Di Li  (IMCS,  Rutgers University)     

li@imcs.rutgers.edu

 

Evaluating essential shellfish habitat of hard clams, Mercenaria mercenaria, during larval settlement and early recruitment

 

Settlement and recruitment to benthic populations depend on the number of competent larvae transported to sites and/or the surface of the substratum. Amphipod tube mats, so-called “tape mud”, made by Ampelisca spp. is a widespread bottom feature of several estuaries in the northeastern US. When amphipods colonize silt/clay sediments in a typical urban estuary they increase the median grain size by incorporating fine sediments into their mucus tubes and fecal pellets, hence changing the roughness and the thickness of the viscous sub-layer of the benthic boundary layer.  There is some evidence that these tape mud habitats have a positive influence on larval settlement and/or the subsequent survival of recruits. Hard clams are more abundant in the southeastern portion of Raritan Bay, New Jersey where amphipod tube mats and fecal pellets dominate the substrate. 

 

This study is designed to determine whether the high abundance of hard clams in tape mud habitats is due to increased larval settlement or due to post-settlement processes.

 

Three approaches have been used: 1) a field study of the abundance of Mercenaria mercenaria juveniles and other macrofauna in contrasting habitat types in Sandy Hook Bay, 2) a racetrack flume experiment on larval settlement of M. mercenaria into different habitat types, and 3) growth of post-settlement juveniles M. mercenaria in different habitat types.

 

Racetrack Flume  (F1) – settlement experiments

 

Competent pediveliger larvae of Mercenaria mercenaria are used for the settlement experiments. Two sets of sediment treatments in a 4-chamber tray are placed flush with the flume floor in the working section including unpelletized mud, tape mud with and without the resident infauna (amphipods), and sand. Larvae are introduced upstream of the sediment trays and circulated around the flume for 24 hours. A free stream flow of 5 cm sec^-1 has been characterized with respect to the vertical and cross stream dimensions using an LDV. Phytoplankton food is added to the flume during the experiment. Temperature is kept at ~20^oC.  At the end of the experiment the different sediment treatments are preserved and sorted promptly to determine the number of larvae settling into the different sediment treatments.

 

Annular Flume  (F5) – growth experiments

 

Recently settled (~1mm) clams Mercenaria mercenaria are used for the growth experiments. Two sets of 4 different sediment treatments in eight dishes are placed flush with the sand covering the flume floor. Dishes are arranged randomly. We add 30 clams to each dish and place the dishes in the flume; the flume is run for ~10 days. A free stream velocity of 5 cm sec^-1 is used. Phytoplankton food is added to the flume daily. Temperature is kept at ~20^oC. The shell length of clams is measured before and after the experiment to determine the effect of habitat type on shell growth.