Ontogenetic patterns and phylogenetic trends in freshwater flatworms (Tricladida); constraint or selection?

Abstract

Semelparity and iteroparity are unevenly distributed among the families of the Paludicola and this implies that there have been taxonomic restrictions on life-history evolution . Species differ in their investments in reproduction and high levels can be related, causally, to reduced life-spans ; i .e. semelparous species invest more in reproduction than iteroparous species. However, there does not appear to be any fundamental reason why the extent and timing of these investments should not be open to modification by natural selection .A major morphological difference between the predominantly semelparous Dendrocoelidae and the predominantly iteroparous Dugesiidae and Planariidae is the presence of an anterior adhesive organ; dendrocoelids have one, but members of the other families do not . A plausible scenario can be formulated relating this structure, causally, to enhanced juvenile survivorship which, in turn, favors the semelparous life history.

Overview of life cycles in model species of the genusDugesia(Platyhelminthes: Tricladida)

“…Benazzi & Gremigni 1982). Calow and Read (1986) distinguished two types of breeding cycles in triclads: semelparity (breed once and die) and iteroparity (breed repeatedly over several seasons). Iteroparity is dominant in the Dugesiidae and Planariidae, *Correspondence: G. A. Stocchino, Dipartimento di Scienze della Natura e del Territorio (DIPNET), Università di Sassari, Via Muroni 25, I-07100 Sassari, Italy.…”

Overview of life cycles in model species of the genusDugesia(Platyhelminthes: Tricladida)

“…Benazzi & Gremigni 1982). Calow and Read (1986) distinguished two types of breeding cycles in triclads: semelparity (breed once and die) and iteroparity (breed repeatedly over several seasons). Iteroparity is dominant in the Dugesiidae and Planariidae, *Correspondence: G. A. Stocchino, Dipartimento di Scienze della Natura e del Territorio (DIPNET), Università di Sassari, Via Muroni 25, I-07100 Sassari, Italy.…”

Overview of life cycles in model species of the genusDugesia(Platyhelminthes: Tricladida)

“…Benazzi & Gremigni 1982). Calow and Read (1986) distinguished two types of breeding cycles in triclads: semelparity (breed once and die) and iteroparity (breed repeatedly over several seasons). Iteroparity is dominant in the Dugesiidae and Planariidae, *Correspondence: G. A. Stocchino, Dipartimento di Scienze della Natura e del Territorio (DIPNET), Università di Sassari, Via Muroni 25, I-07100 Sassari, Italy.…”

Overview of life cycles in model species of the genusDugesia(Platyhelminthes: Tricladida)

“…Benazzi & Gremigni 1982). Calow and Read (1986) distinguished two types of breeding cycles in triclads: semelparity (breed once and die) and iteroparity (breed repeatedly over several seasons). Iteroparity is dominant in the Dugesiidae and Planariidae, *Correspondence: G. A. Stocchino, Dipartimento di Scienze della Natura e del Territorio (DIPNET), Università di Sassari, Via Muroni 25, I-07100 Sassari, Italy.…”