Studies on Palaemon affinis M.-Edw. 1837. (Crustacea, Decapoda, Natantia.)
Part II.—Variation in the Form of the Rostrum
Zoology Department, Victoria University College.*
[Received by the Editor, April 13, 1956.]
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
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.
[Footnote] * This study is part of a series assisted by a grant-in-aid of research from the University of New Zealand Research Grants Committee.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
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.
Nature of Rostral Variation in Palaemon affinis.
In Palaemon affinis the following different types of rostral variation are found:
1. Variation in the number of teeth on the dorsal edge.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
2. Variation in the number of teeth on the ventral edge. (As the number of teeth on the dosal mid-line of the carapace did not vary from 2 in all specimens examined in this study, for the sake of simplicity all dorsal teeth will be given together—i.e., 8/4, which then includes those on the rostrum and those on the carapace over those on the ventral surface.)
3. Variation in the profile of the rostrum.
4. Variation in the ratio of the length of rostrum to length of carapace.
5 Variation in the position of the anterior or 1st dorsal tooth in relation to the dorsal edge of the rostrum and to the position of the 1st ventral tooth.
Text-fig. 1—Relationship between the number of dorsal and the number of ventral teeth on the rostrum of Palaemon affinis. Fig. 1.—50 specimens from throughout New Zealand. Fig. 2.—50 specimens from Island Bay, Wellington. Fig. 3.—50 specimens from Point Howard, Wellington.
Variation in the rostral formula.
Scatter diagrams showing the different rostral formulae and the numbers of specimens having each formula, have been constructed (Text-fig. 1) for three samples of Palaemon affinis. In selecting the material for analysis, obviously damaged specimens and those showing malformed or recently damaged and regenerating rostra (e.g., Text-fig. 2, Figs. 15–21) were rejected. These will be discussed later. Fig. 1 shows the variation in sample 1, a random collection of 50 specimens as follows:
Waiheke Island, Auckland (10); Ohope Beach, Bay of Plenty (3); Wellington Harbour (27); Portobello, Dunedin (7); Ocean Bay and estuary of River Nairn, Chatham Islands (3).
A small amount of variation is represented, only 7 different formulae being present. All these specimens would fit into a general formula of 7 to 9/3 to 5, while 50% have the formula 8/4. The Wellington Harbour specimens (just over 50% of the sample) were neither from Island Bay nor Pt. Howard and were in the restricted range 8/3 and 8 to 9/4 with 50% having the formula 8/4.
Fig. 2 shows the rostral variation in sample 2, of 50 specimens from one intertidal pool at Island Bay, Wellington, collected on 3rd October, 1953. The scatter shows a much wider range of variation than that in Fig. 1. Not only are there more formulae.
represented, but the sample is more evenly spread among several different formulae. In this sample all the specimens would fit into the general formula 5 to 9/2 to 4 while the formulae 6/3, 7/3 and 8/4 each comprise about 20% of this sample.
Fig. 3 shows the variation in sample 3, also of 50 specimens from one intertidal pool at Pt. Howard, inside Wellington Harbour, and approximately 8 miles from Island Bay, which is on the coast outside the harbour. This sample collected on 21st November, 1953, shows more variation than sample 1, but not as much as in the Island Bay sample. All specimens fit into the formula 7 to 10/3 to 5 while almost 50% have the formula 8/4. Sample 3 has a very similar frequency distribution to that of sample 1, but both are quite different to sample 2, where almost 50% have the formulae 6 or 7/3 which were found on less than 5% of each of the other two samples.
Therefore a general formula covering the whole 150 specimens represented in the graphs would be 5 to 10/2 to 2 with the commonest individual formula being 8/4 (about 40% of the 150 specimens). This variation in the rostral formula does not bear any relation to sex. The general formula given above has no specific value, it probably does not establish the full range of variation in this species, and no doubt further collecting would reveal several more unusual formulae. It is, however, useful to give an indication of the variation to be expected in the rostrum of Palaemon affinis and probably in any other closely allied species.
It is interesting to compare here Thomson (1903). He “examined an immense number (of P. affinis) from various parts of the Colony” and found the variation to be 3 to 6/7 to 9–and states that he “never, however, found a specimen with only six.
Text-fig. 2.—Examples of different rostra of Palaemon affinis. All to same scale, setae omitted. Fig. 3.—Waiheke Island, Auckland. Figs. 1, 2, 4–6, 8, 9, 12, Island Bay, Wellington. Figs. 7, 13, 20, Lyall Bay, Wellington. Figs. 10, 11, 17, 18, 21, Pt. Howard, Wellington Figs. 15, 16, 19, Wellington. Fig. 14,—Portobello, Dunedin.
teeth on the upper margin of the rostrum”. Thomson's papers show that it was his practice to include carapace teeth in the rostral count. Fig. 2 shows that 25% of the Island Bay sample had six dorsal teeth.
Variation in the shape of the rostrum.
The length of the rostrum when graphed against the greatest depth, in sample 2, showed only a small amount of variation. There was no significant change in proportion in these dimensions, the variation at any one size being small, and there was no sexual differentiation in the sample.
The profile of the rostrum varied least of all throughout the entire collection. With the exception of the three forms described below, all others showed the slightly upturned profile described and illustrated by Yaldwyn (1954) and illustrated here in Figs. 1, 3, 6–8 and 10 on Text-fig. 2. The other three forms seen are illustrated in Figs. 9, 13 and 14. Fig. 9 is a down-curved rostrum which was seen only once in this study, Fig. 13 is an upturned sabre-shaped form and Fig. 14 is a long, straight, attenuated form. Both the latter appear to be uncommon, since in the 300 specimens examined each was seen on not more than three specimens.
Ratio of rostral length to carapace length.
The length of the rostrum, measured as c in Text-fig. 3, Fig. 1, was compared with the length of the carapace, measured as d, for samples 1 and 2. The rostrum increased in length in proportion to the length of the carapare, but while these proportions were variable, variation was within narrow limits. For sample 1, the rostrum expressed as parts per 100 of the carapace varied from 77 to 140 and for sample 2 from 82 to 130 (see Text-fig. 5, Fig. 1). In this sample 32% of the specimens had the rostrum shorter than the carapace, 60% had the rostrum longer, while the remaining 8% had the rostrum and carapace subequal. Thus the length of the rostrum was sometimes shorter but more often a little longer than that of the carapace.
Position of 1st dorsal tooth.
The position of the anterior tooth of the dorsal series is very variable. Milne-Edwards (1837) in describing Palaemon affinis from New Zealand implies that the extremity of the rostrum is bifid. But when describing P. quoianus, also from New Zealand (1837), he writes, “Rostre droit, robuste,.… armé de six dents en dessus et de trois en dessous, et point bifide à l'extrémité, mais terminé par une seule pointe, à la base de laquelle sont placées, immédiatement au-dessus l'une de l'autre de la première dent de la rangée supérieure et celle de la rangée inférieure”. Milne-Edwards therefore separates two forms of Palaemon from New Zealand, one with a bifid tip to the rostrum and one without. It has long been recognised that P. quoianus is a synonym of P. affinis. Miers (1876) states that “the number of teeth varies slightly in a large series of specimens, while the bifid appearance is caused by the greater or less approximation of the anterior tooth of the upper series to the apex of the rostrum… also a variable character”. The present material bears this statement out completely. All intermediate stages between the almost bifid appearance of what must be taken as typical P. affinis and the single acute point of typical “P. quoianus” have been seen. Text-fig. 2, Figs. 1–12, illustrate the varying position of the 1st dorsal tooth. These camera lucida outlines include the dorsal aspect of the carapace. They are taken from adult P. affinis with rostra showing no signs of damage or malformation and represent different rostral armament. It is not known if regeneration ultimately reproduces the original formula of a specimen. The figures are arranged in the plate in the following order:—Figs. 1—5 have 3 ventral teeth: Figs. 6—10 have 4 ventral teeth; Figs. 1 and 6 have the 1st dorsal tooth so near the tip of the rostrum that it appears bifid; Fig. 11 is similar, but has 5 ventral teeth; Figs. 2 and 7 have the 1st dorsal tooth immediately subterminal, thus the tip of the
rostrum is almost bifid; Figs. 3 and 8 have the 1st dorsal tooth further from the tip but anterior to the 1st ventral tooth; Figs. 4 and 9 have the 1st dorsal-tooth immediately above the 1st ventral; Fig. 12 is similar but has only 2 ventral teeth; Figs. 5 and 10 have the 1st dorsal tooth posterior to the 1st ventral tooth.
Fig. 7, with the anterior tooth of the dorsal series immediately subterminal, making the tip of the rostrum almost bifid, is a typical P. affinis as implied by M. Edwards and with the most frequently met formula of 8/4. Fig. 4 is a typical “P. quoianus” as described by M.-Edwards, the 1st dorsal tooth being immediately above the 1st ventral and the formula being 6/3. Figs. 9 and 12 show other “P. quoianus” types having 4 and 2 ventral teeth respectively.
From the above it can be seen that the 1st dorsal tooth can vary in position from the terminal and bifid condition of Figs. 1 and 6 through all intermediate stages to a position posterior to the 1st and 2nd ventral teeth as in Fig. 5. This fully confirms Miers's opinion (1876) that, due to his strict adherence to the idea of rigid rostral formulae, Milne-Edwards gave different names to two forms of the one species. P. affinis, having page priority, is the valid name.
Text-fig. 4 is an attempt to express the relative frequency of the different positions of the 1st dorsal tooth in relation to the 1st ventral tooth. The camera lucida outlines of the rostrum and carapace of the 50 specimens of sample 2, from Island Bay, were drawn and the following measurements (see Text-fig. 3, Fig. 1) were made from each drawing: a, the horizontal distance between the tip of the rostrum and the anterior edge of the base of the 1st dorsal tooth; b, the horizontal distance between the tip of the rostrum and the anterior edge of the base of the first ventral tooth; c, the length of the rostrum, measured as the horizontal distance between the tip of the rostrum and the posterior margin of the orbit. A ratio of the distance of the 1st dorsal tooth from the tip of the rostrum relative to the length of the rostrum a/c is graphed against the similar ratio of the 1st ventral tooth b/c in Text-fig. 4. Sexual significance was tested for but there was no correlation between sex and position in the 1st dorsal tooth. The 45° diagonal, on the graph, is the line along which points have the same co-ordinates—i.e., where the two ratios are equal. Specimens which fall on this line thus have the 1st dorsal tooth immediately above the 1st ventral and are of the “quoianus” type. This diagonal also divides those specimens whose 1st dorsal tooth is posterior to the 1st ventral (group D) from those where it is anterior to the 1st ventral (groups A, B and C). The line from the zero point approximating 10° to the horizontal, as drawn on the graph, separates those with the 1st dorsal (group A) so near the tip of the rostrum that they appear bifid, from the others. The specimens in group B have the tip almost bifid and form nearly 50% of the sample, those in group C have the 1st dorsal further from the tip but still anterior to the 1st ventral, as in Figs. 3 and 8, Text-fig. 2.
It can be seen that while there is a great deal of variation present, nearly 50% lie in one close group (B), and the extremes are uncommon. Except for the single specimen with 2 ventral teeth, shown on Text-fig. 2, Fig. 12, for which an explanation will be given later, the remaining specimens fall into two patterns. Those with 4 ventral teeth lie entirely on or below the 45° line and show a much smaller range of variation than those with only 3 ventral teeth which occur anywhere in the scatter and form the only representatives in group D—i.e., with 1st dorsal posterior to 1st ventral. There is a much smaller range of variation in the position of the 1st ventral than in the position of the 1st dorsal.
Causes of Extreme Variation
Cases of extensive damage have been seen, even where the entire rostrum has been broken off and a new rostrum and teeth have begun to regenerate. Text-fig. 2, Fig. 15, shows an extreme stage, where the entire rostrum has been lost, leaving only.
the posterior three dorsal teeth and regeneration has begun. Fig. 16 is a case where a portion of the rostrum has been broken and some of the dorsal teeth have been damaged. A slight malformation in the regenerating portion can be seen. Figs. 19 and 21 are examples where the broken rostrum has begun to regenerate a new anterior portion and teeth, though these are still smaller and more crowded than on a normal rostrum.
Only a few cases showing original damage were observed. This damage could be divided into two different classes—i.e., those with loss of the anterior portion of the rostrum including the 1st dorsal tooth, and those with loss of almost the entire rostrum. Only damage where the scars had become healed over with a chitinous.
Text-fig. 3.—Fig. 1—Diagram of rostrum and dorsal surface of carapace, to show measurements taken. Fig. 2—Outline of rostrum, before ecdysis, of specimen from Pt. Howard, Wellington. Fig. 3—Enlargement of anterior portion of Fig. 2, showing new rostrum under old. Fig. 4—Portion of ventral surface of rostrum of a ♀ specimen from Pt. Howard, showing split ventral tooth. All specimens are Palaemon affinis and setae are omitted.
membrane was included, thus excluding those specimens which had been damaged during collection or after death. The numbers can be summarised as follows:—
|Unregenerated After Damage|
|Locality.||Date.||No. of Spp.||Loss of anterior portion rostrum.||Loss of entire rostrum.||Early regeneration after loss of entire rostrum.||Total.|
|Island Bay (same pool as 2)||19/6/54||50||–||–||–||–|
|Sample 3, Pt. Howard||21/11/53||62||2||–||1||3|
|Lagoon, Chatham Is.||27/1/54||24||–||–||–||–|
|Kaingaroa, Chatham Islands||31/1/54||20||–||2||1||3|
|Damage as % of total (approx.)||2.3%||1.7%||1%||5%|
Therefore 5% of all specimens examined showed damaged and unregenerated rostra, or in the case of some showing loss of the entire rostrum, not more than one moult had occurred. Those showing loss of the anterior portion of the rostrum including the 1st dorsal tooth were most common, 2.3% of all specimens, while 1.7% showed loss of the entire rostrum and 1% had passed through one moult since losing the entire rostrum. Therefore the combined incidence of loss of the entire rostrum (2.7%) was slightly higher than the incidence of loss of the anterior portion only. Thus damage to the rostrum is commoner than has been previously recognised. The Island Bay samples showed little original damage, for example sample 2 had only 1 specimen showing loss of anterior portion and none with loss of the entire rostrum, while a collection taken in the same pool as sample 2, about 8 months later, showed no original damage at all. The sample from the rock pools of Kaingaroa, Chatham Islands, had the greatest percentage of damaged specimens (15%) while the collection from the sandy, protected lagoon at the Chathams showed no damage at all. The percentage of damage, however, in the Kaingaroa sample, is probably abnormally high due to the small size of the sample.
Different stages in the regeneration of the rostrum give rise to a large number of the variations in the position of the most anterior dorsal tooth and in the number of ventral teeth. That is to say, these are not stable features, during the next few moults they may be expected to develop closer to the genetic formula. If in Fig. 1 (Text-fig. 2) the tip was broken off, the “quoianus” type, as in Fig. 4, would be an intermediate step in regeneration. If in Fig. 2 the tip, including the 1st ventral tooth was broken off, then Fig. 12 would be one of the early stages in regeneration. That this has, in fact, happened in the case of the specimen from which Fig. 12 was drawn is the more probable because of its singular position in the scatter diagram (Text-fig. 4) and by the fact that its rostrum is much shorter than its carapace (0.82:1). Figs. 2 or 6 are from specimens which on losing their tips would give a rostrum as in Fig. 5 at one stage in regeneration. Many more examples could be given and all other types could be derived from specimens such as Figs. 1, 2, 6 and 7, or specimens of the same type but with different relative positions of the dorsal and ventral teeth.
Text-fig. 3, Figs. 2 and 3, from the one specimen, gives an example of restorative regeneration taking place at a moult. The integument about to be lost at ecdysis has the formula of 9/4, both the 1st dorsal and the 1st ventral teeth are very small in size and the anterior portion of the rostrum is crowded and small. But in Fig. 3 the new integument showing within the old, which has pulled away slightly from it as one of the first signs of an impending moult, has large 1st dorsal and ventral teeth.
and appears to have a new dorsal tooth (T), anterior to the old 1st, which would give the new rostrum a formula of 10/4 and an almost bifid tip.
This demonstrates that in regeneration the dorsal and ventral rostral teeth are reformed posteriorly first so that the 1st dorsal of the above specimen would become the 2nd dorsal after ecdysis. In fact the teeth taken as 1st dorsals in the specimens of groups C and D of the diagram (Text-fig. 4) including the “quoianus” types, are only temporarily 1st dorsals, and after one or more ecdyses would no longer be the most anterior teeth on the dorsal surface of the rostra.
In Text-fig. 4 one can see clearly one major grouping which contains the specimens with undamaged or completely regenerated rostra, and two minor, which contain individuals having the rostra in various incomplete stages of regeneration. The major group includes both groups A and B, and forms 62% of the 50 specimens taken as the sample. The two minor groups are smaller, the first, consisting of group C and including the “quoianus” types, forms 24%, and the second, group D and the individual with two ventral teeth, forms 14%. In the full sample of 55 specimens there was only one specimen with the anterior portion of the rostrum broken off and showing no regeneration. As the specimens forming groups C and D could be regenerating from damage of any degree, it is difficult to gauge from these figures the percentage of damage to be expected in a sample or the number of moults needed to complete the regeneration after any type of damage. It is indicated in the diagram that it takes at least two moults to regenerate after loss of only the.
Text-fig. 5.—Fig. 1—histogram of the rostral length expressed as parts per 100 of the carapace length for Palaemon affinis. Single line—50 specimens from Island Bay (sample 2). Double line—16 post-larval specimens from Island Bay, Wellington. Fig. 2—Relationship between the number of dorsal and the number of ventral teeth on the rostrum of 16 post-larval specimens of Palaemon affinis from Island Bay, Wellington.
anterior portion of the rostrum, including the 1st dorsal tooth. Regeneration of rostral length is generally prior to regeneration of rostral ornamentation (i.e., dentition). Thus the rostrum will usually regain its normal length before the true 1st dorsal has reformed.
Range of Genetic Rostral Variation
A collection of 16 small specimens in an early post-larval stage from Island Bay, Wellington, gives a good example of the genetic variation in rostral formulae. These specimens had carapace lengths ranging from 2.5 to 3.25 mm and showed no signs of damage or regeneration. The rostra would have been unable to regenerate completely during the short post-larval life of these specimens.
Text-fig. 5, Fig. 1, gives the range of variation of the ratio of rostral length to carapace length. The rostrum expressed as parts per 100 of the carapace varied from 112 to 141 with 40% falling above the upper limit of the sample 2 adults. This indicates that the ratio of rostrum to carapace length possibly diminishes slightly during the growth of the shrimp, but the extension to below the 100 mark (i.e., below the point where the rostrum is equal in length to the carapace) is mainly due to the fact that these rostra are incompletely regenerated and are consequently shorter than they would normally be. Thus 40% of the specimens have rostral lengths between 77 and 102 parts per 100 of the length of the carapace, and this percentage compares with that of the two minor groups (C and D) on Text-fig. 4, discussed above, which together make up 38% of sample 2.
These post-larval specimens were divided between the formulae 8/3, 8/4 and 9/4 with a single specimen with 10/4 (see Text-fig. 5, Fig. 2). Thus the variation cannot be expressed by a single formula, such as 8 to 10/3 to 4, but only by the double formula 8/3, 8 to 10/4.
In sample 1, 88% fall within this range, while in sample 3 the figure is 78%. This number is reduced in sample 2 to only 40%. Other samples from New Zealand and the Chatham Islands agree essentially with the scatters shown in Figs. 1 and 3 (Text-fig. 1). However, the other sample taken from the same intertidal pool in Island Bay as sample 2, has a scatter almost identical to Fig. 2, and thus differing from the others in a wider range in low numbers of dorsal teeth with 3 ventral and in the small number of specimens with 4 ventral teeth.
It can now be seen that a general formula such as 5 to 10/2 to 5 arrived at above has no real meaning, as it includes regeneration stages after damage, and that these stages are especially numerous in the Island Bay samples. However, from the above, a genetic rostral variation range for P. affinis of 8 to 10/3, 8 to 10/4 would be indicated. The formulae 3/9 to 10 and 10/4 are, however, quite uncommon.
Thus specimens showing a combination of most or all of the following characters, seen in the Island Bay post-larval specimens, can be presumed to be undamaged, or if formerly damaged to have completely regenerated: 8 to 10 dorsal teeth and 3 to 5 ventral; ratio of rostral length to carapace length of at least 1:1; presence of a true 1st dorsal tooth—i.e., in the almost bifid position at least; no crowding of, or reduction in size of, the anterior dorsal or ventral teeth; an attenuated, unarmed, anterior portion of the rostrum between the 1st and 2nd dorsal teeth, as opposed to a short or blunt portion.
Malformation and abnormal regeneration.
Malformation arising spontaneously or through abnormal regeneration is rare in P. affinis, but some interesting examples will be described. Text-fig. 2, Fig. 17, shows dorsal curving of the regenerating portion of the rostrum. Fig. 18 shows a more common slight malformation in the regeneration of the extreme anterior end of the.
rostrum. Text-fig. 3, Fig. 4 is a case of apparently spontaneous malformation, a ventral tooth has been divided into two by a long narrow split and the new integument showing inside has widened the division considerably. So a specimen with 3 ventral teeth is changed into one with 4, if this malformation is perpetuated in later moults. Duncker (1900, Plate II, Fig. 13) illustrates a similar split ventral tooth in Palaemonetes vulgaris. Text-fig. 2, Fig. 20 illustrates a malformation where a great increase in the number of teeth has resulted probably from abnormal regeneration after damage. Here subdivision of ventral teeth appears to have taken place similar to that described by Richardson (1953) for teeth on the posterior margin of the telson of the stomatopod Squilla armata from New Zealand waters.
Against this background of great variation in rostral dentition, the other ornamentation and armament on the body and appendages are extremely stable, with the single exception of the spines on the dorsal surface of the telson. Nearly 10% of sample 1 had irregularities in the arrangement of these spines. The normal is two transverse rows of 2, while irregularities seen were: one member of a pair not in line; the posterior pair absent; one member from each pair on the same side absent or 3 spines absent.
1. A considerable amount of variation in the form of the rostrum is described for Palaemon affinis. Variation in the rostral formula is wide and differs in three different samples taken; a general formula covering the 150 specimens of these three sample is 5 to 10/2 to 5 with the commonest individual formula being 8/4.
2. Only a small amount of variation in the shape of the rostrum, when expressed as ratio of length to depth and also degree of curvature, was found.
3. The ratio of the length of the rostrum to the length of the carapace varied, the rostrum being sometimes smaller but more often a little longer than the carapace.
4. The position of the 1st dorsal tooth varies from being so close to the tip that the rostrum is described as bifid, to a position posterior to the 1st or even the 2nd ventral tooth, themselves variable in position. It was found from a scatter diagram that the commonest position for the 1st dorsal tooth was near the tip, making the rostrum almost bifid and that rostra with only 3 ventral teeth had a much greater range of variation than those with 4. Miers (1876) is followed in placing P. quoianus M.-Edw. as a synonym of P. affinis M.-Edw., the differing positions of the 1st dorsal teeth in the two forms being within the range described for P. affinis.
5. A small but significant amount of unregenerated damage was seen (5% in a total of 307 specimens) and a number of individuals showing non-genetic variation are described as stages in regeneration of rostra after damage. Change in the rostral formula of an individual during regeneration is described as well as some malformations arising either spontaneously or through abnormal regeneration.
6. Regeneration of rostral length usually precedes regeneration of rostral ornamentation (i.e., dentition).
7. A collection of specimens in an early post-larval stage had a rostral formula range of 8/3, 8 to 10/4. These specimens showed no signs of damage or regeneration. All these specimens had the rostrum longer than the carapace and 40% were above the described upper limit for the rostrum to carapace ratio of a sample of adults from the same area.
8. A genetic rostral variation, as distinct from one including regeneration stages, of 8 to 10/3, 8 to 10/4 is found, with the formulae 9 to 10/3 and 10/4 uncommon.
I wish to thank Mr. G. A. Knox, leader of the Chatham Islands Expedition, January-February, 1954, for permission to examine the collections of Palaemon affinis from the Islands, and especially Professor L. R. Richardson for his continued inspiration and guidance during this study.
Brôzek, A., 1912. Über die Variabilitat bei Palaemonetes varians Leach aus Kopenhagen. Sitzungsber. d. k. bohmischen Ges. d. Wiss. in Prag, 1912: 1–20.
Duncker, G., 1900. On Variation of the Rostrum in Palaemonetes vulgaris Herbst. Amer. Nat. XXXIV: 621–633, 3 Pl.
Gurney, R., 1923. Some Notes on Leander longirostris M.-Edwards, and other British Prawns. Proc. Zool. Soc. Lond. 1923: 97–123.
Kemp, S., 1925. Notes on the Crustacea Decapoda in the Indian Museum XVII: On Various Caridea. Rec. Ind. Mus. XXVII: 249–343, Figs. 1–24.
De Man, J. G., 1915. On some European species of the genus Leander Desm., also a contribution to the fauna of Dutch waters. Tijdschr. Nederl. Dierk. Vereen. (2) XIV: 115–179, Pls. X-XII.
—— 1925. Contribution à l'étude des Décapodes Macroures marins et fluviatiles du bassin du Congo Belge. Ann. Mus. Congo Belge Zoo, III, Arth. III, Crust. I (1): 1–54.
Miers, E. J., 1876. Catalogue of the Stalk- and Sessile-Eyed Crustacea of New Zealand. Col. Mus. Geo. Sur. Dept. London: 1–130.
Milne-Edwards, H., 1837. Histoire naturelle des Crustacéa comprenant l'anatomie, la physiologie et la classification de ces animaux. II: 1–532.
Richardson, L. R., 1953. Variation in Squilla armata M.-Ewd. (Stomatopoda) suggesting a distinct form in New Zealand waters. Trans. Roy. Soc. N.Z. 81 (2): 315–317, 1 Fig.
Serventy, D. L., 1938. Palaemonetes australis Dakin in South-Western Australia. Jour. Roy. Soc. West. Aust. XXIV: 51–57.
Thomson, G. M., 1903. On the New Zealand Phyllobranchiate Crustacea–Macrura. Trans. Linn. Soc. Lond. Zool. VIII (II): 433–453, Pls. XXVII–XXIX.
Weldon, W. F. R., 1890. Palaemonetes varians in Plymouth. Jour. Mar. Biol. Ass. U.K. N.S. 1 (4): 459–461.
Yaldwyn, J. C., 1954. Studies on Palaemon affinis M.-Edw., 1837. Part I. Synonymy and External Morphology. Trans. Roy. Soc. N.Z. 82 (1): 169–187, 2 Figs.
J. C. Yaldwyn,M.Sc.,
Department of Zoology,
Victoria University College,
Box 196, Wellington, N.Z.