Effect of Spartina alterniflora Loisel, 1807 on growth of the stout razor clam Tagelus plebeius (Lightfoot, 1786) in a SW Atlantic estuary

• Ecosystem engineer effects of Spartina alterniflora are evaluated.
• S. alterniflora generates changes in sediment characteristics relevant for clams.
• There is an uncoupling in clam's somatic and shell growth.
• There is a positive effect of S. alterniflora on condition index of clams.

Aquatic organisms that produce epibenthic structures such as saltmarsh plants play key roles in habitat functioning through their ecosystem engineering effects. Intertidal Spartina species are well known for modifying tidal hydrodynamic and sediment processes, affecting cohabiting infaunal organisms. The stout razor clam Tagelus plebeius is an infaunal filter-feeding species that inhabits a broad range of mudflats (MF) and Spartina saltmarshes (SM) along the American Atlantic coast. Through descriptive and experimental approaches conducted at the Bahía Blanca estuary (Argentina, 38°47′S, 62° 20′W), we evaluated ecosystem engineer effects of Spartina alterniflora (due to changes in sediment and water characteristics) on somatic and shell growth of T. plebeius. Comparison of bulk sediment parameters indicated that the sediment is more easily eroded in the SM than in the MF, but softer in the latest. In addition, food supply for clams (i.e., total particulate organic matter) was higher in the MF although food quality (i.e., percentage of organic matter in the water column) was in general higher in SM. Shell growth was higher but clam condition index (CI) was lower in the MF, and the opposite pattern was found in the SM. These results represent an uncoupling between shell and somatic growth with a positive effect of S. alterniflora on the CI of clams. Nevertheless, results of field experiments manipulating stems and roots of plants separately suggested that the effect of Spartina on clam's CI was not due to the structural changes generated by plant presence itself, although a better food quality in the SM may be related to the larger clams CI.