Powerful Hurricane Irma altered the marine fauna of Ten Thousand Islands, Florida as it blew through on Sept. 10, 2017. One of the most prominent observations in its aftermath was the appearance of a marine invertebrate species that is not commonly found in this area, Molgula manhattensis. This species was found in high abundance attached to submerged mangrove debris and formed patchy mats on the sandy bottom of barrier island passes.
Molgula manhattensis belongs to the subphylum Tunicata (Urochordata) commonly referred to as tunicates or sea squirts. They are notable invertebrates (animals without backbones) because they constitute a subphylum of the phylum Chordata (animals with a notochord or backbone, such as ourselves); that is, they are our closest living relatives among the invertebrates. Sea squirts exhibit a backbone type structure called the notochord in their larval stage that is used for swimming to find an ideal location to attach. Once they attach, the notochord is reabsorbed and shrinks to a simple nerve ganglion. The animal then metamorphoses into an adult. While developing, a “tunic” or covering is formed for protection. The name tunicate comes from the outer covering. Tunicates are filter feeders, feeding on tiny particles especially bacteria from the water. They are found attached to all types of hard substrate including mangrove roots. Importantly for this story, there are no known freshwater species, all are marine (all live in seawater), but a small number of species are capable of living in brackish water.
Molgula manhattensis, the species that appeared in large numbers after the storm, is of interest because there are no former records for the Ten Thousand Islands region, despite long-term research on the turtles that live there. Van Name (1945) indicated M. manhattensis occurred along the U.S. coast from Maine to Texas but its distribution appeared to be interrupted by the Florida peninsula. Regarding this discontinuity, however, Frey (1965) warns that “lacking really adequate data on distribution one must always be cautious that a presumed interruption in range reflects the actual condition in the species rather than merely the patchiness of fortuitous collections”. The latter appears to be the case with the perceived distribution in western Florida.
Published (Dragovich and Kelly, 1964) and online information indicate that M. manhattensis occurs as far south as Tampa Bay. The Department of Invertebrate Zoology database shows additional records from Charlotte Harbor, a major estuarine system south of Tampa Bay. The southernmost records (Captiva Pass in Pine Island Sound and the Caloosahatchee River) are only around 90-100 km north of the recent collection in Ten Thousand Islands. We have since received unconfirmed reports of sea squirts (most likely M. manhattensis) being collected during fish trawl studies in the backwater bays of the Ten Thousand Islands prior to the hurricane (P. O’Donnell, RBNERR, pers comm.). Collectively, these data suggest M. manhattensis has occurred cryptically in the less-saline backwaters throughout southwest Florida and it took a major hurricane to bring it out of hiding.
Prior to Hurricane Irma’s landfall, southwest Florida was already experiencing an excess of 15” of rainfall (138%) for year-to-date historical amounts. This amount increased to +24” (150%) after the storm’s passage. All the nutrient-rich freshwater draining into the estuaries and the churning of waters by Irma’s strong winds created hypoxic (no oxygen) conditions for weeks in the Ten Thousand Islands. M. manhattensisis one of the few sea squirts that will live in polluted, low-salinity waters (Van Name, 1945) so it makes sense that this species flourished after the storm. Now that conditions have improved, it will be interesting to see whether M. manhattensis continues to thrive in the outer zone of this estuary or retreats to obscurity in the backwater bays.
By Linda Cole – National Museum of Natural History, Dept. of IZ and Jeff Schmid – Conservancy of Southwest Florida, Dept. of Environmental Science
Dragovich, A. and J.A. Kelly, Jr. 1964. Ecological observations of macroinvertebrates in Tampa Bay, Florida 1961-1962. Bulletin of Marine Science of the Gulf and Caribbean 14:74-102.
Frey, D.G. 1965. Other invertebrates-an essay in biogeography. In: H.E. Wright Jr. and D.G. Frey, The Quaternary of the United States. A Review Volume for the VII Congress of the International Association for Quaternary Research, pp. 613-632. Princeton University Press, New Jersey.
Van Name, W.G. 1945. The north and south American ascidians. Bulletin of the American Museum of Natural History 84:1-476.