Deckenia imitatrix Hilgendorf, 1869

Reed, Sadie K. & Cumberlidge, Neil, 2006, Brachyura: Potamoidea: Potamonautidae, Platythelphusidae, Deckeniidae (), Zootaxa 1262 (1), pp. 1-139 : 1-139

15. Deckenia imitatrix Hilgendorf, 1869 (Fig. 187, plate XV)

Deckenia imitatrix Hilgendorf, 1869a: 2 ; Ortmann, 1902: 306; Bott, 1955: 219, fig. 6, pl. 1 fig. 1a– d; Pretzmann, 1977: figs 17–20; Ng et al., 1995: 583, tables 1,2; Cumberlidge, 1997: 574; 1998: 194–195.

Type material examined: TANZANIA: Zanzibar, female cw 35.4, cl 29.2 mm (lectotype, designated by Bott 1955: 220) (von der Decken) ( ZMB 3216). There is reason to believe that the type locality is actually in Kadiaro, Kenya ( Marijnissen et al. 2005).

Additional material examined: TANZANIA: South Pare Mountains, Bumba, near Gonja , from rice fields, 29.v.1968 (J. N. Raybould) ( NMU TRW1968.04 ) . KENYA: Malindi , male, vi.1934 (A. Loveridge) ( USNM 70915 ) ; Mombasa, from temporary rainwater pool in the center of town, male, vii.1903 (Ch. Alluaud) ( USNM 32298 ) ; Mombasa, ( Joy ) ( NMU 23.02.1972.1) ; Simakeni Dam, west of Rabai , 15 km northwest of Mombasa, rainfed water body, no inlet or outlet, about 30 m wide, 1.7 m deep, highly turbid, crabs caught by sweep net from depths of 30 cm or more, some crabs seen swimming, 16.vii.1980 (A. W. R. McCrae) ( NMU 1980.03 ) ; Mombasa, ( Joy ) ( NMU 07.2001 .e.1) ; Coast Province, Kilifi District, Arabuko­Sokoke Forest , 1998 (J. Ashe via R. C. Drewes) ( NMU 1998 ) . SOMALIA: Giohar , adult male, vii.1968 (Lanza) ( NHMW 4369 ) .

Diagnosis. Exorbital tooth pointed; infraorbital margin lined with long sharp spines; epibranchial tooth sharp spine; ischium of cheliped with sharp spine; inferior margins of merus of cheliped lined with sharp spines; upper margin of merus of cheliped with row of pointed teeth, distal tooth longest; anterior inferior margin of merus of cheliped with large distal tooth; outer margin of carpus of cheliped with two sharp spines; first, second carpal teeth on inner margin of carpus of cheliped subsequal sharp spines; inner/outer margins of carpus of p2 lined with sharp subdistal spines; dorsal margin of merus of p2–p5 with sharp subdistal spine; interior margin of propodus of p2–p5 with row of sharp spines; terminal article of gonopod 1 clearly separated from subterminal segment by distinct line; proximal region of terminal article slim, subconical, tapering to cylindrical distal part. Chelipeds of adult males equal­sized, not markedly heterochelous.

Distribution. East African coastal region, from northeast Tanzania to Taru, Kenya, to Giohar and Eil, Somalia and inland in Kenya as far as Nairobi. Marijnissen et al. (2005) argued that the type locality is Kadiaro, Kenya (and not Zanzibar), and that the report of D. imitatrix from Zanzibar is most probably the result of a mistake arising from the replacement of a lost label.

Remarks. Bott (1955) and Pretzmann (1977) included photographs of this East African species, while Ng et al. (1995) provided illustrations of the gonopods, the mandibles and the unusual mouthparts. Deckenia imitatrix is distinguished from D. mitis by the teeth on the infraorbital margin (which are sharp spines in D. imitatrix and low, broad and blunttipped in D. mitis ) and by the inner and outer margins of the carpus of the cheliped (which possess sharp spines in D. imitatrix and low broad, blunt­tipped teeth in D. mitis ).

Natural history and conservation status. The conservation status of D. imitatrix is currently categorized as least concern (LC) ( Table 4) because it has a range of occurrence and area of occupancy well in excess of the thresholds for vulnerable (VU) ( IUCN 2004). Its population levels were estimated to be stable based on indirect measures such as the fact that it has been collected recently from more than one locality and that it is well represented in museum collections. However, recent field surveys by S. Marijnissen (pers comm.) have revealed that this species may be potentially more vulnerable that previously thought. Although this species has a wide extent of occurrence that is much greater than 20,000 sq. km. (which is above the threshold for vulnerable, VU), it may have an increasingly restricted area of occupancy due to habitat disturbance that would fall into the range for vulnerable (VU). While it is difficult to estimate the population status and trends of this species, its population is estimated to possibly in decline due to the fact that its habitat is restricted to marshes and wetlands, and these are fragmented and declining in this part of East Africa ( IUCN 2004; Table 4). The dependence of this species on wetland habitats that are vulnerable to human disturbance and that are associated with a growing human population in the region would argue for the upgrading of the conservation status of D. imitatrix from least concern to vulnerable in the light of the new data.