Behavioral variability of hatchlings modifies dispersal potential in crown conch (Melongena corona): Why do larvae crawl away but sometimes swim?

The diversity and consequences of development in marine invertebrates has,
for a long time, provided the opportunity to understand different
evolutionary solutions to living in variable environments. However,
discrete classifications of development can impede a full understanding of
adaptation to variable environments when behavioral, morphological, or
physiological flexibility and variation exists within traditionally
defined modes of development. We report here novel behavioral variability
in hatchlings of a marine gastropod, the Florida crown conch (Melongena
corona), that has broad significance for understanding the correlated
evolution of development, dispersal, and reproductive strategies in
variable environments. All hatchlings crawl away from egg capsules after
emergence as larval pediveligers. Some subsequently swim for a brief
period (seconds to minutes) before crawling again. From detailed
observations of 120 individuals over 30 days, we observed 28 (23.3%)
hatchlings swimming at least once (8 – 50% per maternal brood). The
propensity to swim was unrelated to time spent encapsulated or size at
hatching, and lasted for 22 days. We manipulated hypothesized
environmental cues, and found that the proportion of hatchlings that swam
was highest in the absence of cues related to habitat or juvenile food,
and lowest when only habitat cues were present. The relative growth rate
of hatchlings was highest when habitats contained a putative juvenile food
source. About 44% of hatchlings were competent to metamorphose at
emergence, but did not metamorphose at this time in the lab or the field.
The rate of metamorphosis increased with age and depended on the presence
of unknown cues in the field. Crawl-away larvae with prolonged swimming
ability may be an adaptation to balance the unpredictable risks of
exclusively benthic or pelagic development and allow the option to
disperse to higher quality habitat.