Rotifer Composition: Which parts are digestible
Nutrition from rotifers can be divided into four functional areas:
- Gut load
- Internal soft tissue
Digestibility and nutritional significance of each of these components is a matter of debate; here is what we believe to be true:
- The Lorica is composed primarily of protein similar to Keratin and is largely indigestible. The Lorica is thin and amounts to no more than 10% of biomass.
- Rotifer eggs are similar in composition to rotifer biomass (15-20% lipid, 50-60% protein) and are also, by-and-large, indigestible. The volume of a rotifer egg is typically 15-20% of the volume of the parent, but egg biomass density is higher (30%) than rotifer tissue biomass (15%). In a culture with an egg to adult ratio of 1:3, 12-15% of biomass is indigestible egg.
- Together, indigestible egg and lorica amount to roughly 25% of biomass. Digestible soft tissue and gut load account for the remaining 75% of rotifer biomass
- Gut-Load: Rotifers can, with ease, continuously consume 10% of their biomass as microalgae every hour. The residence time of feed in the gut can be up to 2 hours. Their consumption rate can double during enrichment. If mean residence time of feed in the gut is one hour, gut load can amount to 15-20% of biomass and 20-25% of digestible tissue. It is possible that this number is higher. Thus, simply in terms of biomass, the composition of the feed in the gut significantly affects the nutritional value of a rotifer.
- Internal soft tissue account for the remaining 50-60% of biomass, and 75%-80% of digestible biomass.
One of the key challenges for larval fish, especially first-feeding larval fish, is that their digestive tracts are very poorly developed and digestion and nutrient uptake are difficult. Most larval fish must begin feeding soon after hatching and cannot rely solely on the egg yolk for nutrition more than 2 to 3 days post hatch (dph).
Larval fish have delicate digestive tracts and poor digestion. When a rotifer is ingested by a larva it is often squeezed and the internal organs and partially digested rotifer feed are forced out of the rotifer lorica into the larval gut. Often the lorica and attached eggs pass through the larva undigested. It is for this reason the most nutritionally valuable parts of a rotifer are its soft internal tissues and the food-loaded gut.
Gut-Loading and Digestive Function
Many hatchery managers strongly believe that the food-loaded gut of the rotifer is critical to larval fish nutrition. There are many reasons why this may be true.
- The rotifer gut can contain a large portion of total rotifer biomass. A microalgae-fed rotifer can ingest more than 10% of its biomass per hour and much of that algal biomass can remain in the gut for up to 2 hours (see footnote below). During enrichment the feed rate can be even higher. This adds up to a lot of biomass.
- When high-value microalgae are fed to the rotifer, this dense, highly nutritious and nutritionally balanced feed is concentrated and delivered straight to the larval gut.
- Much of the gut content of the rotifer is partially digested, as evidenced by the significant free fatty acid portion of an algae-enriched rotifer’s total lipid content (ca. 5%). Partially digested proteins, carbohydrates and other nutrients are also present in rotifers fed microalgal and other nutritionally-balanced diets.
- Rotifer guts contain essential digestive enzymes and acids that can aid the larval fish in digesting and assimilating nutrition.
- Active bacteria in the rotifer act as digestive probiotics to help establish the gut bacterial flora of the larval fish.
Much has been made of the fact that rotifer eggs and lorica are not digested by many marine fish. Some suggest this demonstrates the poor value of rotifers. Instead, it should be viewed as a testimony to the challenges that first-feeding marine fish have digesting their food and taking up nutrition; there can be no doubt about of the high value of rotifers and other live feeds.
 In a continuous rotifer system fed RotiGrow Plus and harvested at 45% per day, the average feed rate is 8.25% dry microalgal biomass/dry rotifer biomass per hour. Just after the daily harvest the feed rate exceeds 10.5%. And yet, feed conversion is just as efficient at the higher harvest and feed rate as it is at lower harvest and feed rates. This suggests complete nutritional uptake at feed rates in excess of 10% per hour.