When is the abalone Haliotis discus hannai Ino 1953 first able to use brown macroalgae?

The dietary value of microscopic algal stages (gametophyte and juvenile sporophyte) of a brown alga Laminaria japonica, Areschoug 1851 and of the benthic diatoms Cylindrotheca closterium (Ehrenberg) Reimann and Lewin 1964, and Achnanthes longipes Agardh 1824 were examined for different developmental stages of Haliotis discus hannai Ino 1953 [0.4-2.9 mm shell length (SL)] to determine the size at which abalone begin to use macroalgae efficiently. Most individual abalone showed active feeding behavior, but there was considerable variation in growth of abalone between different algae and developmental stages of abalone. The growth rates of smaller post-larvae (0.4-1.2 mm SL) fed gametophytes and juvenile sporophytes of L. japonica, or A. longipes were significantly lower than those fed C. closterium. In contrast, juvenile sporophytes of L. japonica and A. longipes produced significantly faster growth in larger postlarval abalone (>1.8 mm SL) than gametophytes of L. japonica or C. closterium. Postlarvae in all developmental stages fed C. closterium actively grazed and efficiently ingested diatom cells. However, the relative dietary value of C. closterium decreased as abalone grew, probably because feeding efficiency on this diatom decreased because of its low cell volume and thin film-like colonies. Smaller post-larvae (0.4-1.2 min SL) grazed repeatedly on the same surface of gametophytes, juvenile sporophytes of L japonica, or on A. longipes without detaching these algae, whereas larger post-larvae (>1.8 mm SL) detached and ingested large amounts of whole cells of these algae. Postlarval abalone (>1.8 mm SL) began to use L. japonica gametophytes and juvenile sporophytes at approximately the same size at which morphologic changes occurred in their radulae, which enabled the ingestion of macroalgae.