Diatoms are widely used in abalone hatcheries to induce larval settlement (=attachment and metamorphosis) but there are few data on the factors that influence their effectiveness as settlement cues. Of 16 diatom strains tested in Experiment 1, half induced ≥80% attachment and >50% metamorphosis within 4 days. Settlement success did not correlate significantly with diatom abundance or adhesive strength (P > 0.05). Several diatom strains interfered with settlement. Examples included (1) smothering by highly mobile diatoms (Nitzschia longissima); (2) shells becoming stuck to sticky secretions (Navicula britannica, Achnanthes longipes); and (3) unstable diatom cells preventing pedal attachment (Licmophora sp.). In Experiment 2, only 1 of 15 diatom strains induced over 70% metamorphosis, and three others induced 22–36% metamorphosis, within 4 days. Older cultures of a strain induced higher attachment and metamorphosis than younger cultures of the same strain (P < 0.001), but some old cultures still gave poor settlement. For the young diatom cultures, larval attachment correlated positively with diatom percent cover (r = 0.89, P < 0.05), and metamorphosis with the growth phase of the diatom film (r = 0.91, P < 0.05). Experiments 3–5 examined the role of bacteria in settlement induction by diatom films. In Experiment 3, films of 8 diatom species from Experiment 1 were regrown from ∼6 individually isolated and rinsed cells, likely altering the associated bacterial flora. Metamorphosis was 179 fold lower on average than in Experiment 1 (P < 0.0001), but attachment and diatom density were not significantly different (P = 0.87 and P = 0.75 respectively). In Experiment 4, Nitzschia ovalis grown and assayed with antibiotics had lower metamorphosis after 2 days than the same strain grown and assayed without antibiotics (6 vs 64%, P = 0.004), whereas attachment after 2 days did not differ (94 versus 92%, P = 0.49). In Experiment 5 bacteria from a Nitzschia ovalis culture induced as much attachment and metamorphosis as the intact diatom film, whereas cell-free supernatant was much less effective. The presence of antibiotics in settlement assays reduced attachment and metamorphosis by biofilms, but not by coralline algae (Phymatolithon repandum) or GABA. In Experiment 5 metamorphosis on diatoms and bacteria occurred gradually over 2 wk, whereas on coralline algae and GABA it occurred within a few days. This study suggests that many diatoms cue rapid larval attachment, but few induce consistently strong metamorphosis within 4 days in laboratory conditions. The bacteria present in diatom films affect the settlement-inducing activity of the film in at least some cases. The effectiveness of diatom films was generally higher for more mature films but overall, physical factors explained little of the variation in the activity of diatoms.
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Vol. 26 • No. 2