NUTRITION OF THE SEA URCHIN, STRONGYLOCENTROTUS PURPURATUS

¹ Hopkins Marine Station of Stanford University, Pacific Grove, California

1. The intestinal tract of the sea urchin, Strongylocentrotus purpuratus, consists of two loops, the first clockwise, the second counter-clockwise (as seen looking in at the mouth). Both loops are well supplied with glands.

2. Experiments present evidence for an amylase and a proteinase in the sea urchin intestine but none for enzymes capable of digesting entire algae or agar. However, the extract of ground gut was capable of digesting iridophycin, a galactan from the red alga, Iridophycus.

3. Agar-digesting bacteria are present in the second loop of the intestine in large numbers from 10⁶ to 10⁷ per ml. of gut contents. Total bacteria as determined by a count with a Petroff-Hauser counting chamber reach the value of 2 x 10¹¹ per ml. They are largely confined to the pellicle which surrounds each particle of alga.

4. The bacteria inoculated from the gut of the sea urchin are capable of completely digesting the alga, Iridophycus flaccidum, in the course of a week. Many of the intestinal bacteria are capable of digesting agar. Pure cultures of agardecomposing bacteria were isolated from the intestine.

5. The isolated bacteria grown on algae or agar do not liberate reducing sugars into the culture medium.

6. While it seems likely that the sea urchin obtains some nutrient from the activities of its intestinal flora, proof for this is lacking.

7. Some glycogen is stored in the tissue of the sea urchin and the body fluid normally contains a small amount of reducing sugar. No striking changes occur in the latter during the breeding season, but the sugar falls to zero or nearly zero on starvation. Glucose injected into the body fluid is removed by the tissues.

8. The body fluid of the sea urchin always contains a small amount of nonprotein nitrogen (NPN), even after two weeks of starvation, and no striking change occurs during the breeding season.

9. The stimulus that sets off the increased effectiveness of the economy of the sea urchin resulting in the growth of the gonads and accumulation of nitrogenous compounds in them is unknown. After gonads reach their maximal size spawning occurs.

10. Several cycles of gonadal growth and spawning are suggested but the data were not collected at sufficiently frequent intervals to make this certain.

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