Intertidal facilitation and indirect effects: causes and consequences of crawling in the New Zealand cockle

Abstract

Bioturbation by the ghost shrimp Callianassa filholi and the lugworms Abarenicola affinis as well as coverage by macroalgae cause the New Zealand cockle Austrovenus stutchburyi (Veneridae) to relocate by crawling longer distances on the sediment surface. On the surface, the cockles become targets for sublethal predation by benthic-feeding fishes, which crop off their feet. This renders the cockles unable to bury for up to 8 wk, thus exposing them to a 5-fold higher predation pressure from shorebirds and the whelk Cominella glandiformis than buried conspecifics. Shell dimensions, foot size, general condition, gender, age, and infections by gymnophallid trematodes do not influence the crawling activity of cockles. However, heavy infection by the digenean trematode Curtuteria australis and the shell-boring spionid polychaete Boccardia acus reduces the distance travelled, and these organisms may hence reinforce the impact of the biogenic disturbers by forcing repeated crawling of the cockles to reach an undisturbed site. The presence of biogenic disturbers and some parasites therefore benefits predating fishes by providing them with an otherwise inaccessible food source, and these in turn benefit birds and whelks through foot-cropping, thus increasing the availability of prey. The gymnophallids also receive an advantage through transmission success to their definitive shorebird hosts. The question as to whether Curtuteria australis benefits, depends on the balance between its loss to fishes (unsuitable hosts) and its increased trophic transmission to its bird hosts. The spionid B. acus on the other hand is negatively affected, since its fate is linked to that of its substrate, which is the shell of the cockle. If the cockle is predated, the attached B. acus will perish also.

Natural selection by pulsed predation: survival of the thickest

“…Only spat up to a size of 3.5 mm is capable of migration in the water column over large distances (Armonies 1992). Larger cockles are capable of crawling over the surface at speeds of 0.6 cm/ d (Flach 1996), but speeds of 50 cm/d have also been reported (Mouritsen 2004). Such speeds will correspond to an average linear movement of 0.08 m, and 7 m at the most, during our short study period.…”

Cockle Cerastoderma edule population mortality: role of the digenean parasite Himasthla quissetensis

“…(1) the amputation of bivalve siphons (Maillard 1976); (2) the alteration of clam valve edges, which then remain open and make the host more sensitive to desiccation or predation (Bartoli 1974); (3) the secretion of ligament proteins under the umbo, preventing closing of the valves (Bowers et al 1996); (4) an immunodepression of parasitized organisms, making them more susceptible to stress (oxygen depletion, pollution, heavy metals, osmotic and/or thermic stress) (Lauckner 1983; or (5) the reduction of burrowing performance , Mouritsen 2004. In the case of Himasthlinae, metacercariae also could induce harmful tissue damage such as: (1) destruction of muscle fibres, as cercariae pass through muscle layers situated beneath the epidermis of the viscero-pedal mass ; (2) tissue lysis caused by cercarial enzymes (Lauckner 1983); (3) loss of body fluids through penetration holes, which interfere with the cockle's filtration capacity ; (4) haemocytes and fibrous tissue accumulation around the cyst secreted by the parasite ); (5) distortion of cells and hypertrophy of the infected organs (Lauckner 1983); and (6) an association with a bacterial load, increasing mortality rates .…”

Positive effects of a dominant invader on introduced and native mudflat species

“…Positive interactions, namely mutualisms (+/+) and 1 side of both commensal (+/0) and exploitative (+/-) relationships, have thus far only rarely been reported between marine invaders (Crooks 2002, Levin et al 2002, Wonham 2003. As the prevalence and ecological importance of positive interactions are increasingly recognized in marine systems (Peterson & Heck 2001, Stachowicz 2001, Bruno et al 2003, Mouritsen 2004, we anticipate an increase in reports of facilitative interactions with invaders as well. Based on observations in other systems (Simberloff & VonHolle 1999, Richardson et al 2000) these facilitations will likely include pairs of introduced species that were previously associated in their native ranges, and non-co-evolved species in their new regions of sympatry.…”