Demography of the corkscrew sea anemone Bartholomea annulata in the Florida Keys and in laboratory culture: a giant sea anemone under pressure.

Abstract

The successful management of species requires information on their patterns of distribution, abundance, and population dynamics. Demographic studies are important for gauging how environmental conditions impact populations; individuals in declining populations may experience low viability as revealed by low recruitment and/or high mortality, indicating potential thresholds for population recovery and key parameters to focus on for sustainable management. Giant corkscrew sea anemones Bartholomea annulata are common throughout the Caribbean Sea, and also are popular commodities in the ornamental aquarium trade, but almost nothing is known about their population dynamics. The harvest of this anemone for the aquariums is largely uncontrolled in the Florida Keys, and therefore detailed population information is needed to form a scientific basis for effective management. Here, I describe the structure of 6 populations in the Florida Keys, and the dynamics of 2 of these populations over 1 year (May 2014 – May 2015). I also present the dynamics of a population cultured under laboratory conditions, which were examined to determine the maximal growth rates and life-spans in this species under optimal culture conditions. Study sites were selected in 3 regions of the Florida Keys (Upper, Middle, and Lower), and at 2 levels of human impact within each region. The Upper Keys sites exhibited significantly higher coral cover and anemone abundance, as well as a larger proportion of small, recently-recruited anemones than did sites in the other 2 regions, indicating potentially more stable populations. Low-impact sites also had significantly more coral cover and anemone abundance than did high-impact sites in both the Upper and Middle Keys, but not in the Lower Keys. At the 2 repeat-survey sites, anemone growth rate decreased significantly with body size; small individuals grew 2-2.5x as rapidly as did large individuals in terms of percent growth in tentacle crown surface area (TCSA). In laboratory culture, the percent growth rates of small individuals were even more rapid, up to 15x more than for large anemones. Overall growth was slower in the field than in lab, with individuals growing on average 3-4 cm2 mo-1 TCSA in the field and 7-8 cm2 mo-1 under laboratory conditions. Under both types of conditions, individuals slowed their growth as they neared maximal body sizes of 100 – 150 cm2 TCSA. Large, relatively old individuals exhibited signs of senescence after they reached peak body size, in terms of shrinkage over several census periods prior to death. Only 33-42% of individuals survived the entire year of study in the field, indicating rapid population turnover of ~1 years, with most mortality occurring in very small or large individuals. Individual lifespans appeared to be longer under laboratory conditions, but still only slightly over half (54%) of individuals survived the entire year of laboratory study. I conclude that field populations of this sea anemone are highly dynamic in terms of their rates of growth, shrinkage, mortality and recruitment; lifespan is short and population turnover is rapid, on the scale of about a year. Under optimal conditions in the laboratory, this sea anemone has the potential to grow rapidly when small, stops growing after reaching about 100 – 150 cm2 TCSA, and then gradually shrinks before death; individuals show little to no locomotion under the conditions examined here. Based on these trends, I make the following management recommendations for the harvest of this species: (1) implement a slot limit in which only medium-sized individuals are harvested, (2) limit harvest seasonally to avoid the spawning season each year, and (3) manage marine reserves to protect some populations from harvest.