Diastylis cf. algoae Zimmer, 1908

Mühlenhardt-Siegel, Ute, 2014, Deep-sea Bodotriidae, Diastylidae and Pseudocumatidae (Cumacea, Crustacea) from the southeastern Atlantic, Zootaxa 3815 (3), pp. 301-341 : 333-336

Diastylis cf. algoae Zimmer, 1908

( Figures 23–25 )

Material. BIOZAIRE 1, Station #21: 1 male with developing pleopods MNHN-IU-2012-1356.

Description. Male (MNHN-IU-2012-1356; Fig. 23 ), 4.1 mm long, in very bad condition due to conservation in formalin.

Carapace smooth, a small tooth per side laterally at frontal lobe; pseudorostral lobes 0.3 times as long as the rest of carapace, meeting in front of ocular lobe; siphonal tube not protruding; ocular lobe reduced; no eye lenses.

Antennal notch shallow; anterolateral tooth rounded, not protruding.

Anteroventral margin of carapace with numerous fine teeth..

Integument decalcified due to fixation.

Five free thoracic segments visible.

Pleon 0.9 times as long as carapace and free thoracic segments combined, two pairs of developing pleopods.

Telsonic segment 1.3 times as wide as long.

Telson shorter than uropod peduncle, with preanal part half as long as postanal part, the latter with four pairs of lateral cuspidate setae and two strong terminal setae.

Antenna 1 ( Fig. 24 , A1): relative length of peduncle articles 1 to 3: 39/26/35; article 1 with a simple and a pappose seta; article 2 with one simple seta; article 3 with three simple setae; accessory flagellum tetra-articulated, almost as long as the two basal articles of main flagellum.

Antenna 2 developing, not figured.

Maxilliped 2 ( Fig. 24 , Mxp2): B/R ratio not given, basis broken during dissection, relative length of articles I to D: 9/19/ 33/27/12; basis with long pappose seta; ischium and merus each with a shorter pappose seta; carpus with seven pappose setae; propodus with 15 shorter pappose setae (the first row of setae not figured), a very long pappose seta inserting in the middle part; dactylus with a strong terminal and a shorter simple seta.

Maxilliped 3 ( Fig. 24 , Mxp3): B/R ratio not given, basis broken during dissection, relative length of articles I to D: 8/17/20 /33/21; basis with four long and nine shorter pappose setae; ischium with three pappose setae; propodus with one pappose and two simple setae; dactylus with a stronger terminal and three simple sub terminal setae sub-terminally; exopod broken off, not figured.

Pereiopod 1 ( Fig. 24 , P1): B/R ratio 0.5, relative length of articles I to D: 6/ 8/30/38 /18; basis with 17 pappose setae; ischium with one pappose seta; merus with two pappose setae; carpus with one hair-like seta; propodus with five small simple setae; dactylus with six simple setae; exopod present.

Pereiopod 2 ( Fig. 24 , P2): B/R ratio 0.7, relative length of articles I to D: 6/13/39 /14/28; basis with a tooth; ischium with a tooth and a simple seta; merus with a pappose and five simple setae; carpus with three simple setae; propodus with one simple seta; dactylus with a long terminal and five shorter simple setae; exopod present.

Pereiopod 3 ( Fig. 25 , P3): B/R ratio 1.2, relative length of articles I to D: 18/ 42/26/9 /5; basis with two pappose setae; ischium with a simple seta; long merus with a long and a very short simple seta; carpus with a hair-like, a simple and a terminally annulated seta; propodus with two terminally annulated setae; dactylus with long simple terminal seta; exopod present.

Pereiopod 4 ( Fig. 25 , P4): B/R ratio 0.9, relative length of articles I to D: 14/23/ 45/14/4; basis with a hair-like seta; merus with two hair-like setae; carpus with two simple and two terminally annulated setae; propodus with a terminally annulated seta; dactylus with a long simple seta; exopod present.

Pereiopod 5 ( Fig. 25 , P5): B/R ratio 0.5, relative length of articles I to D: 19/20/ 42/14/4; basis with three simple setae; ischium with one simple seta; merus with two simple setae; carpus with four simple setae and two terminally annulated setae; propodus with one terminally annulated seta; dactylus with a long simple seta.

Uropod ( Fig. 25 , U): peduncle 1.1 times as long as endopod, with four cuspidate setae along inner margin; exopod as long as endopod; endopod tri-articulated, basal article with ten cuspidate setae along inner margin, longer than the two distal articles combined; article 2 with two cuspidate setae, article 3 with one seta laterally and one terminally.

Distribution. Lüderitz Bay to East London, 20–200 m; Angola Basin, 365 m depth.

Remarks. The primary diagnostic characters of Diastylis algoae are the smooth carapace surface, a pair of small teeth at the frontal lobe, the preanal part of the telson being much shorter than the postanal part (half as long in the present specimen), and a tri-articulated uropod endopod. All these characters are clearly discernible in the subadult male from the upper slope of the Angola basin.

However, in the ovigerous females described by Zimmer (1908) and Day (1980) and the adult male described by Day (1980) the postanal part of the telson bears 15 (adult male) or 16 (ovigerous females) lateral telson setae, but only four are discernible in the subadult male from the Angola Basin. The elongated edges of the last pereionite, figured in Zimmer (1908) as well as in Day (1980), are not discernible in the subadult male, possibly due to the younger stage of the male and/or to the bad condition of the material.

The uropod rami are equal in length in the present specimen and the adult male described by Day (1980). However, the exopod is longer in the ovigerous female figured by Day (1980).

The proportion of the preanal part to the postanal part of the telson is 0.5 in the Angola Basin specimen but 0.6 in the material of Day (1980) and Zimmer (1908).

The pseudorostrum in D. algoae is straight and pointed in Day’s adult male and slightly upturned in the ovigerous female. However, in the present subadult male it is blunter and with a considerable angle between carapace and pseudorostrum.

These minor differences do not justify the erection of a new species for the single subadult male which is in a poor condition. Day (1980) stated “As well as being highly variable morphologically, D. algoae is one of the most widely distributed southern African Cumacea ”. Therefore, it seems not unlikely that this species is distributed to as far as the upper continental slope of the Angola Basin.

Pseudocumatidae

There is one representative of the family Pseudocumatidae in the Ifremer collection from the Angola Basin. Although there is a wide geographical distance between the type locality, off the Senegal coast, and the new locality, the new specimen is obviously conspecific with Jones’ specimens.

Figures

FIGURE 23. Diastylis cf algoae, subadult male; habitus, scale bar: 2 mm.

FIGURE 24. Diastylis cf algoae; A 1: antenna 1, accessory flagellum in higher magnification; Mxp 2: maxilliped 2; Mxp 3: maxilliped 3; P 1: pereiopod 1; P 2: pereiopod 2; scale bars: 0.1 mm.

FIGURE 25. Diastylis cf algoae; P 3: pereiopod 3; P 4: pereiopod 4; P 5: pereiopod 5; U: pleonite 6, telson and left uropod, tip of telson in higher magnification; scale bars: 0.1 mm.