Variation in Squilla armata M. Edw. (Stomatopoda) Suggesting a distinct form in New Zealand waters.
[Read before a meeting of the Wellington Branch, September 10, 1952; received by Editor, September 15, 1952]
The stability and symmetry of crustacean ornamentation and armament are well recognised and extensively utilised in generic and specific diagnoses. Variation in details of teeth, spines, etc., is more commonly an exception than a rule. In the Stomatapoda there is variation in some features to an extent far exceeding that found normally in groups such as the Brachyura and Macrura. For example, the long curving teeth of the dactyl of the enlarged second thoracic leg are recorded generally as varying in number (Barnard, 1950). In Squilla investigatoris, these prominent teeth may occur as an exceptional minimum of 10, through a general range of 13 to 16, to an exceptional maximum of 18. Descriptions of many species refer to limited ranges of variation in ornamentation on the body and appendages. In examining Squilla armata mostly from Cook Strait, I found that this species in our waters shows variation in three respects apparently not yet recognised but to an extent worthy of careful description. In each case there is a common tendency in the form of an incision of teeth which is not elsewhere described and may indicate a possible variety here.
Barnard (1950) gives a description of S. armata with which local material is in excellent agreement. Bigelow (1895) noted the occurrence on the hind margin of the fifth abdominal segment of small sharp teeth, one to four in number in a group on either side “half-way between the median and intermediate carinac.” Chilton (1911) observed these spines, as described by Bigelow, on New Zealand material, and shows a pair on either side in his figure (Fig. 1) of the terminal abdominal segments and telson. Barnard reports that such spines are lacking from specimens taken at the Cape of Good Hope. His account gives the dactyl of the enlarged second thoracic leg as armed with “7 (rarely 6 or 8) teeth, including the terminal one.” Bigelow gives the range as from 7 to 9. Their accounts of the telson give the variation in number of the teeth between the submedian and lateral teeth as ranging from 8 to 12. Chilton's figure of a telson from a New Zealand specimen shows 7 such teeth on the left, eight on the right, between the submedian and intermediate teeth.
The material available to me is only a small collection of twelve specimens. eleven of which are from Cook Strait and mostly brought together by Mr. J. A. Garrick. These include seven females and four males. One female is 74 mm. in length, the rest range between 108 mm. and 123 mm. The twelfth specimen was sent me from Napier by Mr. L. S. Rickard. This is a female, 93 mm in length. We have much other material from fish stomachs since S. armata is an important item in the food of ling and dogfish; but this material is not in fit condition for study. S. armata is so far the only stomatopod we have obtained from Cook Strait.
The dactyls of the second thoracic legs are missing from two specimens. Six specimens have the teeth on this element, including the terminal one in the count as 8 on the left, 8 on the right; two, 7/7; and two, 6/7. The variation occurs in the deletion of a tooth from the proximal end of the series. In specimens showing the full number, 8, the two most proximal teeth are crowded (Fig. 1B). In one specimen, a female 115 mm. in length, as measured from the tip of the rostium to the tip of the submedian spine, there are two well formed and sharp processes on the one base (Fig. 1A). The evidence of this small colletion suggests that 8 should be regarded as the common number for these teeth in this species in our waters, that 7 is also common, and 6 is not rare. It is noteworthy that while 8 teeth and 7 teeth occur symmetrically in these specimens, none showed 6/8.
The variation in number of spines rising from the posterior border of the tergum of the fifth abdominal segment is very marked. In five of the twelve specimens, the spines are symmetrical in number. Two of these have the groups of spines represented by one spine on either side; one specimen, two spines on either side; one with the spines 3/3; and one specimen with the spines as 5 on the left, 5 on the right. The latter exceeds the number
previously reported. The number of spines in these groups in the remaining specimens is unequal. One specimen has the spines, 1/2; four, 2/3; two, 3/4. So that in the majority, the spines are not symmetrical. A total of seven spines in the two groups includes all the specimens excepting the one which has five spines on either side, a total of ten. The tendency for individual elements to be bifid, as seen in the proximal tooth of the dactyl dealt with above, is repeated here, as is shown in Fig. 2 which represents the specimen described above as having ten teeth, two of which are bifid, so that there is a total of twelve points.
Extensive variation occurs in the form and dentition of the posterior margin of the telson. This is incised in the midline, the incision being generally broad with parallel sides, as has been figured elsewhere; but in some it is narrow, the sides subparallel or more or less approaching one another. In one case (Fig. 3b) the sides of the incision are in firm contact for nearly three quarters of their length so that the “incision” is reduced and has more of the appearance of a perforation. The teeth between the submedian and lateral teeth are described as ranging from 8 to 12 in number. In the present material, the range is from 7 to 11 between the submedian and intermediate teeth. An equal number of teeth on either side was present in only three specimens, these each having 8/8. In all others these teeth were asymmetrical. Excluding one specimen with a damaged telson, these teeth were arranged as follows:—one specimen, 7/8; two, 7/9; two, 9/10; one, 9/11; two, 10/11. The variation shows no relationship with size or sex.
Striking variation in this dentition shows on closer examination. It is visible even to the naked eye that some of these teeth are more or less incised. The telson of only one specimen, a male 108 mm. long, had the same number of points as teeth, 10/11, i.e. no incised teeth which is the condition as figured by Chilton, Barnard, etc. In the rest, the number of incised teeth ranges from one to five on either side. Where the number is high, the appearance is rather that of a slightly coarse comb than of a dentate margin. Occasionally a tooth is trifid. The degree of subdivision is quite variable. Some teeth are but barely incised; others deeply, so that almost equal teeth are produced. In many cases, the subdivision produces one long and one short tooth. Due to this very constant tendency, although the teeth as counted at the base range from 7/9 to 10/11, the points range from 9/11 to 13/14 in number.
It can be noted also that in one specimen the margins between the median incision and the submedian teeth instead of being plain as is described and figured, showed incipient toothing, but only with two very low, wide-based rounded teeth on either side. These in no way approach the well-formed acutely pointed teeth reported in other species.
The degree of variation described above is not paralleled in other features in the animals in this collection so far as was noted. Otherwise, the animals agree with the published accounts of S. armata in a manner leaving no doubt in the assignment of our material to this species as currently recognised; but the colour, while generally that described in Barnard (1950) differs slightly from his account sufficient that if further examination of material elsewhere confirms previously published accounts, it is probable we may recognise the animals in our waters as a distinct variety. This variety may be characterised by a tendency to:—a higher number of spines on the dactyl of the second thoracic leg; a wider range in spination of the rear margin of the tergum of the fifth abdominal segment: and subdivision of the teeth on the margin of the telson between the intermediate and submedian teeth. In each of these features there is the tendency to subdivision of spines or teeth. If valid, the variety may be named S. armata var. schizodontia.
Barnard, K. H., 1950. Descriptive list of South African Stomatopod Crustacea (Mantis Shrimps). Ann. S. Afr. Mus XXXVIII: 838-864.
Bigelow, H. P., 1895. Report upon the Crustacea of the order Stomatopoda collected by the steamer “Albatross” etc. Proc. U. S. Nat. Mus. 117. 489-550.
Chilton, Charles, 1911. Revision of the New Zealand Stomatopoda. Trans. N.Z. Inst. XLIII: 134-139.
Kemp, Stanley, 1913. An account of the Crustacea Stomatopoda of the Indo-Pacific Region based on the collection in the Indian Museum. Mem Ind. Mus. IV: 1-127.
Fig. 1.—Dactyl of second thoracic leg showing (A) 7 teeth, the proximal tooth incised; and (B) the fully-developed condition of 8 teeth. (A, from a female 115 mm., B, from a male 120 mm.) Fig. 2.—Spines on the posterior margin of the fifth abdominal segment of a female 119 mm. long. Fig. 3.—Posterior margin of telson (of A, female, 117 mm.; B, female, 124 mm, C, male, 113 mm.; D male, 112 mm.) showing variation in number of teeth, and incision of teeth to a bifid of trifid condition, between the submedian and intermediate teeth.