Aspects of Egg Attachment in Austropotamobius Pallipes (Lereboullet, 1858) (Decapoda, Astacidae)
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ASPECTS OF EGG ATTACHMENT IN AUSTROPOTAMOBIUS PALLIPES (LEREBOULLET, 1858) (DECAPODA, ASTACIDAE) BY W. J. THOMAS 6 Stewarthall Gardens, Lockerbie Road, Dumfries, DG1 3AS, Scotland, U.K. ZUSAMMENFASSUNG Die Eier von A. pallipes sind am Weibchen durch einen spiraligen Funikulus befestigt, der bei der Eiablage gebildet wird. Bei seiner Enstchung kommt es zu einer Fusion der weichen Außenschicht eines frish abgelegten Eies mit einer Gruppe von Borsten (Ooseten), die sich auf den Pleopoden und Sterna des Hinterleibs befinden. Seine endgültige Form erhält der Funikulus in einer späten Phase des Eiablagegeschehens. INTRODUCTION Apart from the early studies of Lereboullet (1860) and Andrews (1906), there are only scattered references to egg attachment in the freshwater crayfish, notably those of Malaczynska-Suchcitz (1956), Cheung (1966) and Mason (1970). Little attention has been focussed on the origin and anatomy of the connection between eggs and female in Austropotamobius pallipes (Lereboullet, 1858) the only British species of the genus. This study is based on observations of spawning, and scanning electromicroscope photographs of ovigerous females taken from the river Darent, Kent, England. The sequence of spawn- ing in A. pallipes has been described elsewhere (Ingle & Thomas, 1974). Further observations of the process showed egg attachment as occurring in mid sequence resulting finally in discrete clusters of eggs becoming tightly bound to the ventral surfaces of the abdomen of females. MATERIALS AND METHODS During the period between October 12th and November 4th 1987 female specimens of A. pallipes carrying spermathecae were collected by hand from the river Darent, Kent, England at points within the National Grid reference 539657 and 533656. Observations of their subsequent spawning behaviour were made from small numbers of specimens kept in aquaria and also in the field, where spawning was often induced by moving the females from the river bed into buckets of river water placed alongside the point of capture. 288 After fixing in Bouin's fluid some eggs were hand sectioned, freeze-dried and stored in a dessicator until required. Portions of the egg carrying pleopods were similarly treated. Material was mounted directly on a standard stereoscan stub with a colloidal silver in iso-butyl ketone solution. The specimens were then coated with carbon. All observations were made using an I.S.I. Super IIIA Stereoscan scanning microscope. OBSERVATIONS All eggs were connected individually to the females by an egg stalk, the funiculus, to specific sites on the underside of the abdomen. Most, about 95 9lo , are attached to each of 6 groups of specialized setae, the oosetae, set on the pro- topod and proximal parts of the endopod and exopod of each pleopod. The remainder become attached to oosetae present on the sterna of abdominal segments 1-5 (figs. 1A, B). The 6th abdominal segment is devoid of oosetae so carries no eggs. This makes a total of 53 groups of oosetae on females of A. pallipes, all potential sites for egg attachment. Essential for attachment, their distribution on the abdomen exactly matched that of attached eggs and is cons- tant for all individuals of the species. With the number of oosetae in each of the 53 groups increasing at each moult, it is clear that the size of the female is closely related to fecundity. Older, larger females carried significantly more eggs than those younger and smaller, due in part to the greater number of oosetae present on their pleopods. It is during the third phase of spawning (Ingle & Thomas, 1974), that eggs are extruded forcibily through the oviducal openings into a glair filled brood chamber. In A. pallipes the glair is produced from 15 to 20 minutes before extrusion. 1'he eggs flow down the mid-line of the reflexed abdomen, between the proximal parts of the pleopods which effectively arch over them. At this stage, the eggs have a smooth surface, soft outer shell with no fixed shape, and move through the glair in floating amoeboid fashion. The pleopods, motionless during the extrusion of both glair and eggs begin to undulate moving the heavy eggs, which having lost their initial momentum, gravitate to the floor of the brood chamber. With the female now supine the eggs are wafted through the groups of oosetae present on the proximal parts of the pleopods and sternal ridges (fig. 1A). The combined effect of the arched roof created by the moving pleopods, the glair enclosed within the brood chamber, and the rolling and turning movements of the female, serve to keep the eggs close to their potential attachment sites. It is here eggs and oosetae make contact, about 36 hours into the spawning period, marking the beginning of funiculus formation. Clearly the funiculus is of dual origin, the meeting of the soft outer egg shell with the distal ends of the oosetae (fig. 1 B) resulting in attachment, made more secure by the flowing of egg shell material onto the oosetae. Often as much as half the length of the oosetae is incorporated within the egg shell to form the base of the funiculus (fig. 1C, E). .