Abstract

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The rostral formula of immediately post-larval specimens is 8 to 10/3 to 4 but the variation in adults is 5 to 10/2 to 5 with 8/4 in 40% of 150 specimens. This wide range of variation results from damage with subsequent progressive regeneration or from malformation. The rostral profile is highly variable, with no significant trends in proportions. The position of the anterior or 1st dorsal tooth varies from sub-terminal in a bifid condition (the commonest) through the “quoianus” type, to posterior to the 1st ventral tooth.

In many natant decapods a study of the armed rostrum in a number of specimens reveals great variation in proportion, profile and armament. A simple specific formula attempting to describe the rostrum does not show this, and is thus of low specific value.

This is especially so in the palaemonids, where strict adherence to rostral formulae, e.g., Kemp's (1925) keys, disregards the variation possible in the dentition of a species. Such a practice led the original describer of New Zealand's common marine littoral shrimp to distinguish two “species” on this criterion.

This study on the range of variation in the dentition and shape of the rostrum in P. affinis shows that the observed range is not primarily genetic in origin, but includes damage and stages in regeneration; it then describes the true rostral variation and form. Three samples of P. affinis, each of 50 specimens having undeformed rostra, are graphed to show the different rostral formulae and their relative frequencies. One of these samples is composed of specimens from several localities throughout New Zealand, and the other two are from localities in the Cook Strait area. Variations in the rostral profile and in the ratio of rostral length to carapace length in these samples are not found to be significant. The position of the anterior, or 1st, dorsal tooth is found to vary from immediately subterminal to a position posterior to the 1st or 2nd ventral teeth, thus confirming the relation between P. affinis and its synonym P. quoianus. Two mechanisms, damage and subsequent regeneration of rostra, and malformation arising either spontaneously or from abnormal regeneration, are shown as causing non-genetic variation. The genetic rostral variation in dentition and form, as distinct from that due to damage and regeneration, is then deduced by analysis of samples and from another small collection of undamaged post-larval specimens. These show a very small amount of variation in the numbers of rostral teeth, a narrow range of variation in the ratio of rostrum length to carapace length and a uniform rostral profile.

Rostral variation has been described in the closely allied genus Palaemonetes. Weldon (1890) found considerable variation in the number of teeth present on 915.

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specimens of P. varians (Leach) from Plymouth, England. He discussed dorsal teeth quite separately from ventral teeth and obviously included cases of gross damage without recognising it. For instance he lists specimens with only one dorsal tooth when some 70% had about 4 or 5 dorsal teeth, and also specimens with no ventral teeth when most of the sample had about 2. His figures, however, “show how enormously the variation… exceed those indicated by the current diagnoses of the species”. Duncker (1900) analysed the variation in 1050 specimens of P. vulgaris Herbst. from Cold Spring Harbour, U.S.A. in order to test the relation between the average value of a varying character and its variability. He gives both dorsal and ventral teeth together but analyses them separately, and he includes all cases of damaged and regenerating rostra in his figures. He suggested regeneration after “traumatic injuries” as a possible cause of the frequent “curious malformations” seen, and considered that the rostrum might regenerate finally to its original shape and dentition. Brôzek (1912) has attempted, by treating rostral formulae statistically, to separate Southern European fresh water forms of P. varians from the Northern brackish water forms. He was concerned only with the frequency of the different numbers of teeth, and included specimens with damaged or regenerating rostra in his data. Gurney (1923) gives some further figures for dorsal teeth only, and summarises the work on this species up to that date. “.… there seems to be no ground for supposing that the number of teeth has anything to do with the geographical situation or salinity. It is, however, not improbable that a statistical study of the rostral teeth based on a large material would show constant local variations, since many populations of this species must be isolated for long periods and subjected to intense selection. The result is hardly likely to justify the great labour necessary”. In South-Western Australia, Serventy (1938) dealt with the rostral formulae in salt and fresh water specimens of P. australis Dakin, and could not correlate the variations in the populations studied with their differing habitats. He analysed the data on the dorsal and ventral teeth separately and, when the rostrum was bifid, did not include the small dorsal tooth as the 1st of the dorsal series. However, he states that “an abnormal specimen… showed the formula of 10/4 a fracture near the tip of the rostrum having resulted in regeneration of the extremity with additional teeth”. In the genus Palaemon (Leander of most authors) rostral variation has only been briefly recorded in general terms by de Man (1915) and Gurney (1923) for the European species P. elegans Rathke [= P. squilla (Linn.)], P. serratus (Pennant) and P. longirostris M. Edw., and by de Man (1925) for several other species from the Congo Basin. Gurney did note, however, in P. longirostris that there was variation in the position of the 1st dorsal tooth, it could be apical and thus included in the dorsal series.