Establishment of the Green Alga Codium Fragile ssp. Tomentosoides on New Zealand Rocky Shores: Current Distribution and Invertebrate Grazers

Abstract

1. The green alga Codium fragile ssp. tomentosoides has recently become established on New Zealand rocky shores in spite of a diverse and abundant assemblage of invertebrate herbivores, many of which consume native species of Codium. 2. The alga was initially reported from the port of Auckland in 1973; it now occurs on many wave-protected, east-coast shores of the North Island but not at wave-exposed, west coast beaches or in the south at Wellington Harbour or Cook Strait. 3. Of 11 common species of grazers tested in laboratory feeding trials, four gastropods and two echinoids consumed the introduced C. fragile ssp. tomentosoides. In the field, the major intertidal grazers on this alga were the snail Turho smaragdus (a generalist herbivore) and the ascoglossan ( = sacoglossan) sea slugs Placida dendritica and Elysia maoria (specialist herbivores). 4. In laboratory experiments, herbivores were offered pairwise choices of the invasive alga C. fragile ssp. tomentosoides and the sympatric, native, encrusting congener C. convolutum. The generalist snail Cookia sulcata and sea urchin Evechinus chloroticus preferred the invasive alga whereas one of the ascoglossan sea slugs (P. dendritica) preferred the native species but the other had no preference. 5. When grazers were offered pairwise choices of the invasive C. fragile ssp. tomentosoides and the native ssp. novae-zelandiae, T. smaragdus and P. dendritica preferred the introduced alga whereas other grazers preferred the native subspecies or exhibited no preference. 6. Feeding preferences were not related to herbivore size, diet breadth, life history, or geographical range, and differences in algal structural morphology were not clearly related to herbivore choice. 7. Field observations and an algal transplant experiment indicated that intertidal herbivores exerted little grazing pressure on C. fragile ssp. tomentosoides. 8. Results of this study suggest that the introduced alga will eventually successfully invade most of the protected to semiexposed shores of New Zealand despite the diverse herbivore fauna.