Percnon planissimum (Herbst, 1804)

Published, First, 2009, New records of crabs (Decapoda: Brachyura) from the New Zealand region, including a new species of Rochinia A. Milne-Edwards, 1875 (Majidae), and a revision of the genus Dromia Weber, 1795 (Dromiidae), Zootaxa 2111, pp. 1-66 : 49-53

Percnon planissimum (Herbst, 1804)

Figs 25a, b, 26a, b.

Cancer planissimus Herbst, 1804: 3 , pl. 59, fig. 3.

Percnon planissimum — Chilton & Bennett 1929: 774. — Richardson 1949a: 32 (key), pl. 1, fig. 12. — Barnard 1950: 138, fig. i, j. — Tweedie 1950: 136, fig. 4d. — Edmondson 1959: 197, fig. 25c, 27a–c. — Forest & Guinot 1961: 163. — Davie 2002: 439.

Leiolophus planissimus — Miers 1876: 46.

Percnon pilimanus — Chilton 1911: 559. [not Acanthopus pilimanus A. Milne-Edwards, 1873 = P. affine (H. Milne Edwards, 1853) ]

Percnon affine — McLay 1989: 106, fig. 1A–D. [not Acanthopus affinis H. Milne Edwards, 1853 ]

Material Examined. Kermadec Is: AQ2789, 2 males, 15.2 mm x 16.6 mm, 18.4 mm x 19.9 mm, 2 females, 18.3 mm x 19.7 mm, 22.2 mm x 23.2 mm, coll. W. R.B. Oliver, 1908 (reported by Chilton, 1911 as P. pilimanus and labelled as such); AQ2397, 1 male, 29.2 mm x 30.8 mm, 3 females, 22.8 mm x 23.6 mm, 24.2 mm x 25.0 mm, 29.1 mm x 30.0 mm, coll. by W. R.B. Oliver, 1908 (reported by Chilton, 1911 as P. pilimanus and labelled as such); AQ2445, 1 female, 15.2 mm x 16.8 mm, coll. J.H. Sorensen Denham Bay, Raoul I., May, 1944; 3 males, 13.0 mm x 14.2 mm, 18.9 mm x 20.3 mm, 24.8 mm x 26.5 mm, 4 ovigerous females, 17.4 mm x 19.0 mm, 20.5 mm x 22.2 mm, 21.8 mm x 22.4 mm, 21.9 mm x 23.8 mm, 1 female, 12.3 mm x 13.7 mm, Oneraki Beach, Raoul I., coll. by DOC staff, 4 Oct. 2003.

New Zealand: Tutukaka, at Poor Knights I: 2 photos taken by dive guide, Dave Abbott, CW about 19.5 mm, (cheliped size suggests a female), 35° 28ʹS, 174° 44ʹE, 4 m, among boulders, 24 Aug. 2001 (see Fig. 26a, b) .

Remarks. Chilton (1911) reported “ two males and several females from Sunday I.” as Percnon pilimanus (A Milne-Edwards, 1873) , but he debated at some length whether they should not be called P. planissimus . I think that these specimens (held as two lots in the Canterbury Museum, AQ2397, AQ2798) should be referred to P. planissimum because they have similar CW/CL ratio and exhibit the same patterns on the dorsal surface of the carapace (see Crosnier 1965: fig. 135). It should be noted that there is some variation in the length of the two spines that terminate the rostrum in P. planissimum with much longer spines in some specimens than is figured by Crosnier (1965: fig. 135).

The occurrence of Percnon planissimum around the New Zealand mainland has always been uncertain. Chilton & Bennett (1929) included it on the authority of Hutton (1882) who had originally listed it as being one which should be deleted from the list of species to be excluded from the New Zealand fauna, but in his personal copy of the list the name was struck off. A double negative, making a positive, is hardly strong evidence for its presence on the mainland! The only substantial evidence seems to be from Miers (1876) who stated that there was a specimen from New Zealand in the Paris Museum collection. It has not been possible to confirm or not whether the specimen is in Paris or whether it belongs to this species. The only evidence that P. planissimum is present are some photos (see Fig. 25a, b) taken by divers in 2001 at the Poor Knights Is off the east coast of the North Is. They were seen again several times between 2001 and 2006. It is not possible to compare the CW/CL ratio with the Kermadec specimens, because of the angle from which the dorsal view photo ( Fig. 25a) has been taken, but the length/width ratio of the merus of the second pereopod is the same (about 4.0) as P. planissimum . The pattern of colours on the carapace and banding on the legs are similar (see Fig. 24) even though the actual colours are not exactly the same. The photos ( Fig. 25a, b) were taken in situ using colour film at a depth of around 4–5 m so that would’ve influenced their apparent colour.

Takeda & Webber (2006) included Percnon affine in their list of Kermadec crabs on the assumption that Chilton (1911) had correctly identified his specimens as P. pilimanus (A. Milne-Edwards, 1873) , a species that after comparing the types, Crosnier (1965) argued was synonymous with P. affine (H. Milne Edwards, 1853) . However, P. affine is not part of the fauna of the Kermadec Is because Chilton’s specimens belong to P. planissimum .

Chilton (1911: 559–560) reported “ P. pilimanus ” as being very common among rocks near the low tide mark. As to the habits of these crabs, it is worthwhile quoting the comments of the collector: “Mr. Oliver makes the following observations on the habits of this species: ‘Fairly common among rocks near low-tide mark. Very quick in its habits. Its colour somewhat resembles the rock, on which it stays perfectly still, but when any one approaches too near it darts into the water. When, after continued westerly winds, sand was driven ashore so as to bury the boulders on the north coast of Sunday I. to about half-tide mark, thousands of these crabs, retreating before the encroaching sand, congregated in heaps amongst the rocks near shore until the sand was washed away again.’ DOC staff who collected specimens from Raoul I. in 2003 found them “washed up dead and dying on sand/rocks near hot pools – many were cooked in hot pools”. It seems that these crabs were victims of volcanic activity.

An ovigerous P. planissimum from Raoul I. reported above (21.9 mm x 23.8 mm) carried around 2000 very small eggs (diameter = 0.30–0.35 mm). As might be expected for a species with such a wide distribution, it is likely that the larval stages spend a long period in the plankton during which they are transported considerable distances. The eggs are carried in two distinct clutches (left and right) in the narrow brood chamber beneath the flattened abdomen. The gonopores are not operculate.

Distribution. Red Sea, East Africa, Cocos-Keeling Is, Japan, southeast Australia, Middleton and Elizabeth Reefs (Tasman Sea), Niue, Tahiti ( French Polynesia), Hawaiian Is, and now the Kermadec Is and New Zealand. P. planissimum is a very widespread Indo-West Pacific species. Inhabits rocky substrates in the depth range 0– 5 m.