In situ quantification of a natural settlement cue and recruitment of the Australian sea urchin Holopneustes purpurascens

In the first study of its kind, the recruitment of the Australian sea urchin Holopneustes purpurascens is compared to quantitative in situ measurements of a naturally occurring settlement cue, histamine. More than 90% of new recruits were found on either the foliose red alga Delisea pulchra or on coralline turf algae while 8% of recruits were found on the brown alga Homeostrichus olsenii. These algae induced the settlement (and metamorphosis) of almost all larvae in laboratory assays after 24 h. No new recruits were found on Ecklonia radiata or Sargassum vestitum and unfouled E. radiata induced low levels of settlement in laboratory assays. Thus, the algae on which we found the most recruits in the field induced the highest rate of settlement in laboratory assays. D. pulchra contained far higher levels of histamine than all other algae and the coralline algae lacked measurable histamine. Seawater collected in situ adjacent to D. pulchra induced low levels of settlement of aged larvae in laboratory assays and contained the highest concentration of histamine (similar to 5 nM). With the exception of coralline algae, variation in settlement and recruitment was consistent with the variation among species histamine contents. Antibacterial treatment of Amphiroa anceps greatly reduced the number of larvae settling in response to the alga. Biofilms of 2 bacterial strains isolated from the surface of coralline algae induced settlement of 20 to 43% larvae in laboratory assays after 96 h and media in which these strains were cultured contained more histamine. These initial results support a biofilm derived settlement cue for larval H. purpurascens from coralline algae.