Predators and Grazers
Codium setchellii forms crusts on rocks in the intertidal zone and is commonly grazed by intertidal animals. There has been extensive work on the herbivorous ascoglossan sea slug Placida dendritica and its effects on C. setchellii, a subject discussed below. Studies from Oregon have identified a number of grazers on C. setchellii, including the gastropod Lacuna marmorata, various gammarid amphipod species, and idoteid isopods (Trowbridge 1992a). The gastropod snail Tegula funebralis was observed on C. setchellii at Carmel (CA). It is unknown whether this species was actively grazing on the alga. The Tegula were likely using the physical structure of the alga as a means of avoiding predation, aggregating within the bulbous ripples of the thallus.
Unique interactions have been recorded between the alga C. setchellii and grazing sea slugs in the coastal waters of Oregon. Here, the ascoglossan sea slug Placida dendritica feeds on green algae (specifically C. setchellii, C. fragile, and Bryopsis corticulans). The slug ingests the chloroplasts from the algae upon which it grazes, however, contrary to popular belief, the slug does not derive any photosynthetic benefit (i.e. the chloroplasts do not continue to photosynthesize sugars to feed the slug); this is different from most ascoglossan slugs (Trowbridge 1992a). The specific algal diet of the slug actually determines the animal's morphology; the cerata (dorsal gut projections on the slug) were shorter and more numerous on individuals feeding on Codium specimens (Trowbridge 1997). Furthermore, those feeding on Codium as opposed to Bryopsis usually had a wider body (Trowbridge 1997), though maximum live mass was lower in individuals living on Codium compared to those on Bryopsis (Trowbridge 1992b).
Studies from the coast of central Oregon found peak densities of P. dendritica in May and June, likely related to environmental stress (Trowbridge 1998). Recruitment of larval slugs occurred continuously in spring and summer, often in significant numbers, with rapid growth, with most ascoglossans typically sexually mature one month after recruitment (Trowbridge 1992b). These aggregations of slugs may actually facilitate feeding (and ultimately growth). Studies from the lab of P. dendritica feeding on Codium found growth generally increased in the presence of conspecifics (Trowbridge 1991). Such results were size-dependent, however, with smaller individual able to grow faster in the presence of any conspecific, but large individuals only exhibiting increased growth in the presence of similarly-sized conspecifics (Trowbridge 1991).
It appears that the large aggregations of P. dendritica occurring during the warmer summer months support the notion that this species forages on C. setchellii during periods of environmental stress. Similarly, the slugs become increasingly abundant in the high intertidal and on the eastern ("landward") face of the alga, where desiccation is highest (Trowbridge 1998). It is interesting to note that P. dendritica may actually set the distribution and range of its host alga, including the range of C. setchellii. Given the rapid nature of recruitment of larval slugs and the efficacy with which the larvae consumed their host, there was rapid consumption of the alga. However, P. dendritica is less tolerant of physical disturbance (e.g. wave exposure and sand scour) than C. setchellii, thus there are functional refuges in highly disturbed habitats (Trowbridge 1992a).This differential tolerance and rapid degradation of the host alga may thus limit the distribution of C. setchellii in the intertidal zone and account for spatial variation of the species.
Codium setchellii in Intertidal Communities
Codium setchellii is a common alga in the intertidal assemblage at low tidal heights. Space is a limiting factor in the intertidal zone, as both animals and plants compete for this limited resource. In most areas, C. setchellii was observed as but one component of the benthic assemblage. It was observed amongst a wide variety of algal forms at Point Pinos, including articulate coralline algae, fleshy red algae (including Mastocarpus papillatus, Mazzaella flaccida, and Prionitis sp.), and the surfgrass Phyllospadix.
At the Carmel site, C. setchellii was observed forming essentially a monoculture. Rocks were completely covered in the alga, seemingly to the exclusion of other benthic species in some areas. However, even here, the C. setchellii crust hosted diverse plant and animal communities. Various epiphytes were observed on the C. setchellii thallii. Interspersed within the C. setchellii crusts were some fleshy algae (including Codium fragile, Egregia menziesii, Mastocarpus papillatus, Mazzaella flaccida). In some areas, sessile benthic invertebrates were also observed in between the C. setchellii crusts, including some unidentified sponges, tunicates, and the anthozoan Anthopleura sp. As mentioned above, the gastropod Tegula funebralis was often observed on the algal crust.
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2005 Raúl Nava. Text and images freely available for personal,
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