"Thymops birsteini (Zarenkov & Semenov, 1972)
Figs. 13, 14
Nephropides birsteini Zarenkov & Semenov, 1972: 599, figs. 1-6.
Material.—Off Buenos Aires Province, Argentina, 39°56'S 55°54’W 800 m; 19 June 1966; WALTER HERWIG sta. 269, 2 males (L). —Off Chubut Province, Argentina: 45°15'S 59°54'W; 600 m; 24 June 1966; WALTER HERWIG sta. 305, 5 males, 1 ovigerous female (W,L) 46°13'S 59°49’W; 805 m; 17 January 1971; WALTER HERWIG sta. 191, 2 females (L). —N.E. of Falkland Islands, 52°05'S 55°20'W; 1200 m; 25 January 1971; WALTER HERWIG sta. 227, 1 female (L).
Description.—The rostrum is slender, it is elongate triangular in dorsal view and reaches far beyond the antennular and antennal peduncles. It has two or three lateral rostral spines. There are no ventral teeth. The upper surface of the rostrum shows a median groove which becomes deeper posteriorly. From the base of the rostrum, where the groove is deepest, it continues posteriorly as a shallow and depressed smooth linear area, which extends over the full length of the carapace. Starting at the middle of the rostrum there are two low subdorsal carinae, which become more distinct posteriorly; they reach slightly beyond the posterior supraorbital spine and diverge posteriorly. Seven or eight distinct sharp tubercular spines are present in the posterior half of these carinae. The carapace is irregularly and rather finely granular. The granules are largest and sharpest anterodorsally. Between the tubercles numerous short hairs are implanted, giving the animal a hirsute appearance. There are two supraorbital spines, one placed behind the other. The anterior of these is placed some distance behind the orbital margin and is the largest; the posterior is often double or replaced by a row of spinules. The antennal spine is strong and placed on the anterior margin of the carapace, some distance behind it there is a small tubercle or spinule.
The postcervical groove is distinct and crosses the dorsum. It first goes down, where it meets with the urogastric groove it curves angularly forward to finally curve down again and with an anteriorly convex curve merge with the hepatic groove. The hepatice groove curves up and connects with the postcervical groove with the cervical, forming a kind of loop. The cervical groove is distinct as fas as point α and then fades out. The urogastric groove is visible as a posterior branch of the upper part of the postcervical groove, it is short and does not reach the middorsal line. The sellar groove meets with the postcervical groove on the median line; from there it curves forward and down. The parabranchial groove is present, but not very distinct, it joins the postcervical groove somewhat above its lower end and is almost parallel with the middle part of the postvervical groove. Antennal and gastro-orbital grooves are present. The incisura clavicularis is distinct and consists of two small overlapping lobes; above the incisura sometimes a third lobe or some granules are visible on the anterior margin of the carapace. The marginal carina reaches up to the icisura clavicularis; it is distinct throughout its length, being widest posteriorly.
The abdomen has the anterior half of the somites (i.e. the part that moves under the previous somite in the fully stretched animal) entirely smooth. The posterior part shows a blunt and smooth longitudinal median carina, which is inconspicuous in the first, but distinct in the following somites. Each half of the tergites of the second to sixth abdominal somites has a broad transverse groove and several deep pits. The pleura of the first somite is small and short, and is broadly widened distally; the posterior part is depressed for the reception of the plauron of the second overlaps both the first and the third, it shows two grooves, the posterior of which is the continuation of the groove of the tergite. These two grooves are parallel in their baswal part and curve towards each other distally. The pleuron ends in a blunt apex. The following pleura are narrower, but have the same general shape, only the grooves are less distinct. The sixth somite has the pleura short and broadly triangular. The posterolateral angle of the somite is almost circularly rounded. The posterior margin bears three strong spines: one in the middle and two that are placed just mediad of the rounded posterolateral angle. The posterior margin of the somite is slightly concave in the middle.
The telson is much longer than wide, and longer than the sixth abdominal somite. Its lateral margins bulge slightly in the middle and may carry a strong spine there, otherwise it is unarmed but for a strong posterolateral spine. The posterior margin of the telson is about as wide as the anterior; it is convex and reached beyond the end of the posterolateral spines. The dorsal surface is granular and shows two low and blunt, rather wide diverging carinae.
The eyes are small and without pigment. There still is a corneal portion with recognizable facets, which is about as wide as the stalk. The stylocerite lies on the basal part of the first segment of the antennular peduncle, it is wide, has rounded margins and ends in the small triangular apex. It is rather high and shows a broad crest. The distal of the two segments of the antennular peduncle are of equal length, they are shorter and narrower than the basal segment. The two antennular flagella are of about equal lenth and somewhat more than half as long as the carapace.
There is no scaphocerite. No spines are present on any of the segments of the antenna. The antennal flagellum is almost as long as body.
The epistome bears, slightly behind the opening of the antennal gland a conspicuous high and broad transverse clavicular ridge, which connects with the incisura clavicularis of the carapace. The anterior margin of the epistome, near the bases of the antennulae, forms a high rounded marginal ridge, which is incised in the middle. The posterior ridge of the epistome is high, rounded, and smooth.
The mandible consists of a heavy molar process without distinct teeth and with a distinct three-segmented palp. The maxillula has the upper lacina much higher than the lower; the palp is two-segmented, with the distal segment narrow and whip-like. The maxilla has the two endites deeply cleft so that four narrow lacunae are formed; the palp is slender and simple; the scaphognathite is large and narrow, it ends posteriorly in a long bundle of hairs. The first maxilliped has the two endites separated by a deep notch; the palp is two segmented; the exopod is single, shorter than the palp and undivided; a large epipod is present. The second maxilliped is pediform. It has a large epipod, but no podobranch. The exopod is present, but is reduced; it consists of a single segment, which reaches about to the middle of the merus. The dactylus is almost circular and carries some strong spines. The propodus is wide and short. The third maxilliped reaches with part of the propodus behind the antennal peduncle. The dactylus is elongate, without spines. It is slightly shorter than the propodus. The carpus is about as long as the dactylus and bears one or two spines in the distal part of the lower margin. The merus is longer than any of the preceding segments; its lower margin bears numerous granules and ends anteriorly in a strong spines; a small spine is present on the outer anterolateral angle. The ischium is about as long as the merus; its outer posterior margin is serate with blunt teeth; the inner posterior margin bears a strong dentate ridge extending over almost its full length. No teeth are present on the posterior margin of either merus or ischium. There is a small, but distinct exopod, which bears a multi-articular flagellum; it reaches slightly beyond the middle of the merus.
The branchial form is as follows:
Maxillipeds
Pereiopods
1
2
3
1
2
3
4
5
Pleurobranchs
-
-
-
-
1
1
1
1
Arthrobranchs
-
-
2
2
2
2
2
-
Podobranchs
-
-
1
1
1
1
1
-
Epipods
1
1
1
1
1
1
1
-
Exopods
1
1r
1
-
-
-
-
-
The first pereiopods are very large and heavy. The right and left are equal and reach with the larger part of the carpus beyond the rostrum. The fingers are about as long as the palm. They bear many spinules which on the outer surface of the dactylus are arranged in a single longitudinal row; on the inner surface of the dactylus and on both surfaces of the fixed finger the spinules are irregularly grouped together in the median area. The upper surface of the dactylus is covered by irregularly placed spinules. The apices of the fingers are sharp, curved, and crossing. The cutting edges of both dactylus and fixed finger are provided with one large tooth, that of the dactylus being placed before the one of the fixed finger; the rest of the edges is crentilate or serrate. The palm bears numerous spinules which are densest in the lower half and on the upper surface, least dense in the upper half of both inner and outer surfaces. The median row of the outer surface consists of larger tubercles than the rest. The basal part of the upper half of the palm, both on the outer and inner surfaces bears closely placed soft hairs. Similar hairs are present on the upper and upper inner surface of the carpus. The outer and lower inner surfaces of the carpus bear numerous spines of various sizes, two on the inner surface are conspicuously larger than the rest. The merus is compressed and has the inner and outer surfaces with hardly any spines. The upper surface bears slightly diverging rows of rather large spines and several small spillules. The lower surface is hairy and bears small spinules along the outer and inner margins.
The second pereiopod reaches with the chela beyond the rostrum. The fingers are about 2/3 of the length of the palm. The fingertips are of a dark horn colour and the cutting edges are provided with numerous very short dark horny spinules. A few scattered tufts of setae are present on the chela but no hairy fringes. The carpus is about as long as the palm. The merus is about twice as long as the carpus. The third pereiopod resembles the second, but is more slender; it reaches with part of the chela beyond the rostrum. The fingers are about half as long as the palm, and have the same arrangement of spinules and hairs as in the previous leg. The carpus is distinctly shorter than the palm and slightly more than half as long as the merus.
The fourth leg reaches with part of the propodus beyond the rostrum. The dactylus is simple and half as long as the propodus. The lower margin of the propodus bears a single spine anteriorly. The carpus is somewhat less than 2/3 as long as the propodus. The merus is of about the same length as the propodus. The fifth pereiopod does not reach the end of the rostrum. The dactylus is about half as long as the propodus. The propodus bears no spines. The carpus is slightly less than 2/3 as long as the propodus. The merus is slightly shorter than the propodus.
The sternites at the base of the first pereiopods form two submedian plates, which are placed against each other; each consists of two rounded lobes. Similar plates are present between the bases of the second pereiopods, they are, however, more longitudinally placed, diverging slightly posteriorly; these plates also are more elongate than those of the first pair of legs. In the next sternite the plates form diverging ridges that end in a rather sharp point. In the male the plates of the sternite of the fourth legs form large oblique lobes that are posteriorly rounded and overhang the next somite, forming a cavity in which the male gonopods rest. No plates are visible in the last thoracic sternite. The female sternum differs from that of the male only in the structure of the sternite of the fourth pereiopods. This forms here a thelycum consisting of a high triangular swollen structure that widens posteriorly and ends in a bluntly rounded margin, which is somewhat concave in dorsal view and is minutely incised in the middle. When seen from behind the structure is |¯|-shaped. The posterior opening is filled by an almost vertical plate which is wider than long and has the lateral margins convex.
The abdominal sternites of males and females show no median spines. The first abdominal sternite of the male shows a large rounded lobe laterad of the base of the pleopods.
The first pleopod of the male consists of two segments which are immovably fused to form a rigid copulatory organ. The distal segment is broad, it is curved mediad and has the distal margin broadly truncate and slightly emarginate in the middle. The segment is about as long as wide and has all angles broadly rounded. The anterior surface of this distal segment is concave. The basal segment is about as wide as, but distinctly longer than the distal; it shows a large blunt lobe on the distal parrt of its mediad margin. The second male pleopod as he endo- and exopodod slender. The endopod is widened in the middle and bears a short and broad appendix masculina, which does not reach to the end of the endopod proper, it is about triangular in shape. The third to fifth pleopods have the endo- and exopod slender, without appendices; however, there is a small lobe visible in the basal half of the inner margin of the endopod; this lobe possible is a verstige of an appendix interna. In the female the first pleopod is present and consists of a single branch formed by two narrow segments, a short basal and a long distal one. The following pleopods resemble the last three pairs of pleopods in the male, the lobe on the inner margin of the endopod is somewhat more distinct.
The protopodite of the uropods has the two lobes over the bases of the endo- and exopod ending in a small spine. The endo- and exopod are broad. The outer margin of the endopod ends in a single spine. The exopod shows a distinct diaeresis, which carries on its anterior margin about 20 sharp spinules of various sizes. The outer margin of the exopod shows about 6 to 10 spines, the largest of which is placed at the end. Both the exo- and endopod carry a median longitudinal carina, the exopod moreover shows an indication of a second carina laterad of the base of the first.
Size.—The males have a carapace length (rostrum included) varying between 50 and 82 mm, without rostrum between 36 and 61 mm. In females these figures are 65 to 99 mm (with rostrum), and 44 to 67 mm (without rostrum). An ovigerous female of which the rostrum is broken had the carapace length (without rostrum) 51 mm. The eggs are 1.7 mm in diameter.
Distribution.—The species is known from the continental slope of the Atlantic coast of southern South America between about 40° and 52°S and between 55° and 61°W. The only previous localities reported for the species are those of the type material given by Zarenkov & Semenov (1972) and all of which lie off Santa Cruz Province, Argentina, north of the Falkland Islands: 47°31.5’S 61°00’W, 243-275 m; 47°48.5'S 60°34'W, 510-540 m; 48°07.3'S 61°17.3'W, 135-145 m; and 49°09.2'S 59°34.8'W, 405 m. The present material extends the known range of the species both to the north and to the south.
Habitat.—The depth at which Thymops birsteini has been taken varies between 145 (actually 135-145) and 1200 m; all of the present material has been caught at distinctly greater depths (600-1200 m) than the type material (135-540m).
Remarks.—Thymops birsteini resembeles Thymopsis nilenta closet, especially in the presence of the parabranchial grooves, in the wide abdominal pleura, and the poorly developed exopods of the second and third different arrangement of the branchiocardiac and sellar grooves, in possessing spines on the lateral margin of the telson and the uropodal exopods, and in having three spines on the posterior margin of the sixth abdominal somite. The presence of exopods, although somewhat reduced in size; on the second and third maxillipeds is one of the main characters to separate the present genus and Thymopsis.
The description and figures of the present species, which I considered new and planned to name in honour of Dr. E. E. Boschi, Mar del Plata, Argentina, had long been finished when I received through the kindness of Messrs. Zarenkov and Semenov a reprint of their resent paper describing Nephropides birsteini. There can be little doubt that the present material actually belongs to that species. The few discrepancies between the original description and the WALKTER HERWIG material on the other, probably are due to different interpretations and slight inaccuracies. Zarenkov & Semenov stated that the fifth pereiopod lacks the pleurobranch, but possesses an arthrobranch, Thymops agrees with all other genera of this family. None of my specimens has an epipod on the fifth pereiopod as indicated by Zarenkov & Semenov. Two arthrobranchs and a podobranch are present on the third maxilliped in my material; according to the branchial formula given by Zarenkov & Semenov all three are lacking in their speciemens.
I want to thank Dr. E. E. Boschi, Instituto de Biologia Marine, Mar del Plata, Argentina, for his kindness in placing this most interesting WALTER HERWIG material at my disposal."
(Holthuis, 1974)
