Macrobrachium Bate, 1868

Unnikrishnan, V., 2013, Description of a new subspecies, Macrobrachium aemulum madhusoodani (Decapoda: Palaemonidae) from Neyyar River, Kerala, S. India, Zootaxa 3722 (1) : -

Genus Macrobrachium Bate, 1868

Macrobrachium is characterised by the absence of branchio-stegal spine, presence of hepatic spine and mandibular palp, dactyl of last 3 legs simple, second chelate leg more robust, mandibular palp 3- jointed, third maxilliped with arthrobranch and pleurobranch, propodus of fifth pereiopod with transverse rows of setae on posterio-distal margin. Second chelate legs equal or sub equal.

Macrobrachium aemulum madhusoodani subsp. nov. ( Figs. 1–4 )

Materials examined. Analysis of the nine specimens of the new subspecies collected from the middle reaches of the Neyyar River (77°7´30.83´´E 8°29´25.65´´N) Kerala, South India, on 24 March 2007, showed 8 males and 1 female. One male specimen is described here as the holotype (no.1, Table 1 ). Holotype and Paratype (no. 2, Table 1 ) are deposited in the faunal depository of the Zoological Survey of India, Western Ghats Regional Centre, Kozhikkode (Reg. nos.- holotype- ZSI/WGRS/IR/INV 2167; paratype- ZSI/WGRS/IR/INV 2168). The paratypes (nos. 3–9) are currently stored in the Natural History Museum of the Department of Zoology and Research Centre, Mahatma Gandhi College, Thiruvananthapuram (MGC/TVM/NHM-INV/3/2012). All length measurements are in mm. and weight in grams.

Measurement of holotype: total length, 60; carapace length, 18; length of rostrum, 12; length of telson, 7.0; first chelate leg: ischium (i) 3.0; merus (m) 6.0; carpus (c) 7.0; propodus (p) 4.0; dactyl (d) 1.5

Left second chelate leg (large): i = 8.0, m = 13.0, c = 22.0, p = 30.0, d = 9.0

Right second chelate leg (small): i = 7.0, m = 11.0, c = 16.0, p = 23.0, d = 7.0

First non-chelate leg (pereiopod 3): i = 3.0, m = 7.0, c = 4.0, p = 6.0, d = 2.0

Second non-chelate leg (pereiopod 4): i = 3.0, m = 7.0, c = 4.0, p = 6.0, d = 2.0

Third non-chelate leg (pereiopod 5): i = 3.0, m = 7.0, c = 4.0, p = 6.0, d = 2.0

Diagnosis. Rostral formula 9–13/2 with 4–5 postorbital teeth; carapace smooth; distal end of rostrum directed upwards and upper margin convex, second chelate legs robust, dissimilar in shape and unequal in size. In large second pereiopod, merus shorter than carpus and about half length of chela; carpus shorter than chela, palm longer than fingers; width of palm slightly greater than width of distal carpus; palm of larger chela narrower than smaller chela; fingers stout and equal. Two large denticles and six weak denticles respectively at the proximal and distal parts of movable finger. Similar pattern of dentition is present in the immovable finger also. In small second pereiopod, merus shorter than carpus, about half-length of chela; carpus shorter than chela, palm longer than fingers; width of palm greater than width of distal carpus. Fingers stout, equal, unarmed and wide gap between fingers when closed. Numerous setae present on the inner margin of both fingers. Four pairs of plumose setae present between inner movable spines of telson.

Description. Rostrum short, extending as far as the middle of distal segment of antennular peduncle; dorsal margin convex with 9–13 teeth, 4–5 postorbital (table 1), distal end directed upwards and ventral margin with two teeth. Small setae present between teeth, on both dorsal and ventral margins.

Carapace smooth, both antennal and hepatic spines characteristic of Macrobrachium , latter situated just below and behind level of former (figs. 3A, 4). Abdomen glabrous, pleurae of somites I–III typical, those of IV and V directed backwards, and of VI ending in spine. Telson conical with two pairs of spines on dorsal surface, proximal pair situated midway and distal at about ¾ length of telson. Posterior end of telson with two pairs of movable spines; inner longer, overreaching tip of telson. Four pairs of plumose setae present between inner pair of movable spines (fig. 2A). Antennules typical, outer lateral spine of antennal scale sharp, pointed forwards.

First pair of chelate legs slender, tips of chelae over reaching antennal scale; ischium half merus, merus shorter than carpus; carpus almost twice length of chela; palm longer than fingers and devoid of spination (fig. 3B).

Second chelate legs dissimilar, unequal and covered with spinules (fig. 1). Length range of larger second chelate leg 19.5 to 73.0 mm, 0.57 to 1.22 times total body length. Merus shorter than carpus (fig. 2B), about half length of chela; carpus shorter than chela; palm longer than fingers. Width of palm slightly greater than width of distal carpus; palm of larger chela narrower than smaller (fig. 2B, D). Fingers stout and equal, two large denticles at proximal part of movable finger, six weak denticles at distal part. Similar pattern of dentition present in immovable finger also (fig. 2C). In small second pereiopod, merus shorter than carpus, about half length of chela; carpus shorter than chela, palm longer than fingers; width of palm greater than width of distal carpus. Fingers stout, equal, unarmed, wide gap between fingers when closed. Numerous setae on inner margin of both fingers (fig. 2E). Second large and small, third, fourth and fifth pereiopods densely spinulose, spines distributed uniformly.

Non-chelate legs and pleopods, normal in structure of the genus (fig. 2F, 3D). Appendix interna less than half length of appendix masculina, spinous setae on inner side (fig. 3C). In the present specimens, telson extends beyond level of outer lateral spine of uropodal exopod.

Remarks. The present specimens share most of its characters with M. aemulum (Nobili, 1906) collected and described from New Caledonia and subsequently recorded from India by Jayachandran and Joseph (1988), but there are perceptible differences with these specimens in certain distinctive diagnostic characters (table 2).

According to Nobili (1906), in M. aemulum , rostrum short, always a little shorter than antennular peduncle, slightly turned down, but elevated a little towards tip with rostral formula 10–13/1–3 with 4–5 dorsal rostral teeth, carapace smooth. Second chelate legs unequal in size, larger second pereiopod with merus shorter than carpus and about half the size of chela (fig.5), carpus shorter than chela. Basal third of the movable finger with a big tooth, preceded at the articulation by two small teeth; followed by another big tooth in the middle; over which follow one another to the ends 15–16 pairs of blackish tubercles, a little more larger than the tubercles lining the finger. The smaller chelate leg repeats nearly the same provision, but the fingers much shorter and more arched and far apart. Ischium shorter than merus, merus about half size of chela, carpus shorter than chela; palm almost twice length of fingers. Telson bears two pairs of distal spines; inner pair much longer than the outer.

FIGURE 2. Macrobrachium aemulum madhusoodani subsp. nov., specimen no. 1, holotype (male). A, telson and uropod; B, 2nd large chelate leg; C, 2nd large chela enlarged; D, 2nd small chelate leg; E, 2nd small chela enlarged; F, 3rd pereiopod, distal part.

Jayachandran and Joseph (1988) recorded M. aemulum from Indian waters, and compared the lengths of the podomeres of second chelate leg of specimens from New Caledonia and India to establish similarities in their morphometric characters and also noticed some other variations in Indian specimens. Width of rostrum just in front of orbit slightly greater, 2nd cheliped unequal, palm width of smaller cheliped greater than that of the larger, gap between fingers when closed wider in smaller cheliped than larger etc. are the variations. Rostral formula 11/2–3 also is within the range 10–13/1–3 seen in New Caledonian specimens.

FIGURE 3. Macrobrachium aemulum madhusoodani subsp. nov., specimen no. 1, holotype (male). A, carapace-right lateral view; B, 1st pereiopod, distal part; C, 2nd pleopod of male; D, 3rd pleopod.

There are similarities and differences between the new subspecies and the M. aemulum recorded by Jayachandran and Joseph. In M. aemulum , rostrum short, extends up to the middle of distal segment of antennular peduncle, tip very sharp, directed slightly upwards. In the new subspecies also the same characters could be seen. Jayachandran recorded 11 dorsal rostral teeth in M. aemulum whereas in the new subspecies, it ranges from 9–13. However, number of dorsal rostral postorbital teeth is 4–5 (interval between first four teeth greater than the remaining teeth), which is similar to the situation in the new subspecies. Moreover ventral margin of rostrum in M. aemulum has 2–3 teeth (situated in between 9th and 11th dorsal teeth), but in new subspecies, it is 2 only, whereas carapace smooth in both specimens.

In first pereiopod of M. aemulum , ischium about half size of merus; merus shorter than carpus, carpus twice size of chela, palm longer than fingers and the new subspecies also agree with these characters. In M. aemulum , and it’s new subspecies, second pereiopods are strong, distinctly unequal and dissimilar in shape. Merus shorter than carpus, about half length of chela in larger second pereiopod of M. aemulum ; carpus shorter than chela (fig. 6). Width of palm slightly greater than that of distal carpus, palm of larger chela, narrower than smaller. Proximal part of cutting edges of movable finger with two denticles, fixed finger with one, rest of cutting edge entire. Whole leg scabrous due to close-set spinules. The new subspecies, share characters with M. aemulum , like merus shorter than carpus, about half-length of chela, carpus shorter than chela, palm longer than fingers; width of palm slightly greater than that of distal carpus. Palm of larger chelate leg narrower than the smaller chelate. Fingers are stout and equal, but in the new subspecies movable finger has two large denticles in the proximal part and six weak denticles in the distal part. Similar pattern of dentition is present on immovable finger also.

In smaller second pereiopod of M. aemulum , ischium shorter than merus, merus about half size of chela, carpus shorter than chela; palm more than twice length of fingers. Fingers stout, equal, distinct gap when closed; wider than that between fingers of larger chela. Cutting edges of fingers with 1–2 weak denticles in proximal part, stiff setae at cutting edges. The new subspecies also show similar characters except that cutting edges of fingers unarmed, numerous stiff setae at cutting edges of fingers.

In M. aemulum telson reaches up to the level of the outer lateral spine of the uropodal exopod, whereas in the present specimens it extends beyond. In both M. aemulum and new subspecies, there are four pairs of plumose setae between inner spines of telson.

FIGURE 5. Macrobrachium aemulum (Nobili, 1906) from New Caledonia. A, 2nd small chelate leg; B, 2nd large chelate leg. (Courtesy, Bull. Mus. Histoire Naturelle, Paris)

FIGURE 6. Macrobrachium aemulum (Nobili, 1906) from India, Jayachandran and Joseph, 1988. A, 2nd small chelate leg; B, 2nd large chelate leg. (Courtesy, Oxford and IBH Publ. Company Pvt. Ltd.).

In the specimens of Nobili (1906) and Roux (1926), fingers of second chelate legs are slender. However, in M. aemulum reported by Jayachandran and Joseph and new subspecies, fingers of second chelate legs are stout. The new subspecies has two large denticles at proximal part of movable finger and six weak denticles at the distal part. Similar pattern of dentition is present in immovable finger also. But the specimens of Nobili and Roux have a strong triangular tooth at about half the length of fixed finger.

The collections during the pre monsoon consisted of M. aemulum (11.8%) and new subspecies (7.3%). However, during the monsoon and post monsoon seasons, representation of M. aemulum (Nobili, 1906) in the collections was 11.8 % and 6.4% respectively whereas new subspecies could not be found. Even though the new subspecies described here share some characters with the specimens of Nobili, whether it can be designated as new species has to be confirmed only after gene sequencing. This was not attempted as prolonged formalin preservation often fails to produce precision results.

Etymology. The subspecies name is in honor of P. Madhusoodanan Pillai, former Professor, Mahatma Gandhi College, Thiruvananthapuram and teacher of the junior author. The name thus is a noun in genitive singular.

Figures

FIGURE 1. Macrobrachium aemulum madhusoodani subsp. nov., specimen no. 1, holotype (male). A, entire animal, dorsal view

FIGURE 4. Macrobrachium aemulum madhusoodani subsp. nov., specimen no. 2, paratype. Carapace left lateral view.

Tables

TABLE 1. Morphometric and meristic features of Macrobrachium aemulum madhusoodani subsp. nov. (length in mm and weight in g.)

Specimen nos.	1	2	3	4	5	6	7	8	9
Total length	60	75	40	43	38	36	34	64	58
Carapace length	18	20	11	13	11	10	10	18	17
Length of rostrum	12	14	7.0	10	9.0	8.0	8.0	14	11
Max. width of carapace	10.5	12.0	5.5	7.0	6.0	6.2	6.0	11.0	9.0
Preorbital teeth	9	6	6	6	6	5	6	6	6
Postorbital teeth	4	5	4	5	5	4	4	5	4
Rostral formula	13/2	11/2	10/2	11/2	11/2	9/2	10/2	11/2	10/2
Length of large 2nd chelate leg	73.0	69.0	-	-	-	22.0	19.5	69.0	-
Length of small 2nd chelate leg	57.0	49.0	14.5	-	-	-	18.0	53.0	49.0
Uropod length	10.0	10.0	7.0	9.0	7.0	6.0	7.0	9.0	8.5
Telson length	7.0	7.0	5.0	7.0	5.0	4.0	6.0	7.0	6.5
Weight	3.94	4.74	0.64	1.28	0.65	0.60	0.56	3.59	2.53
Sex	♂	♂	♂	♂	♂	♀	♂	♂	♂