Four new species of the genus Rhopalophthalmus (Mysidacea: Crustacea) from the northwest coast of India

Four species belonging to the genus Rhopalophthalmus—R. mumbayensis, R. anishi, R. murudana, and R. vijayai—are described as new to science. All four species are distinguishable from each other and from other related species by the combination of characters afforded by the number of spines on the antennal sympod, number of carpopropodal segments of the thoracic endopods, relative length of the vestigial endopod of the eighth thoracic limb, number of lateral spines on the telson, and the relative length of the distal spines.


Introduction
The genus Rhopalophthalmus was instituted by Illig (1906) for the reception of type species R. flagellipes captured by the S.S. Valdivia off Banana in the Congo Estuary. Of the 18 species which have up to the present been referred to this genus, only five species-R. chilkensis Tattersall, 1957, R. kempi Tattersall, 1957, R. tattersallae Pillai, 1961, R. indicus Pillai, 1961and R. macropsis Pillai, 1964-have been recorded from Indian waters. By the present work four more species, R. mumbayensis, R. anishi, R. murudana, and R. vijayai, have been added to the list, which are all new to science. The study area included coastal waters off Murud, Daman, Dahej and creeks around Mumbai (Dharamtar, Thane, Bassein, Kasheli). Currents within the creek system are almost entirely due to the tidal ebb and flow and the temporal and spatial variations in salinity are governed by the quantum of sea water influx and the riverine fresh water flow. The interior part of this complex creek system is fringed by rich mangrove vegetation and salt marsh and receives large quantities of waste water, thereby enhancing the nutrient level which in turn supports high plankton production. Kasheli, the area of study, is the mid-estuarine zone of Ulhas estuary. The materials were collected with a Heron Tranter Net (Tranter et al. 1972) having a mouth area of 0.25 m 2 and a mesh size of 0.33 mm. The common and specific characteristics of the known species of Rhopalophthalmus from the coastal waters of India are given in Table I. All the type specimens are stored in the reference collection of the Indian Ocean Biological Centre (RC, NIO, Kochi). The descriptive terminology employed follows to a great extent Pillai (1973).

Description
Body robust and strongly built. Carapace with dorso-median nodules; anterior margin lacking rostrum, postorbital spines prominent, keels prominent, cheeks sinuous. Eyes stout, extending to distal end of first segment of antennule, cornea occupying little more than half of eye and wider than stalk (Figures 1, 2).
Antennule more robust in male, first segment longer than rest of peduncle, outer distal angle drawn out and armed with few stout plumose setae, outer margin armed with row of 10 long curved plumose setae, inner margin with five to six short slender plumose setae; second segment short and armed with two long and three short hooked setae on inner distal angle, four setae on outer distal angle and three setae on dorsal margin; third segment broader and armed with one spinous seta on outer margin, and four hooked setae and group of seven long plumose setae on inner distal margin; mid-dorsal lobe between flagella present, base of outer flagellum swollen and thickly hirsute ( Figure 3). In female antennular peduncle longer, first segment with row of 13 long curved plumose setae along outer margin; second segment with setae on outer and inner distal angles and on mid-dorsal margin; third segment with group of long plumose setae extending from inner distal angle to inner middle margin, hooked setae absent ( Figure 4). Antennal scale slightly overreaching antennular peduncle, nearly five times as long as broad, outer margin straight and terminating in strong spine extending beyond rounded apex; antennal peduncle short, less than half of scale; inner distal angle of sympod armed with two long and two short spines, second spine longest ( Figure 5). In young and immature specimens one or two secondary spinules present on second longest spine.
Labrum wider than long with transverse anterior margin and without any process in front ( Figure 6). Mandibular palp slender, second segment four times as long as third and with row of barbed setae; third segment armed with group of modified setae on distal border  Basis of first thoracic endopod with prominent lobe, claw present (Figure 11) second; thoracic endopod stout, dactylus armed with strong peculiarly barbed spiniform setae ( Figure 12). Endopods of third to seventh thoracic limbs slender, becoming longer in posterior pairs; third and fourth endopods with three to four, fifth endopod with four to five, and sixth and seventh endopods with five to six propodal segments (Figures 13-16). All endopods profusely setose, setae on seventh endopod spinulose ( Figure 17); outer distal corner of basal plate of exopod rounded, flagelliform part 11-14-segmented. Endopod of eighth limb three-segmented, second segment short with five long plumose setae on inner margin, third segment long and bent, shorter than basal segment of exopod lacking setae ( Figure 18). In female, endopod unsegmented, tapering, shorter than basal plate of exopod and armed with short simple seta on inner margin ( Figure 19).
Telson broad, 1.3 times longer than last abdominal segment, 2.3 times as long as basal width, abruptly constricted near base to form waist, slightly broadening towards middle and gradually narrowing towards broadly rounded apex; lateral margins armed along distal half with 11-12 stout spines in male, gradually increasing in length towards posterior, apex with two pairs of long stout spines, outer pair slightly longer than inner pair and nearly onequarter length of telson, each spine with row of bilaterally arranged subsidiary teeth progressively more flattened towards tip of spine (Figures 26,27). In female, lateral margin of telson armed with 12-14 pairs of spines.
Uropods longer than telson, two-segmented and setose all round; endopod with stout spine near middle on inner margin; exopod longer than endopod ( Figure 25).

Etymology
This species was collected from areas around Mumbai and hence the species name mumbayensis.

Remarks
Rhopalophthalmus mumbayensis sp. nov. is related to R. kempi O. Tattersall, 1951, R. orientalis O. Tattersall, 1957, R. tattersallae Pillai, 1961, R. macropsis Pillai, 1964, and R. longipes Ii, 1964 in having two long and two short spines on the antennal sympod, but they differ from the new species in the following points. In R. kempi, there are only three carpopropodal segments in the third to seventh thoracic endopods, and the vestigial endopod of the eighth thoracic limb of both sexes is longer than the basal plate of the exopod. In R. tattersallae, one of the long spines on the antennal sympod is barbed; thoracic endopods three to seven have only four carpopropodal segments and the inner pair of apical spines of the telson are longer than the outer pair. In R. macropsis and R. longipes, the carpopropodus is four-segmented and the telson is long and narrow. In R. orientalis there are four carpopropodal segments, and the telson has 15-16 lateral spines and the apical pairs of the telson spines are equal in length. The combination of characters afforded by the spines on the antennal sympod, the carpopropodal segments of the thoracic endopods, and the armature of the telson will serve to identify this species.

Ecological note
At Bassein, this species occurred in large numbers during April. Ninety-five per cent of the samples were constituted by adult males and females, and 80% of the females were carrying 6-20 eggs or larvae in the brood pouch. It occurred at 5-10 m depth, where the bottom sediment was sandy or silty clay. The temperature and salinity recorded ranged from 21 to 33uC and from 29.4 to 38.0 psu, respectively. Its occurrence at 5-10 m depth proves it is a littoral species, and the large number of specimens in the samples indicate that this species is a gregarious form.

Description
General form slender. Carapace broadly rounded in front, covering basal part of antennules, post-orbital spines small, continuing backward into faint keels; antero-lateral angles produced into strong spines; cheeks sinuous. Eyes stout, reaching beyond first antennular segment; cornea as broad as stalk and occupying distal third of eye ( Figure 28).
Basal segment of antennule in male as long as rest of peduncle, third segment short and stout, but short basal part of outer flagellum swollen and hirsute ( Figure 29). In female, basal segment much longer than rest of peduncle. Antennal scale long and narrow reaching clearly beyond antennular peduncle, five times as long as broad; antennal peduncle short, less than half length of scale; sympod with two long and two short spines (Figure 30).
Mouthparts and first and second thoracic endopods as in R. mumbayensis sp. nov. Thoracic endopods three to seven with three to five carpopropodal segments (Figures 31-33). Vestigial endopod of male eighth thoracic limb straight, three-segmented, almost as long as basal segment of exopod, second segment with three to four plumose setae on outer distal corner, apex broadly rounded (Figure 34). In female, eighth endopod longer than basal segment of exopod, slender, straight and unsegmented, distal end produced at apex into minute cusp with short seta (Figure 35).
Pleopods in male biramous, first pleopod with unsegmented endopod and 10-segmented exopod ( Figure 36); second pleopod with 10-segmented endopod and 12-segmented exopod, distal segment of exopod with third long barbed setae ( Figure 37); third, fourth, and fifth pleopods similar with 10-segmented exopod and endopod ( Figure 38). Telson moderately broad, longer than last abdominal segment, about twice as long as broad at base, distal half armed with 11 pairs of graduated spines, spines on distal border long and sharply pointed, outer pair distinctly longer than inner pair, subsidiary teeth sharp ( Figure 39).

Etymology
This species is named after the first author's son, Anish.

Remarks
This species can be readily distinguished by the combinations of the following characters: (1) thoracic endopods have three to five carpopropodal segments; (2) vestigial endopod of eighth thoracic limb is longer than the basal segment of the exopod in female and in male almost as long as the basal segment of exopod and (3) spines on the distal border of the telson are sharply pointed and the outer pair of spines are distinctly longer than the inner pair.
Rhopalophthalmus kempi O. Tattersall, 1957 is the only other species of the genus in which the endopod of the eighth thoracic limb in both sexes is longer than the basal segment of the exopod. But R. kempi differs from the new species in having a shorter antennal scale, only three carpopropodal segments and the inner pair of distal spines on the telson are longer than the outer pair.

Ecological note
The new species is a shallow-water form and the temperature and salinity recorded at the time of collection were 25.4uC and 16.4 psu, respectively. more slender than male, first segment 1.5 times longer than rest of peduncle. Antennal scale as long as antennular peduncle, five times as long as broad; antennal peduncle onethird length of scale; antennal sympod with two long and two short spines (Figure 43).

Rhopalophthalmus murudana
Mouthparts and first and second thoracic endopods as in R. mumbayensis. Third to seventh endopods slender, third with two, and fourth to seventh with three propodal segments (Figures 44, 45). Eighth endopod three-segmented; second segment short with five long plumose setae on outer distal margin; third segment cylindrical and bent at right angle, with one stout seta at tip and two on inner margin, when straightened shorter than basal segment of exopod ( Figure 46). Eighth endopod in female unsegmented, straight, shorter than basal segment of exopod ( Figure 47).
Pleopods in male biramous, first pleopod with unsegmented endopod and 10-segmented exopod ( Figure 48). Endopod of second pleopod 11-segmented; exopod long and 12segmented, segments becoming longer posteriorly, distal segment with three long barbed setae (Figure 49). Pleopods three to five with 10-segmented exopod and endopod ( Figure 50). In female, pleopods simple, unsegmented and rod-shaped becoming longer in posterior pairs. Telson 1.5 times longer than last abdominal somite, 2.7 times as long as broad at base, more than distal half of lateral margin armed with 15-16 slender spines gradually increasing in length distally, first two spines very small; inner pair of distal spines slightly longer than outer pair, subsidiary teeth becoming flattened distally (Figure 51).

Etymology
This species is named after the type locality.

Remarks
This species closely resembles R. orientalis, O. Tattersall. The vestigial endopod of the eighth thoracic limb, the number of carpopropodal segments of third to seventh thoracic endopods, and the spines on the telson are very similar in the two species. But in R. orientalis the rostrum is relatively low, eyes are longer, and the subsidiary teeth on the telson are broader and truncate. This species also resembles R. mumbayensis sp. nov. but differs in having fewer carpopropodal segments, spines on the lateral border of the telson are more slender and more in number, and the inner pair of distal spines on the telson are longer than the outer pair.

Ecological note
This species occurred in temperatures and salinity of 25.2-28.5uC and 35.7-36.3 psu, respectively, and was collected from 5 to 9 m depth where the bottom was clayey silt.

Description
General form slender. Carapace short, leaving last three thoracic somites exposed; anteriorly produced into broadly rounded rostrum covering basal one-quarter of antennular peduncle and basal part of eyestalks; two small dorso-median nodules present, postorbital spines small, cheeks sinuous. Eyes extending to distal end of first antennular segment, cornea occupying one-quarter of eye and little narrower than stalk ( Figure 53).
Antennule robust in male, first segment as long as distal two segments combined and with row of 9-10 plumose setae on outer margin, outer distal corner produced and tipped with few setae; second segment short with two hooked setae on inner margin, outer distal corner produced and tipped with setae; third segment broad with one spinous seta on outer margin, four hooked setae on inner margin and five to six long plumose setae on inner distal angle; dorsal lobe tipped with few setae; base of outer flagellum swollen and densely hirsute ( Figure 54). In female antennular peduncle slender and longer, first segment longer than other two segments combined, hooked setae absent; third segment with seven long plumose setae on inner distal angle and four setae on inner margin. Antennal scale as long as antennular peduncle, 4.6 times as long as broad, outer margin straight, terminating in spine projecting beyond rounded apex, inner margin setose; antennal peduncle extending to middle of scale; antennal sympod with two long and two short spines on inner distal corner (Figure 55).
Mouthparts and first and second thoracic endopods as in R. mumbayensis.
Endopods of third to seventh thoracic limbs slender, increasing in length posteriorly; propodus three-segmented in third and five-segmented in fourth to seventh endopods (Figures 56, 57). Eighth endopod in male three-segmented, shorter than proximal segment of exopod, second segment short with three long plumose setae on outer distal corner, second segment long and finger-like with single simple seta at tip (Figure 58). In female eighth endopod unsegmented with two simple setae and shorter than proximal segment ( Figure 59).
Pleopods biramous and natatory. First pleopod with 10-segmented exopod; endopod unsegmented and short, sympod with three plumose setae on inner distal margin and row of long plumose setae along mid-dorsal line ( Figure 60). Second pleopod with 10segmented endopod; exopod 13-segmented, segments increasing in length posteriorly, first five segments with usual pair of plumose setae, distal eight segments without setae, terminal segment ending in three long barbed setae, two at tip, one slightly away from tip ( Figure 61). Third to fifth pleopods in male similar, endopod and exopod 10-segmented. Pleopods in female simple, unsegmented and rod-shaped, distal pairs progressively increasing in length ( Figure 62). Telson 1.4 times longer than last abdominal somite, 2.3 times as long as maximum width at base, abruptly narrowing near base to form waist, lateral margins almost parallel up to distal half and gradually narrowing towards rounded apex and armed along distal half with seven slender subequal spines, distal margin with two pairs of long sub-equal spines, inner pair of spines with 12 and outer pair with 10 pairs of closely set subsidiary teeth, proximal teeth spiniform and distal teeth broader ( Figure 63).
Uropods: exopod and endopod two-segmented, setose all round and longer than telson. Endopod with one strong spine on inner margin ( Figure 64).

Etymology
This species is named in honour of Dr Vijayalakshmi R. Nair, Scientist (retired), NIO.

Remarks
This species can be easily distinguished from all the other species of the genus except R. africana O. Tattersall, 1957 by its slender form and in having fewer spines on the lateral border of the telson. In R. africana, the lateral margin of the telson is armed with six to eight spines only, but differs from the new species in having only two large spines on the antennal sympod, four carpopropodal segments in third to seventh thoracic endopods and in its more robust form. The largest male and female of R. africana measured is 10 and 10.2 mm, respectively, whereas the maximum size of the present species is only 8.6 mm.

Ecological note
The specimens were collected from depths between 8 and 25 m, where the bottom was silty sand, and occurred in temperatures and salinity ranging from 26.9 to 29.0uC and from 23.1 to 29.8 psu, respectively.