Why size matters: mouth gape, larval feeding efficiency, rotifer mass
Considerable attention has been given to obtaining small rotifers. This is because the small mouth size (gape) of many marine fish larvae requires a small prey. While the relationship between measured mouth gape and prey size would seem to be straightforward, there are significant difficulties in relating measured gape to larval food preference. Studies have shown that copepods are consumed even when they are much longer than larval gape, as long as the copepods are not as wide as the larval gape. Other studies have shown that larvae prefer prey organisms significantly smaller than their gape.
- Lorica length and width data for 70 strains of L, S and SS-Type rotifers from Hagiwara and Kuwada in The Second Hatchery Feeds and Technology Workshop, Sydney, September 30-October 1, 2004, p. 28
- Additional data points were included for the RMI “Mini-L 160” and UNCW 120 µm SS-Type rotifer available from Aquatic Ecosystems Inc.
- The orange line shows rotifer mass, derived from lorica length. It is calculated by simple volumetric projection from the RMI “Mini-L 160” length to mass ratio. The curve represents L-Type, not S-Type rotifers (the two strains have different body shapes).
Brachionus plicatilis tend to be longer than they are wide whereas B. rotundiformis is more nearly round. If larval prey capture is limited by the width of the prey, then a longer body shape (L-Type) potentially delivers more biomass per rotifer. If however, prey capture is limited by prey length, then a rounder body shape delivers more biomass for the same rotifer length.
Some additional size and shape information:
Puvanendran et al. measured first feeding Atlantic cod at 4.3 mm with a gape of about 160 µm. Larvae were fed small (192 X 150 µm) and large (242 X 181 µm) rotifers under a number of different conditions. First-feeding cod seemed not to ingest the large rotifers until day 8, and preferred the small rotifers until day 20, when the rotifers were 5.7 mm long with a gape of about 290 µm. Significantly, larvae were able to ingest rotifers longer than their gape, presumably because rotifer width was less than the larval gape and/or because the rotifers are deformable.
Data on other species are scanty. First-feeding Red Drum have been shown to consume relatively large prey so long as the width is less than the gape (220 µm). Hamasaki et al. demonstrated that Amberjack (a particularly small-mouthed fish) prefer 140 µm rotifers at first feeding, but will consume rotifers over 200 µm when offered larger rotifers.
Metabolic Cost of Rotifer Size for Larval Fish
However, smaller rotifers come at a metabolic cost associated with feeding. To obtain comparable nutrition, a larval fish must consume four 120 µm rotifers to equal the value of each 190 µm rotifer they consume. A recent publication by SINTEF in Norway found higher survival and specific growth rate when cod larvae were fed larger (270 µm) instead of smaller (180 µm) rotifers.
Rotifer Size and Feeding and Enrichment Protocols
It is standard practice to state feeding and enrichment protocols as if all rotifers are the same. Yet clearly, if you change rotifer strains to a strain twice is long as your are accustomed to, the you can expect the dry weight biomass to increase by 2 cubed (8-fold). Naturally you should expect feed and enrichment requirements to expand 8-fold as well.
The effect of Rotifer size on feeding and enrichment protocols is discussed more fully in the section on production.
 V. Puvanendran, K. Salies, B. Laurel, and J.A. (2004) Size-dependent foraging of larval Atlantic cod (Gadus morhua) Can. J. Zool. 82: 1380–1389.
 Justin M. Krebs & Ralph G. Turingan (2003). Intraspecific variation in gape–prey size relationships and feeding success during early ontogeny in red drum, Sciaenops ocellatus. Environmental Biology of Fishes 66: 75–84,.
 K. Hamasaki et al. (2009) Aquaculture 288:216–225
 Gunvor Øie, Ingrid Overrein, Sunniva Wannebo Kui, Yngvar Olsen, Kjell Inge. SINTEF Fisheries and Aquaculture AS, Trondheim, Norway http://www.sintef.no/upload/Fiskeri_og_havbruk/Faktaark/Different%20rotifer%20(Brachionus)%20size%20in%20firstfeeding%20of%20cod%20(Gadus%20Morhua).pdf