Douglas E. Conklin

Ontogenetic development of the digestive system in California halibut (Paralichthys californicus) with notes on feeding practices

Abstract The development of the digestive tract and accessory glands in California halibut (Paralichthys californicus) is described from hatching to metamorphosis (42 days post-hatch, dph) at 18 °C. Differentiation of the alimentary canal into the buccopharynx, esophagus, pre- and postvalvular intestine, and rectum was complete by 3 dph (2.7±0.1 mm standard length, SL) coinciding with the time of first feeding. Zymogen granules (pancreatic enzyme precursors) were detected in the exocrine pancreas 1 day before the onset of exogenous feeding and their number increased after first feeding, confirming the importance of pancreatic secretions during the agastric period of larval development and their genetically rather than dietarily induced secretion. The stomach was morphologically differentiated at 27–30 dph (7.2–7.5 mm SL) coinciding with the onset of eye migration. At this stage, gastric glands were abundant in the fundic region and cardiac and pyloric regions were also clearly distinguishable. Supranuclear bodies were observed in the postvalvular intestine throughout the study period, although the number decreased as the stomach differentiated and extracellular digestion took place. The reduction of supranuclear bodies might be due to a change in the protein digestion mechanism. Histological observations showed that Artemia-fed larvae had almost intact nauplii in the postvalvular intestinal lumen and rectum, suggesting a fast intestinal transit in California halibut larvae, and a limited availability of nutrients from brine shrimp nauplii. Special attention might be required in rearing tanks during the first week of exogenous feeding, since the desquamation of the esophageal epithelium and the absence of mucous secretion due to the lack of functional goblet cells may promote bacterial infections under poor water quality conditions.

A successful purified diet for the culture of juvenile lobsters: The effect of lecithin

Abstract A purified diet that provides excellent survival (≥ 85%) and good growth of juvenile lobsters (Homarus americanus and H. americanus × H. gammarus hybrids) through 120 days is presented. When various natural foods (frozen shrimp tails, frozen mussel, frozen liver, and live Artemia salina) are substituted for the purified diet every fourth day, the Artemia supplement produces significantly greater increases in wet weight after 90 days of growth than the other supplements. The inclusion of soy lecithin in the purified diet is critical. Its absence reduces survival dramatically (55% survival within 30 days). Investigations reveal that different forms of lecithin have variable effectiveness and that choline, a significant constituent of lecithin, is probably not the active component. An optimal vitamin mix level for this artificial diet has been determined. The artificial diet has practical application for crustacean aquaculture.

Sterol requirement of cultured juvenile crayfish, Pacifastacus leniusculus

Abstract Changes in the sterol composition of an extruded diet revealed that juvenile Pacifastacus leniusculus requires at least 0.4% (dry weight) of dietary sterols for survival. A mixture of plant-derived sterols (phytosterols) is as effective as an equal amount of cholesterol in the partial satisfaction of the requirement. The experimental diet is composed primarily of unrefined feedstuffs and has been used in the intensive raceway culture of juvenile P. leniusculus . Growth was comparable to that achieved with a natural food diet composed primarily of vegetables and fish. The extruded diet can be formulated from readily available ingredients and should serve as a prototype for either intensive nursery or pond culture.

Relationship between dietary phosphatidylcholine and serum cholesterol in the lobster Homarus sp.

The absence of soy phosphatidylcholine (PC) in purified diets fed to juvenile lobstrs causes a significant decrease in the concentrations of total cholesterol and phospholipids in the serum. When egg PC, bovine PC, phosphatidylinositol, and ovine cephalin are used as substitutes for the soy PC, cholesterol and phospholipid levels in the serum are also significantly reduced. Cholesterol and phospholipid concentrations in the serum are highly correlated. Analysis of levels of hepatopancreatic cholesterol indicate that intestinal absorption of cholesterol is not impaired by the absence of dietary PC or by the presence of a phospholipid substitute. We suggest that the PC molecule is an important component of a lipoprotein complex that effectively transfers cholesterol, an essential nutrient for lobsters, from the hepatopancreas to the hemolymph.

Sterol requirement of juvenile lobsters, Homarus sp.

Abstract Juvenile lobsters were fed purified or unrefined diets in which the qualitative and quantitative sterol composition varied. Dietary cholesterol was required. A level of dietary cholesterol as low as 0.12% (dry weight) in a purified diet was satisfactory for normal growth and survival. No significant increases in growth were observed when lobsters were fed unrefined diets with cholesterol levels ranging from 0.19 to 0.59% (dry weight). Total cholesterol content of the whole body tissue lipid ranged from 0.012 to 0.046% (wet weight), lowest values being characteristically associated with those animals fed purified diets virtually devoid of cholesterol. Serum sterol levels were similar except for reduced levels associated with the sterol-free diet. Total replacement of cholesterol with a mixture of phytosterols composed primarily of β-sitosterol did not yield good growth and survival. Phytosterols composed large proportions of the total body sterols of those lobsters fed diets containing phytosterols exclusively. At least a portion and possibly all of the sterol requirement of juvenile lobsters is specific for cholesterol.

Food consumption and growth of juvenile lobsters

Abstract Experiments conducted on juvenile lobsters ( Homarus americanus ) measured the effects of several environmental variables on growth and ingestion of food. These variables consisted of feeding frequency, photoperiod, and temperature, and were selected on the basis of their potential importance to intensive culture systems. Specific consumption rates, defined as percent of body wet weight ingested daily, and determined by measurements of the ingestion of natural foods, were found to follow an inverse log-log relation to live weight. Significantly faster growth was obtained when juvenile lobsters were kept in near-constant darkness, and when they were fed 7 days per week instead of 5 days per week. Increased food consumption was observed when juvenile lobsters were maintained in near-constant darkness, as well as when temperature was increased. Feeding twice daily did not increase the amount of food consumed daily. The proportion of food consumed during the first 4h indicated that the lobsters ate periodically throughout the day. Although starvation for a 48-h period caused a significant increase in the proportion of food consumed during the next feeding period, this increase was not sustained more than 2 days. The evidence suggests that lobsters are slow, periodic feeders, and that food consumption and growth can be readily increased by manipulation of particular environmental factors.

Nutrient leaching from coated crustacean rations.

Abstract The use of ethyl cellulose as a coating agent to retain water soluble nutrients in pelleted feeds was investigated. When the entire ration pellet was coated, minimal nutrient leaching occurred, even after 6 h. Inclusion of individually coated micronutrients was less effective than total pellet coating for nutrient retention, but did help prevent losses due to leaching.