82
H.U. Riisgard and P. Kamermans
growth in M. balthica when limited to suspension feeding only. Exclusive suspension feeding for M. balthica is unlikely because the requisite phytoplankton concentrations are far above in situ values. Thus, a mean value of
51lg chi a I-I applies for coastal Danish waters (Riisgard 1998), and in, e.g., the
Marsdiep tidal inlet (Dutch Wadden Sea), the annual average is about
81lg chla I-I while the monthly averages vary from Illg chi a in winter to
30 Ilg chi a during the spring peak period (CaMe and Hegeman 1993, see also
Beukema and Cadee 1997). Other studies by Specht and Lee (1989) suggest
that a related species M. nasuta is not a very efficient suspension feeder
because of a low weight-specific pumping rate compared with rates for
obligate suspension-feeding bivalves. Further, in contrast to Reid and Reid
(1969), Hylleberg and Gallucci (1975) found that M. nasuta is primarily a
deposit feeder that indiscriminately sucks the top millimeter of the sediment
surfaces rich in settled organic material, and selection of food material subsequently takes place on the gills and palps. Meyhofer (1985) measured both
filtration rate (F,l h- I ) and gill area (G, cm 2 ) as a function of size (W, g body
soft weight) of M. nasuta and found: F=0.08Wo.86 and G=2.85Wo.63. This shows
that the weight -specific filtration rate is about 100 x lower than found for the
blue mussel Mytilus edulis, and other true suspension-feeding bivalves (e.g.
M0hlenberg and Riisgard 1979; Riisgard 2000). Further, the approach velocity
of water to the M. nasuta gill surface can be estimated as F/G=O.1 mm S-I
which is about 10 times lower than found for M. edulis (Riisgard and Larsen
2000). All together this indicates that the gills of M. nasuta are not adapted
to suspension feeding in the traditional sense. For a detailed account on
present knowledge on switching between deposit and suspension feeding in
M. baltica, see 'Example II' (Sect. 4.3).
Wilson (1990) examined the mode offeeding by both Tellina tenuis and T.
fabula and found that they could function as suspension feeders, although
Trevallion (1971) suggested that suspension feeding alone would not suffice to
meet the energy demands of T. tenuis.
4.1.4 Amphipods
The amphipod sibling species Ampelisca vadorum and A. abdita are widespread in the shallow marine waters of North America, and two methods of
feeding are common in these amphipods (Mills 1967). Occasionally, the
second antennae pick up sand grains from the sediment surface and cast
them inwards to the midline of the body to be held by the gnathopods, and the
micro flora is then scraped off with the mouthparts and discarded (Fig. 4.10.
An animal may also tip forward and grasp sand grains with its mouthparts.
The second method of feeding involves currents set up by the pleopods and
second antennae that whirl rapidly besides the body. The beat of the pleopods
H.U. Riisgard and P. Kamermans
growth in M. balthica when limited to suspension feeding only. Exclusive suspension feeding for M. balthica is unlikely because the requisite phytoplankton concentrations are far above in situ values. Thus, a mean value of
51lg chi a I-I applies for coastal Danish waters (Riisgard 1998), and in, e.g., the
Marsdiep tidal inlet (Dutch Wadden Sea), the annual average is about
81lg chla I-I while the monthly averages vary from Illg chi a in winter to
30 Ilg chi a during the spring peak period (CaMe and Hegeman 1993, see also
Beukema and Cadee 1997). Other studies by Specht and Lee (1989) suggest
that a related species M. nasuta is not a very efficient suspension feeder
because of a low weight-specific pumping rate compared with rates for
obligate suspension-feeding bivalves. Further, in contrast to Reid and Reid
(1969), Hylleberg and Gallucci (1975) found that M. nasuta is primarily a
deposit feeder that indiscriminately sucks the top millimeter of the sediment
surfaces rich in settled organic material, and selection of food material subsequently takes place on the gills and palps. Meyhofer (1985) measured both
filtration rate (F,l h- I ) and gill area (G, cm 2 ) as a function of size (W, g body
soft weight) of M. nasuta and found: F=0.08Wo.86 and G=2.85Wo.63. This shows
that the weight -specific filtration rate is about 100 x lower than found for the
blue mussel Mytilus edulis, and other true suspension-feeding bivalves (e.g.
M0hlenberg and Riisgard 1979; Riisgard 2000). Further, the approach velocity
of water to the M. nasuta gill surface can be estimated as F/G=O.1 mm S-I
which is about 10 times lower than found for M. edulis (Riisgard and Larsen
2000). All together this indicates that the gills of M. nasuta are not adapted
to suspension feeding in the traditional sense. For a detailed account on
present knowledge on switching between deposit and suspension feeding in
M. baltica, see 'Example II' (Sect. 4.3).
Wilson (1990) examined the mode offeeding by both Tellina tenuis and T.
fabula and found that they could function as suspension feeders, although
Trevallion (1971) suggested that suspension feeding alone would not suffice to
meet the energy demands of T. tenuis.
4.1.4 Amphipods
The amphipod sibling species Ampelisca vadorum and A. abdita are widespread in the shallow marine waters of North America, and two methods of
feeding are common in these amphipods (Mills 1967). Occasionally, the
second antennae pick up sand grains from the sediment surface and cast
them inwards to the midline of the body to be held by the gnathopods, and the
micro flora is then scraped off with the mouthparts and discarded (Fig. 4.10.
An animal may also tip forward and grasp sand grains with its mouthparts.
The second method of feeding involves currents set up by the pleopods and
second antennae that whirl rapidly besides the body. The beat of the pleopods