The linking of polyp and medusa stages in Obelia and other coelenterates

Abstract

Although medusa‐release was first seen in Obelia in the mid‐18th century (Baster, 1762b), the true nature of medusa‐release in any coelenterate was not understood until Sars' (1829) observations on a scyphozoan scyphistoma. A flourish of work followed during which many coelenterate life‐cycles were worked out, that of Obelia being among the first (by van Beneden, 1843). The impact on some areas of zoological thought of the mid‐19th century was considerable. Around the 1880s Obelia became used in teaching to illustrate and introduce the concept of alternation of generations and, alongside Hydra, as a “type” example of the Hydrozoa and of the Coelenterata. It remained important as a teaching organism for many years.

Evolution in leptolid life-cycles (Cnidaria: Hydroida)

“…The term of course wrongly implied a two-state life-cycle rather than a succession of generations (i.e., including the planula), but the phrase caught on and has been widely used in cnidarian and other literature in the English language. Since in biological teaching the concept of 'alternation of generations' is introduced early on through the study of cnidarians (Cornelius, 1977), the mis-translation must have had a misleading influence on countless biology students using English-language texts. Thus Hyman (1940Hyman ( , 1959 expressly stated that the phrase 'alternation of generations' was an alternative to "metagenetic' but this completely ignored the third, important, planula stage.…”

“…Some species, notably Orthopyxis integra (Macgillivray, 1842) and the athecate Halocordyle disticha (Goldfuss, 1820), release their morphologically-reduced medusae on some occasions and not on others (references in Brinckman-Voss, 1970;Cornelius, 1977Cornelius, , 1982. Thus in Mediterranean populations of O. integra it was reported by Stefani (1956Stefani ( , 1959) that different environmental factors seemed to trigger the two alternative processes of medusa development and medusa suppression.…”

Towards a phylogenetic classification of Leptothecata (Cnidaria, Hydrozoa)

“…The classification was also based on skeletal characters of the polyps and, when present, on the medusa stage. Classification problems were recognized for some groups, including uncertainties due to the lack of information on the life cycle of the polyp and the medusa stages (Allman 9 , Hincks 8 , Cornelius 10 , Calder 11 ). In 1871, Allman 12 proposed the taxon Calyptoblastea based on characters of the skeleton surrounding the polyp stage and variations in the life cycle: “Calyptoblastic ([...] covered; [...] bud).…”

Individuals at the Center of Biology: Rudolf Leuckart’s Polymorphismus der Individuen and the Ongoing Narrative of Parts and Wholes. With an Annotated Translation

“…Thus the term ''alternation of generations'' implied both that the different stages comprised different individuals (of different ''generations'') and that the mode of generation (sexual or asexual) alternated as well. The Norwegian Michael Sars later described a similar phenomenon in the jellyfish Cyanea and Aurelia (Figures 3 and 4), in which the colonial polyp stage of the life cycle releases a c LYNN K. NYHART AND SCOTT LIDGARD medusa that sexually produces eggs, which are fertilized by sperm from another adult medusa; from these eggs emerges a different larval form that begins the life cycle anew (Sars, 1841;Cornelius, 1977). The polyp does not itself develop into an adult form, but asexually produces a bud that grows into the medusa form.…”