Settlement cues in the locally dispersing temperate cup coral Balanophyllia elegans.

Most studies of settlement cues in sessile marine invertebrates have focused on species with the potential to disperse over great distances. This persistent focus has perpetuated the idea that long-distance dispersing species may rely on cues to settle in specific habitats, whereas shortdistance dispersers are delivered directly into a favorable habitat. I tested the effects of water movement and substrate on the settlement of the temperate solitary cup coral Balanophyllia elegans, a species whose crawl-away larvae disperse just centimeters before settling. Ninety percent of larvae settled within 3 days of release in the presence of both natural rock substrate and moving water, but fewer than 11% settled in the same period when either or both factors were absent. An additional experiment revealed that when natural rock substrate was available, water velocities of less than 25 cm s 1 triggered a 5-fold increase in settlement rate relative to standing water. When settling, the larvae of B. elegans respond strongly to the interactive effects of water flow and substrate. Thus, settlement cues may play a significant role in generating the patchy distribution of B. elegans, a pattern previously attributed to short-distance dispersal. Larval settlement is a critical stage in the life history of sessile marine invertebrates. This transition from a mobile larval stage to a sessile juvenile stage represents a commitment to a particular location, and determines the subsequent conditions that an individual will encounter, including predation pressure (1), severity of physical stress (2), competition for space (2), competition for food (3), and fertilization success (4). Numerous studies have revealed that two factors influence the spatial pattern of settling larvae: a variable supply of larvae, which can lead to a correlated variable settlement pattern (5, 6); and selective larval settlement in response to patchy environmental cues, which can refine settlement patterns set by larval supply (7, 8). Variable larval supply can lead to heterogeneous settlement patterns in both long-distance (scale of kilometers) (6) and short-distance (scale of cm or m) (9, 10) dispersing species. Settlement cues, on the other hand, have been observed primarily in long-distance dispersers (but see 10, 11), and reviews on the topic typically ignore settlement cues in species whose larvae disperse and settle within the area immediately surrounding parent individuals, particularly those with crawl-away larvae (7, 8). The patchy distribution of short-distance dispersers remains largely attributed to patterns of larval supply (12–16); however, few studies have investigated the alternative hypothesis that larval settlement of species with limited dispersal is influenced by environmental cues. The stony (scleractinian) aclonal cup coral Balanophyllia elegans is an ideal species with which to test the widely accepted hypothesis that species with limited dispersal do not show strong settlement response to environmental conditions. Unlike the planktonic larvae characteristic of most marine invertebrate species, which can disperse kilometers, the wormlike planula larvae of B. elegans disperse locally by crawling along the substrate (13). Adults of B. elegans are patchily distributed at a scale of meters—a pattern largely attributed to an average dispersal distance of just 39 cm, though larvae are capable of traveling more than 1.3 m before settling (13, 17, 18). Thus, larvae generally settle closer to parent individuals than their dispersal potential would predict, indicating that environmental factors may influence the settlement behavior of larvae and contribute to the patchy distribution of adults. Settlement cues have been largely overlooked as a potential cause of the patchy settlement distribution of B. elegans and many other species with short-distance dispersal, perhaps because short-distance disReceived 30 September 2002; accepted 14 April 2003. *Current Address: Department of Ecology and Evolutionary Biology, Box G-W, Brown University, Providence, RI 02912. E-mail: [email protected] Reference: Biol. Bull. 204: 241–245. (June 2003) © 2003 Marine Biological Laboratory