The Distribution of Pariambus Typicus Var. Inermis Mayer (Amphipoda, Caprellidae) on the Common Starfish Asterias Rubens L
Total Page：16
File Type：pdf, Size：1020Kb
THE DISTRIBUTION OF PARIAMBUS TYPICUS VAR. INERMIS MAYER (AMPHIPODA, CAPRELLIDAE) ON THE COMMON STARFISH ASTERIAS RUBENS L. BY M. B. JONES Department of Zoology, University College of Swansea, Wales, Great Britain INTRODUCTION Although the association of Pariambus typicus (Kroyer) (Amphipoda, Caprelli- dae) with Asterias rubens Linnaeus, 1758, has been recorded by a number of authors ( Cuenot, 1912; Chevreux & Fage, 1925 ; Mortensen, 1927; Harrison, 1944; Marine Biological Association, 1957), none of them refer to which of the varieties of this species was involved. The two varieties of P. typicus likely to occur in Britain are var. armata Mayer, 1903, and P. typicus var. inermis Mayer, 1903, which can be separated anatomically on the form of the third segment of the peduncle of the first antenna, and the second gnathopod of the males (see Sars, 1895; Chevreux & Fage, 1927; Harrison, 1944). P. typicus var. inermis has not been recorded in Britain previously. This paper reports on the frequency distribu- tion of P. ty PiCl4J var. inermis, newly recorded in Britain, on A. rubens, and compares the association with that between the newly described Caprella grahami Wigley & Shave (Caprellidae) and the North American starfish Asterias forbesi (Desor) (Wigley & Shave, 1966; Patton, 1968). MATERIALAND METHODS Starfish were collected from a large population of A. rubens under the pier at Mumbles, Swansea during August, September and October 1967. They were placed separately into polythene bags and each bag sealed with a rubber band. In the laboratory the starfish were measured, the aboral surface graded for colour using a simple light, medium or dark scale, sexed, and examined for caprellids. The position of each caprellid on the starfish was noted. RESULTS The variety of P. typicus found on A. rubens at Mumbles was identified in all cases as inermis Mayer and this is its first definite record in British waters and on A. rubens. A total of 942 caprellids was collected from 150 starfish. Their frequency distribution was plotted and compared with the theoretical curve for a random 90 distribution (i.e., Poisson Curve) (fig. 1). The observed distribution does not fit the theoretical curve and the differences between observed and expected values are too large to be attributable to sampling variation. The large percentages of starfish with 0 and high numbers of caprellids, and the low percentage of starfish with intermediate numbers suggests that the caprellids are aggregated in their occurrence, i.e. they show a "clumped distribution" (see Mather, 1967). There does not appear to be a relationship between the diameter of a starfish and the Fig. 1. The observed percentage frequency of occurrence of the numbers of Pariambus typicus var. inermis Mayer on each Asteriaf rubens L. plotted against the expected Poisson distribution. number of caprellids found on it (fig. 2) but in general the smaller starfish lack any infection by caprellids (fig. 3). During the three month collection period the - - percentages of infection of the starfish were August 50%, September 67%, - and October 52qo. Each collection contained caprellids of all sizes, including small juveniles and ovigerous females. The incidence of the caprellids could not be related to the sex or the colour of the aboral surface of the starfish. DISCUSSION This investigation carried out on a single population of starfish reveals that the caprellids are aggregated on to certain starfish and are not randomly distri- buted. The absence of a planktonic larval stage in the life-cycle of Pariambtls may partly account for the clumped distribution, since the young are retained in the brood lamellae of the females and are released as miniature adults. Most young probably stay on the same starfish resulting in the observed clumped distribution, .