Palaemon powelli, Ashelby, Christopher W. & Grave, Sammy De, 2009

Ashelby, Christopher W. & Grave, Sammy De, 2009, A new species of Palaemon (Crustacea, Decapoda, Palaemonidae) from West Africa, with a re-description of Palaemon maculatus (Thallwitz, 1892), Zootaxa 2085, pp. 27-44 : 33-42

publication ID	https://doi.org/ 10.5281/zenodo.187392
DOI	https://doi.org/10.5281/zenodo.6220429
persistent identifier	https://treatment.plazi.org/id/03D6135B-0A0A-C574-39AC-57FCFE9FFB41
treatment provided by	Plazi
scientific name	Palaemon powelli
status	sp. nov.

Treatment

Palaemon powelli View in CoL sp. nov.

Figures 3–8 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8

Palaemon View in CoL species A.– Powell 1983: 273, fig. 15. Palaemon View in CoL sp— Powell 1985: 235.

Material Examined. Holotype: ovigerous Ψ, pocl. 7.7 mm; Oguck, 1 ft bank, ½ km from entrance to Bonny River, Niger Delta, Nigeria; 04˚39’40”N 07˚09’20”; leg. C.B. Powell; NHM1980.325a. Paratypes: 5 ɗɗ, pocl. 3.6 – 4.7 mm; 7 ΨΨ, pocl. 4.6 – 6.5 mm; 4 ovigerous ΨΨ, pocl. 5.8 – 6.5 mm; 1 juvenile pocl. 2.4 mm; same data as Holotype; NHM1980.325b-d. 4 ɗɗ, pocl. 3.6 – 4.8 mm; 5 ovigerous ΨΨ, pocl. 5.9 – 7.6 mm; medium salinity mangrove creeks, Niger Delta; leg. C.B. Powell; RMNH.D 49892. 7 ɗɗ, pocl. 2.2 – 4.1 mm; 4 ovigerous ΨΨ, pocl. 5.5 – 6.0 mm; 18 ΨΨ, pocl. 2.6 – 4.8 mm; medium salinity mangrove creeks, Niger Delta; leg. C.B. Powell; RMNH.D 49887. 2 ɗɗ, pocl. 3.6 – 4.8 mm; 3 ovigerous ΨΨ, pocl. 5.9 – 7.6 mm; medium salinity mangrove creeks, Niger Delta; leg. C.B. Powell; OUMNH-ZC 2008-01-0017.

Comparative Material Examined. Palaemon adspersus : 7 ΨΨ, pocl. 6.3 – 10.3 mm; 2 ovigerous ΨΨ, pocl. 10.5, 10.7 mm; Ria Formosa, Algarve, Portugal; 01/04/2004; leg. A. Whitaker; OUMNHZC 2004-16 - 0 0 0 1.

Palaemon longirostris : 35 ɗɗ, pocl. 5.4 – 9.5 mm; 3 ΨΨ, pocl. 9.7 – 10.9 mm; Guadalquivir Estuary, Spain; 13/11/2004; leg. E. González-Ortegón; OUMNHZC 2005-03-0001. 11 ɗɗ, pocl. 9.5 – 10.8 mm; 2 ΨΨ, pocl. 12.3, 13.6 mm; Tilbury Power Station, Thames, UK; 51°27'6.5"N 0°23'19.0"E; 24/01/2007; leg. R. Kowalik; NHM 2008.872-881.

Description. A small sized Palaemon species.

Carapace glabrous. Rostrum ( Figures 3 View FIGURE 3 A–B, 8A–F) slender, ascendant in distal half, sometimes, slightly concave in middle; approximately equal to pocl., overreaching scaphocerite by 0.125 times length; armed with 6–8 dorsal teeth and 3–4 ventral teeth; proximal three dorsal teeth with weakly constricted bases; second tooth situated above or slightly behind margin of orbit; proximal most tooth 1.5 more distant than gaps between other teeth; distal portion (approximately one third) unarmed but with bifid tip. Double row of setae present in ventral unarmed portion, single row of setae present between teeth. Antennal and branchiostegal teeth present ( Figure 3 View FIGURE 3 C); antennal tooth marginal, branchiostegal tooth variable in position, usually submarginal but may be slightly displaced from margin of carapace by up to 1–1.5 times its length. Branchiostegal groove originating dorsal to branchiostegal tooth, evenly curved and finishing just in front of half carapace length, slightly lower than at its origin. Sub-orbital lobe subquadrate ( Figure 3 View FIGURE 3 C), pterygostomial angle rounded. Bec ocellaire ( Figure 3 View FIGURE 3 C) with convex anterior margin, pronounced beak pointing upwards at approximately 45˚, dorsal surface convex with slight anterior concavity.

Eye ( Figure 3 View FIGURE 3 B) well developed, with pigmented cornea; cornea slightly wider and longer than stalk; ocellus present on dorsomesial side.

Antennular peduncle ( Figure 4 View FIGURE 4 C) extending to level with base of tooth of scaphocerite, or slightly further in males. Basal segment 2 times as long as wide with strongly convex outer margin, stylocerite acute; anterodistal tooth falling short of laminar portion; inner ventro-mesial tooth present; statocyst with statolith; ultimate segment 1.8 times as long as penultimate, their combined length being slightly less than 0.75 times that of basal segment.

Dorsal flagellum of the antennula ( Figure 4 View FIGURE 4 D) fused for just under half its length (approximately 7–10 segments fused, 8–12 free); free portion bearing 2 or 3 aesthetascs per segment.

Scaphocerite ( Figure 3 View FIGURE 3 F) laminar and just over three times as long as broad; outer margin straight or slightly concave terminating in tooth, falling short of distal margin of lamina; basal segment of antenna with large mesial tooth; antenna articulated to lateral margin of this segment. Flagellum of antenna about twice length of body.

Abdominal pleurae ( Figure 3 View FIGURE 3 D) furnished with plumose setae on ventral margin; fifth pleuron posterodistal angle acute, sometimes prolonged into small laterally directed tooth; sixth segment approximately 1.5 times length of fifth; lateral margin ( Figure 3 View FIGURE 3 e) with small tooth and notch disto-ventrally; median lobe blunt, lacking ventral submedian process.

Thoracic sternal armature sexually dimorphic. Fourth thoracic sternite in females armed with sharp tooth, flanked by two submedian bosses, remainder with low transverse ridge; in ovigerous and post-ovigerous females eighth sternite with flattened setose plate. Eighth thoracic sternite in males with small, median tubercle.

Abdominal sternal armature sexually dimorphic. Females with blunt process on fourth abdominal sternite and acute longitudinal ridge on fifth; in males first to third abdominal sternite armed with broad flattened teeth with emarginated tips; longitudinal ridges present on fifth and sixth abdominal sternite. Pre-anal carina unarmed in both sexes.

Mandible ( Figure 4 View FIGURE 4 A) with two segmented palp; terminal segment about 1.5–2 times length of proximal segment; terminal segment bearing two apical simple setae and two lateral setae; proximal segment with slightly swollen inner margin, bearing a single seta distally. Incisor process of mandible with 3 teeth on right mandible, the middle of which is the smallest, and 4 teeth on left mandible, the middle 2 of which are smaller than the outer ones; molar process with 6 teeth of varying sizes. Paragnaths covering about half the mandibles; alae formed by broad, transverse more or less oval distal lobes, ventromesial lobes triangular. Corpus short, narrowly separated; base with two posteriorly diverging carinae. Epistome triangular with rounded anterior angle and strong anteromedial carina. Labrum broadly rectangular, flanked by triangular lobes on each side. Maxillula with lower lacinia near oval, smaller and narrower than upper lacinia; upper lacinia provided with several distal cuspidate and stout setae but otherwise naked; palp with bifid tip; upper process naked, lower process broad with small ventral tubercle that bears a single recurved setiform process. Maxilla with upper lacinia deeply cleft, ending in a number of stout setae, two simple setae on its upper margin; palp well developed, broad and naked except for a few plumose setae proximally on its outer margin; scaphognathite is large, fringed with plumose setae; lower lobe narrower than the upper. First maxilliped with endites separated by distinct notch; palp slender, slightly twisted and provided with single apical seta; exopod well developed, slender and furnished with plumose setae distally; caridean lobe well developed; epipod large and bilobed. Second maxilliped with slender rectangular ultimate segment; penultimate segment broadly triangular, with convex, semicircular upper margin; exopod well developed; well developed podobranch present. Third maxilliped ( Figure 4 View FIGURE 4 B) pediform; ultimate segment 0.8 times length of penultimate; antepenultimate and preceding segment fused, with strongly curved dorsal margin; single spine subdistally; exopod reaches just over half length of antepenultimate segment; epipodal plate cupped, ear shaped; well developed arthrobranch and reduced pleurobranch present.

Well developed pleurobranchiae present on all thoracic legs. First pereiopod ( Figure 5 View FIGURE 5 A) overreaching scaphocerite by length of fingers and half palm; basis 0.5 length of ischium; merus 1.7 length of ischium; carpus slightly longer (1.1x) than merus; chela 0.6 times length of carpus, fingers approximately equal to palm, with tufts of setae; carpal-propodal brush well developed. Second pereiopod ( Figure 5 View FIGURE 5 B) extending beyond scaphocerite by full length of chela and 0.75 length of carpus; ischium 5 times length of basis; merus 1.1 length of ischium; carpus 1.1 length of merus; chela ( Figure 5 View FIGURE 5 C–E) 1.25 times length of carpus, fingers about 1.2 times palm (but may be equal to palm in males and immature females), fingers slender, forcipitous, palm slightly swollen, strong dentition proximally between fingers ( Figure 5 View FIGURE 5 D). Ambulatory pereiopods increase in length, pereiopod 3 being shortest. Third pereiopod ( Figure 6 View FIGURE 6 A) overreaching scaphocerite by 0.5 length of its dactylus; ischium 2 times basis; merus 2.5 times length of ischium; carpus 0.5 length of merus; propodus 2 times length of carpus, approximately equal to merus, ventral margin armed with 6 pairs of cuspidate setae; dactylus simple, about 0.3 length of propodus. Fourth pereiopod overreaching scaphocerite by one sixth of its propodus; ischium 3 times length of basis; merus 2.8 times length of ischium; carpus 0.5 length of merus; propodus 2 times length of carpus and subequal to merus, ventral margin provided with 6 pairs of cuspidate setae; dactylus about 0.3 length of propodus. Fifth pereiopod ( Figure 6 View FIGURE 6 C) extending beyond distal margin of scaphocerite by 0.6 of propodus; ischium 2 times length of basis; merus 2.8 times length of ischium; carpus 0.6 length of merus; propodus 1.8 length of carpus and longer than merus, ventral margin armed with 6 pairs of unevenly spaced cuspidate setae, grooming brush comprises 2 rows of serrulate setae and extends for 0.1 length of propodus.

First pleopod sexually dimorphic in proportions, lacking appendix interna in both sexes; in males endopod 0.8 times length of exopod; both exo- and endopods fringed with plumose setae but mesial portion of inner margin of endopod devoid of plumose setae, with 6 spiniform setae ( Figure 7 View FIGURE 7 A); in females, endopod approximately one third length of exopod. Second to fifth pleopods broadly similar with endopod being slightly shorter than exopod, with appendix interna. Second pleopod of males with appendix masculina; appendix masculina subequal in length to appendix interna ( Figures 7 View FIGURE 7 B–C), furnished with 8 lateral and 5 apical setae; both apical and lateral setae smooth; apical setae longer than lateral setae.

Telson ( Figure 4 View FIGURE 4 E) approximately 1.2 times length of sixth pleonite; length:width ratio 2.8:1 proximally narrowing to 8.8:1 distally; dorsal surface with two pairs of cuspidate setae, proximal dorsal tuft of setae (few in number 2–4) and one pair of simple setae subdistally on median process; proximal pair of cuspidate setae situated at approximately 0.5 length, distal pair at 0.75 length; marginal setae present in distal portion only; posterior margin ( Figure 4 View FIGURE 4 F) prolonged into acute process, with 1 pair of plumose setae, and 2 pairs of stout setae, inner pair more than 3 times longer than outer pair; outer pair finish approximately level with median process.

Uropods broadly ovate, overreaching telson by 0.2 times length of endopod; exopod slightly longer than endopod, weak diaresis present; mobile lateral spine of exopod overreaching the fixed tooth by length of tip.

Eggs numerous; 0.8x 0.5 mm.

Colour pattern. “Transparent shrimp with thin dark transverse line across posterior edge of 3rd abdominal somite, more intense than any such line (if present) on other somites; and a short dark vertical (not oblique) line in middle of posterior half of side of carapace. Legs (including chelipeds) uncoloured, lacking reddish pigment at joints. Eggs dark olive, appearing black. Pleura of ovigerous females with 4 vertical bars (the last one an extension of the transverse abdominal line), with large conspicuous white patches” ( Powell 1983).

Distribution and habitat. Currently only known from medium salinity mangrove creeks in the Niger Delta, Nigeria. Powell (1983) states that it occurs in salinities between 5 and 20 ppt.

Derivation of name. The new species is dedicated to the collector of the type series, the late Charles Bruce Powell (1943–1998), in recognition of his work on the Decapoda of Nigeria. The name is genitive.

Remarks. Palaemon powelli sp. nov. is characterised by a combination of features. The shape of the chelae of the second pereiopod, having the grooming brush of the fifth pereiopod comprising just two rows of setae, the absence of a pre-anal spine and having the proximal most tooth of the dorsal rostral series 1.5 times removed are diagnostic for the species. A number of more subjective features may, likewise, have some diagnostic value; these include the relative length of the apical setae of the appendix masculina compared with the lateral series and the shape of the antennular peduncle.

In addition to P. powelli sp. nov. a further 12 species of Palaemon are reported from marine and brackish waters in the Atlantic: P. adspersus Rathke 1837 , P. elegans , P. floridanus Chace 1942 , P. longirostris , P. macrodactylus Rathbun 1902 , P. maculatus , P. northropi , P. paivai Filho 1967 , P. peringueyi Stebbing 1915 , P. rosalesi Rodriguez de la Cruz 1965, P. serratus and P. xiphias Risso 1816 . With the exception of P. adspersus , P. floridanus , P. longirostris and P. maculatus , all these species possess a strong pre-anal spine easily separating them from P. powelli sp. nov. Currently, P. paivai and P. roselesi are poorly known and the pre-anal spine is not mentioned in their original descriptions. Palaemon roselesi (currently known only from Mexico) has a downwards sloping rostrum with dorsal teeth along its full length and P. paivai (from Brazil) has broad straight rostrum which would serve to separate them from P. powelli sp. nov. which has an upturned, slender rostrum that is unarmed distally. In P. elegans the pre-anal spine is occasionally reduced or lacking however it may separated from the new species by having a quadrate versus acute posterodistal angle to its fifth abdominal pleura. Palaemon powelli sp. nov. can be separated from P. m a c u l a t u s by the shape of the rostrum (upturned versus straight and slender), the proportions of the second pereiopod (compare Figure 2 View FIGURE 2 b with Figure 5 View FIGURE 5 b) and the grooming brush of the fifth pereiopod, which extends much further and comprises 11 rows of setae compared to 2 in P. powelli sp. nov. The western Atlantic, P. floridanus has a greater number of ventral rostral teeth that P. powelli sp. nov. (5–9 versus 3–4). From P. adspersus it can be separated by the shape of the rostrum (straight and broad in P. adspersus versus slender and upturned), the shape of the chelae (less elongate in P. adspersus ) and the number of rows in the grooming brush on the propodus of pereiopod 5 (4 rows in P. adspersus versus 2 rows in P. powelli sp. nov.). The taxonomy of P. longirostris is not fully resolved with González-Ortegón and Cuesta (2006) reporting two morphological forms. The northern form has a straight, deep rostrum with dorsal teeth along its length and is not easily confused with P. powelli sp. nov. The southern form has a more slender and slightly upturned rostrum and is therefore more likely to be confused with the new species. It may be distinguished from P. powelli sp. nov. by the number of rows of setae in the grooming brush of the fifth pereiopod (4 versus 2), the shape of the antennular peduncle as well as its larger size.

As is normal in Palaemon (e. g. Yaldwyn, 1957; Gutu 1971; De Grave 1999; De Grave and Al-Maslamani, 2006), P. powelli sp. nov. shows variation in its rostrum shape and dentition. Some of this variation is shown in Figures 8 View FIGURE 8 A–F but this should be regarded as indicative rather than comprehensive. Although there are usually 3–4 ventral rostral teeth, one specimen (RMNH.D 49892) has 6 ventral teeth.

Powell (1983) provides a brief diagnosis of this species which is based largely on colour pattern (see description of colour pattern, above) but also he mentions some rostral features. The rostral features he mentions agree well with those described here.

Although formally undescribed until now, the presence of P. powelli sp. nov. in the Niger Delta has been known for many years and, in accordance with the suggestion of Powell (1983), some possible references to it are found in the literature as ‘ Palaemon sp. A Powell’ ( Marioghae 1987; Ajayi et al. 1995; Adeyeye and Adubiaro 2004). Despite its small size, Powell (1983, 1985) stated that the species has some fisheries potential and is often present in catches of P. maculatus and there is evidence ( Ajayi et al. 1995) that it may have limited commercial importance and may be a by-catch species of the fishery for Nematopalaemon hastatus (Aurivillius 1898) although Marioghae (1987) suggests that the small size of this species makes it unsuitable for culturing.