Polymorphism in the Rotifer Asplanchna Sieboldi: Biomass, Growth, and Reproductive Rate of the Saccate and Campanulate Morphotypes

Abstract

Saccate and campanulate ♀ ♀ of the 12C1 clone of Asplanchna sieboldi were cultured at 25 degrees C, the former on Paramecium aurelia and the latter on Asplanchna brightwelli or A. sieboldi together with 10(—7) M d—alpha—tocopherol. The dry weights of newborn and gravid, adult saccates were 0.49 and 2.14 mg/♀; corresponding weights for A. brightwelli—fed campanulates were 2.14 and 6.37 ♀♂. Thus, the campanulates were 4.37 and 2.98 ° heavier than saccates as neonates and gravid adults, respectively. The disproportionately great birth size and slight postnatal growth of campanulates may be adaptative in decreasing mortality from canibalistic adults and in maximizing the ability of young individuals to capture and eat large prey available to adults. Gravid, adult campanulates fed on conspecific saccates were heavier than those fed on the smaller A brightwelli–7.03 and 6.37 mg/♀, respectively. The ability of ♀ ♀ of this and the other morphotypes to alter their size in direct response to the size of their prey may be adaptive in maximizing the efficiency of prey capture and ingestion. In four experiments conducted at 22 degrees , 25 degrees , and 28 degrees , the intrinsic rates of natural increase of A. brightwelli—fed campanulates were significantly greater (p = 0.0260 and 1.10—1.29 ° higher than those of P. aurelia—fed saccates; at these temperatures, the r—values for the campanulates were 0.717, 0.854, and 1.058 or 1.216 per day, respectively. The campanulates lived slightly longer and produced more offspring than the saccates; their net reproduction rates varied from 9.08 to 12.18 and were significantly greater (p = 0.023) as well as 1.19 to 2.01 ° higher than those of saccates. The mean generation times of the saccate and campanulate ♀ ♀ were similar. The productivity of the campanulate ♀ ♀ at 25 degrees C was estimated to be about 4 x that of saccate ♀ ♀. Campanulates fed from birth on the rotifer Brachionus calyciflorus had life—table—parameter responses similar to those fed on A. brightwelli. Saccate ♀ ♀ of the 10C6 clone fed on B. calyciflorus produced about 2 x as many offspring and lived about 2 x as long as those fed on P. aurelia; the respective r—values were 1.298 and 1.664. The superiority of the rotifer—diet suggests that, in the experiments with the 12C1 clone, the reproductive potential of campanulates was greater than that of saccates primarily because of the diets used to maintain the two morphotypes. The well—known, inverse relationship between the size and the reproductive potential of an organism does not hold for the genetically identical @V @V of the saccate and campanulate morphotypes. Transformation of saccate ♀ ♀ to campanulate ♀ ♀ is not at a reproductive cost to the population. Both morphotyes appear to be designed for comparably rapid, parthenogenetic reproduction, and in concert they may maximize the ability of a population to utilize a wide variety of prey in a very large size range.