Nilenta Lobsterette

Thymopsis nilenta, new species

Figs. 10, 11, 12

Material.—S.E. of the Falkland Islands, 55°01'S-55°10'S, 39°55'W-39°46'W; 2886-3040 m; 10 feet Blake trawl; 8 February 1966; ELTANIN 22, sta. 1537, 1 male, 1 female paratypes (W: U.S.N.M. no. 141258). S of South Georgia, 60°04'S-60°08'S, 35°59'W-36°04'W; 1976-2068 m; 10 feet Blake trawl; 15 February 1966; ELTANIN 22 sta. 1555, 2 females, (L,W). The female specimen (cl. 72mm) preserved in the collection of the Smithsonian Institution (U.S.N.M. no. 141257) is the holotype.

Description.—The rostrum is slender, reaching slightly beyond the antennular peduncle. Its dorsal surface is flattened and bears five to eight lateral spines. From the posterior part of the rostrum two low and inconspicuous subdorsal carinae extend posteriorly, being still slightly visible in the anterior part of the carapace; they bear about seven or eight spinules one or two of which are placed on the rostrum proper. The rostrum is triangular in cross section, its lower margin bears four teeth in the extreme distal part and is irregularly serrate in the rest of its length.

Apart from the two inconspicuous subdorsal carinae the carapace bears on either side a short longitudinal supraorbital carina, which ends anteriorly in a strong supraorbital spine, behind which there are three somewhat smaller spinules. There is also a strong antennal spine,which is placed on the anterior margin of the carapace and is followed by a short granular antennal carina. The lateral rostral carina continues in the orbital carina. The anterior margin of the carapace forms a blunt rectangle at the lower orbital angle. The anterolateral margin of the carapace shows a triangular clavicular incision; the ventral lobe of the incision is distinct and almost rectangular, the dorsal lobe is inconspicuous and merges gradually with the margin. The incision tits around a ridge of the epistorne.

The postcervical groove is wide and does not reach the dorsum, it is replaced there by the sellar groove. The branchiocardiac groove is distinct and reaches far posteriorly. A beginning of the intercervical groove is visible. The parabranchial groove is well developed and reaches almost to the branchiocardiac groove. The postcervical groove connects through the distinct hepatic groove with the cervical. The latter reaches up to and slightly beyond point α.. The gastro-orbital groove is indistinct, but the antennal groove is well marked in its basal part and branches distally. There is no median carina on the carapace, but an irregular smooth, slightly sunken median area before the postcervical groove and a smooth median line behind it indicate the median groove. A marginal groove is present.

All abdominal somites show a distinct transverse groove, which separates the smooth anterior part which disappears under the previous somite (or under the carapace in the case of the first somite) and a rougher posterior part. In the first somite the posterior part shows as a broad rounded transverse carina which has some shallow and inconspicuous depressions on its dorsal margin_. In somites 2 to 5 there is a distinct median carina in the posterior part. This carina is rather low and rounded, but well discernable. On each side of this carina the posterior part of the tergum shows a transverse groove extending to the base of the pleura. Before this groove, in the lateral part of the tergum, some irregular depressions can be seen. The groove is distinct in somites 2 and 3, less conspicuous and interrupted in somites 4 and 5. The posterior margin of all these somites is straight and unarmed. The tergum of the sixth somite shows the median carina in the anterior half. The rest of the tergum shows irregular depressions. The posterior spine of the sixth somite is rather irregular and ends in a sharp median no spines are present in the lateral part of this margin. The pleura of the first abdominal somite are very short, rounded and with irregularly and inconspicuously crenulate margins. The pleura of the second somite are large, oval, longer than wide, and end distally in a small posteriorly directed sharp tip. The surface is faintly granular. In the middle part of the base there is a deep depression followed by a ridge-like elevation. The pleura of the third to fifth somites are similar to that of the second, but smaller; they too end in a posteriorly directed tooth. The pleura of somite 3 still show a median depression at their base, in somites 4 and 5 there is a ridge over the pleural base, which is interrupted posteriorly. The pleura of the sixth somite are blunt and rather smooth; the posterolateral angle of the somite is rounded. The telson is elongate quadrangular. Its surface is granular and shows two indistinct, posteriorly diverging carinae, which end in the posterolateral spines. The lateral margin of the telson is armed with one or two small lateral and one large posterolateral spine; the posterior margin is convex and setose, but not spinose or granular.

The eyes are strongly reduced. The cornea is small, being distinctly shorter than, and about as broad as the peduncle. It is unpigmented and no distinct corneal elements can be seen.

The antennular peduncle consists of three unarmed segments; the basal is the longest and widest. The stylocerite lies as an oval body on the upper surface of this basal segment, and shows a blunt tooth antero-dorsally. The inner part of the anterior margin of the stylocerite is incised. The flagella are short, being about half as long as the carapace and rostrum together.

The antenna has the segments very short and stubby, the ultimate and penultimate segments are of about the same length. There is no antennal scale.

The epistome ends anteriorly in three short ridges, the median of which is separated from the two submedian by very deep grooves, in such a way as if the middle risge is part of another somite. The submedian ridges are rounded, the median bears a distinct tooth. The posterior margin of the epistome is marked by a high ridge with a narrow groove just in front of it. The upper margin of the ridge is smooth or shows a few tubercles. The clavicular ridge is high externally, less conspicuous medially.

The mandible consists of a heavy molar process, which bears a few teeth only, and a three-segmented palp. The maxillula has the upper lacinia somewhat larger than the lower, tha palp is two-segmented, with the ultimate segment whip-like but undivided. The maxilla has the two endites deeply incised so that four slender laciniae are formed, the upper and lower of which are widest; the palp is unsegmented and well developed; the scaphocerite is large. The first maxilliped has the two endites separated by a deep incision. The palp is large, divided into two segments and triangular in transverse section. The exopod is slightly smaller than the palp and undivided; the epipod is large. The second maxilliped is pediform, it has a large epipod, but no podobranch, and the exopod is virtually absent, being reduced to a minute knob. The dactylus is short and semicircular, it is provided with strong dark spines along the distal margin. The propodus is slightly wider than the dactylus and is considerably wider than long. The third maxilliped reaches with part of the dactylus beyond the rostrum. The dactylus is elongate, widest somewhat above the basis and it ends in a sharp dark point. The propodus is about as long as the dactylus. The carpus is somewhat shorter and bears a spine in the ventral part of its distal margin. The merus is about 1.5 times as long as the dactylus; it bears a distinct antero-dorsal spine, while another large spine is present at the antero-ventral angel of the outer surface; behind this last spine the lower margin of the outer surface is somewhat serrate. The ischium has a very small anterodorsal spinule; a dentate row is present on the inner posterior margin; the outer posterior margin is slightly serrate. The exopod is even more strongly reduced than in the second maxiliped and is hardly visible, for practical purposes it may be considered absent

The brachial formula 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

-

-

-

-

-

-

-

The left and right pereipods of the first pair are equal; they reach with the larger part of the carpus beyond the rostrum. They are very heavy and spinous all over. The fingers are slightly longer than the palm. The chela is laterally compressed. The cutting edges of both fingers are provided with numerous (20 to 25) small rounded teeth of about equal size. Both fingers has the tips curved so that these are crossing. The outer surface of the fingers shows a longitudinal ridge with several spinular, at each side of this ridge there is an area without or with a few spinulea only. The lower margin of the fixed finger show a single row of spunlues, the upper margin of the dactylus has several rows of spinules. The carpus is also covered by scattered spinules, of which some on the upper margin and a few on the inner surface are larger than the rest. The merus has the inner surface smooth and also shows a large smooth area over the middle of the outer surface; the upper and lower part of the outer surface of the merus bear several spinules. The upper margin bears three or four larger spines, one of which is subdistal; these spines are not placed in a distinct row.

The second pereiopods reach with part of the carpus beyond the rostrum. The fingers are slightly shorter than the palm. The merus is slightly longer than the chela and about twice as long as the carpus. The third leg is somewhat more slender than the second. It reaches with part of the palm beyond the rostrum. The fingers are half as long as the palm. The carpus is about half as long as the chela, while the merus is somewhat shorter than the chela.

The last two pereiopods have no chela, but simple dactyli. The dactylus is about half as long as the propodus. The carpus is about 2/3 as long as the propodus, and the merus is about as long as the propodus. The second to fifth pereiopods have no spines or spinules, but are smooth with scattered tufts of hairs.

The first and second legs of the female are placed close together. Between the bases of the third legs there are two oblique, anteriorly converging submedian ridges, which end posteriorly in a rather sharp tooth and show a blunt notch in the middle; they end rectangularly anteriorly. The thelycum, situated between the fourth pereiopods, consists of a triangular smooth, somewhat swollen structure. Its anterior end is bluntly truncated, it widens posteriorly and has the posterior margin V-shapedly incised in the middle. Behind this anterior structure there is a rounded flat posterior median plate on the last thoracic somite. This plate fits against the posterior surface of the anterior structure, it lies somewhat lower and is directed obliquely upward. The posterior margin of the last thoracic sternite is straight.

In the male the sternal plates of the first three pereiopods are as in the female, but narrower, less swollen and slenderer. Those of the fourth leg are strongly produced backward and their posterior parts reach over the base of the last thoracic sternite as oblique circular lobes, their anterior parts being low and slightly convex. The fifth sternite shows no special structures.

The first pleopod of the male has become a short and stubby rigid copulation organ. The two segments are so closely fused that the line separating the two could not be detected. The distal part of the organ is directed mediad and has the top broadly truncated, it is of about equal width throughout its length and is concave anteriorly. It is placed obliquely on the lower part, which is triangular with broadly rounded angles, narrowing towards the base. This lower part also is hollowed anteriorly; the hollowed anterior surface shows a ridge in the middle distal part, which ridge continues in the inner margin of the distal part. The second pleopod of the male has the appendix masculina equilateral triangular; the outer margin is naked, the inner bears many long, rather stiff hairs. The exopod and the distal part of the endopod are articulated.

The first pleopod of the female consists of a single branch formed by a short basal segment and a much longer distal segment, which is subdivided into a large basal and six or seven smaller distal subsegments. The following pleopods of the female as well as the third to fifth pleopods of the male have the endo- and exopod elongate, without any appendix; the inner margin of the endopod shows a lobe in the basal half, this lobe bears some stiff hairs and may be a remnant of the appendix interna. The endo- and exopod are elongate with the greatest width slightly below the top.

The abdominal sternites of neither male or female show any spines.

The uropods are large and wide, they are slightly longer than the telson. The two dorsal lobes of the protopod each end in a sharp spine. The endo- and exopod both have a strong posterolateral spine. The exopod shows a distinct and complete diaeresis, which is provided with a row of spinules. The posterior part of the exopod articulates movably with the anterior part along the diaeresis. A longitudinal carina is present on both exo- and endopod. The outer margin of the exopod is slightly serrate.

Size.—The carapace length (rostrum excluded) of the examined females is 53 and 57 mm, in the male it is 54 mm.

Remarks.—The present genus differs from all known genera of Nephropidae in the absence of exopods on the second and third maxillipeds. In the absence of scaphocerite it resembles Thymops, Nephropsis and Nephropides, and agrees with the first and the last of these genera in having granular claws and carapace, and in the relatively wide abdominal pleura. It finds its closest relative in Thymops and at first was thought to be identical with that genus. It differs from Thymops, however, in the presence of ventral teeth on the rostrum, in the postcervical groove which does not reach the dorsum of the carapace, in the absence of lateral spines on the posterior margin of the sixth abdominal somite, and in the complete absence of exopods on the second and third maxillipeds. It is remarkable that the ranges of Thymopsis nilenta and Thymops birsteini are so close together in the South Atlantic. However, Thymopsis occurs far deeper than Thymops. The depth range of the former is (1976-) 2068 to 2886 (-3040) m, that of the latter (135-) 145 to 1200 m.

(Holthuis, 1974)

Thymopsis nilenta is a species of lobster and the only species in the genus Thymopsis. It is found around the Falkland Islands and South Georgia at depths of 1,976 to 3,040 metres (6,483 to 9,974 ft). It reaches a total length of 15 centimetres (5.9 in), of which the carapace makes up about 5 to 6 cm (2.0 to 2.4 in). It is known from a total of four specimens collected from two localities.