimens, 475 to 3530m) and off south­eastern Brazil (108 specimens, 1227m). Later, Puri and Hulings (1976) erected as the lectotype of B. reniformis a subfossil RV (length 1.09mm) from Culebra Islands (713m). The last authors erroneously designated topotypes (1 RLV and 1 LV) from Prince Edward Island (91 to 274m). The lectotype differs from the illustrated [sic] topotype in the more equilateral lateral outline, more straight anterodorsal margin (instead of smoothly rounded), and more concave ventral margin of the former. The lectotype also differs from the specimen illustrated by Brady (1880, Pl. 5.1b) by the more truncated anterior margin, and more rectilinear outline, especially the straight anterodorsal margin presented by the former species. The specimens collected off south­eastern Brazil (Maddocks 1969, Fig. 45.J–M) differs from the lectotype of B. reniformis because the former: (1) are more smoothly rounded, subtriangular valve outline; (2) have a more arched dorsal margin; (3) present maximum height at mid­length (instead of anterior to it); (4) present much larger size (1.51 to 1.62mm instead of 1.09mm). The specimens from the Mozambique Channel (Maddocks 1969, Fig. 45.N–U) differ from the lectotype of B. reniformis, because the former: (1) have a more sub­reniform outline; (2) have a more obtuse posterior angle; (3) have a less concave ventral margin; (4) have a more broadly rounded anterior margin; (5) present maximum height posterior to mid­length (instead of anterior to it). Finally, Briggs (1978) reported B. reniformis from Ross Island, Antarctica (no illustration or description was provided).

Specimens from Marion Island (Dingle 2003) also vary from the lectotype of B. reniformis. The RV (Dingle 2003, Pl. 1.4) shows: (1) more smoothly rounded dorsal margin (the lectotype presents a more arched dorsal margin, with straight anterodorsal margin and conspicuous anterodorsal angle); (2) posterior angle of the first species is more obtuse than the last species; (3) ventral margin is concave in the former sp. and broadly rounded anteriorly and upswung posteriorly in the type material. The LV (Dingle 2003, Pl. 1.3) lacks the conspicuous anterodorsal angle, has a more smoothly rounded dorsal margin, slightly convex (instead of concave) ventral margin, and more obtuse posterior angle than the topotype from off Marion Island.

Based on the above, I conclude that specimens previously recorded as B. reniformis should be included in at least 4 different species (1. type locality – Caribbean; 2. off Canada; 3. off Brazil; 4. off Mozambique Channel;?5. off Australia).