The Ruination of the Ship: Shipworms and their Impact on Human Maritime Travel Trevor Hoberty Wittenberg University Marine Science Honors Thesis Spring 2020 Thesis Committee: Dr. J. Welch, Advisor; Dr. K. Reinsel, 2nd Reader; Dr. M. Mattison, 3rd Reader Hoberty 2 Abstract Shipworms, family Teredinidae, are woodboring mollusks that have evolved specialized feeding strategies to glean nutrients from the consumption of wood. Through this feeding strategy, with the assistance of symbiotic Teredinibacter bacteria in the gut, the shipworm breaks down structurally dense wood in the marine ecosystem – introducing previously stored energy back into the system. Historically, this wood consumption has proved disastrous for human seafaring efforts. The destruction caused by means of shipworm feeding is heavily referenced in the historical record from the ancient to modern periods. Most all sailors or marine builders have faced disastrous damage because of the shipworm. Despite the tumultuous relationship present between the shipworm and humanity, this simple clam continues to fascinate researchers across the world. Hoberty 3 Introduction The vessel though her mast be firm, Beneath her copper bears a worm… Far from New England’s blustering shores, New England’s worm her hulk shall bore, And sink her in the Indian seas, Twine, wine, and hides, and China teas.1 (Thoreau) As illustrated by these bars, and others throughout the written humanities, humans have long held a close connection to the ocean. From the gift of countless resources to an effective means of travel and commerce, the ocean has captivated our collective culture. However, the most interesting reference in this poem is not in regards to robust international trade. Instead, perhaps the most peculiar reference lies with a “worm” – a shipworm. The shipworm, a colloquial misnomer, is a woodboring marine bivalve mollusk of the family Teredinidae. Occupying a masterfully evolved niche of the marine world, the Teredinidae have long plagued human efforts in the ocean. While often forgotten in our modern world, the shipworm remains as both a fascinating organism and a symbol of persistence in the natural world. This endeavor explores the specialized biological life strategies of the shipworm, member of the family Teredinidae, within their wooden niche of the ocean ecosystem. These shipworms demonstrate an adaptive presence of feeding and survival, developed long before recorded time, that has long brought ruin to human efforts of maritime travel and exploration. In highlighting 1 Excerpt from Though all the Fates by Henry David Thoreau Hoberty 4 this interconnectivity, this paper will work to show the direct connections between this intricate biology and rich history, both persisting throughout the ages. A true interconnectivity that, like many secrets of the ocean, has began to show the potential to benefit modern industries – both medical and beyond. Hoberty 5 Chapter 1: Biology of the Shipworm The shipworm is a truly specialized marine organism. Commonly referred to as the “termites of the sea,” shipworms have developed a specialized feeding strategy that allows them to sustain themselves on both nutrients from the water column and the wood in which they live (Cobb 2002). This unique feeding methodology has allowed the shipworm to carve out a significant niche in the marine ecosystem. This niche, centering around the management of wood in the ocean, has functioned to both manage an important ecological good and draw the distain of human seafarers throughout history (Cobb 2002; Masser and Sedell 1994). Shipworm Larval Biology and Lifecyle Shipworms, through their specialized feeding process, actively destroy the wooden habitat in which they live. As such, the organism must effectively reproduce and disperse offspring to new areas of available habitat (MacIntosh et al. 2014). In order to manage this, shipworms are selective in their reproductive strategies. These selective processes largely vary among differing species of shipworm. For example, members of the genus Bankia are more rapid in their means of reproduction. This species, and the majority of the Teredinidae, reproduce through oviparous means – projecting gametes into the water and allowing for external fertilization. In this method, a lengthy larval period of more than 20 days follows this fertilization event (MacIntosh et al. 2014). However, some species, such as Teredo, are more labor intensive with regard to reproduction. These species are larviparous – brooding eggs within the mantle cavity (MacIntosh et al. 2014). This brooding allows for further developed larva at the time of dispersal. Interestingly, these species, reaching sexual maturity around the eight-week mark, are able to invert their sex in order to create a more balanced population (Grave 1942). This sexual inversion works to keep the community from becoming overpopulated with one Hoberty 6 given sex. Regardless of the reproductive method, newly developed shipworm larvae are promptly left free-swimming in the water column and tasked with finding an appropriate place of settlement. As with many young marine invertebrates, these newly formed shipworm larvae are dependent on the physical workings of the ocean to find and colonize new areas. These organisms are able to achieve this goal through the effective use of ocean currents (Scheltema 1971). It has been found that the natural global circulation patterns of the ocean, coupled with the relative hardiness of the species as a whole, have allowed the transport of shipworm larvae across large distances. In fact, this successful larval transport has been hypothesized to be a driving factor behind the global distribution of the Teredinidae (Scheltema 1971). However, it must be noted that this successful transport is limited to those species that have longer phytoplanktotrophic life stages. These species, typically those of the oviparous variety, are able to remain pelagic for longer periods of time – moving farther distances with the current (Scheltema 1971; MacIntosh et al. 2014). For species lacking this long pelagic phase, dispersal is much more limited. As a result, these organisms, more developed after being brooded in the body of the parent, are able to settle more quickly and wherever possible (Scheltema 1971). Dominated by these factors, the larval stage of the shipworm can last anywhere from mere minutes to extended months (Toth et al. 2015). In both instances, larva remain in the water column until they are prompted to settle through environmental cues. The most direct of these settlement cues proves to be presence of wood. It has long been hypothesized that shipworm larvae settle in response to chemical cues emitted by wood. In an experiment conducted by Toth et al. (2015), this hypothesis was tested by examining the settlement of shipworm larvae on covered and exposed pieces of wood. In all, it Hoberty 7 was found that larvae were more likely to settle in fresh, exposed wood substrates. Furthermore, larvae were more likely to settle in fresh wood adjacent to already invaded samples (Toth et al. 2015). While the exact compounds are not yet known, this experiment suggests that larval settlement is impacted by chemical factors. Once this appropriate wood habitat is found the shipworm larvae need only the smallest of holes to begin infestation (Cobb 2002). Once embedded in the wood, the bivalve is able to grow and feed from the safety of its burrow. Shipworm Diversity and Anatomy Within the family Teredinidae, there are approximately 65 recognized species across the globe (Turner 1966). Found in virtually all non-artic waters, the shipworm has long been championed for their resilience to changing environmental factors and physical conditions. For example, Teredo navilis, the common naval shipworm, has been known to tolerate temperatures ranging from 0 to 30℃ and salinity levels between 7-39 ppt (Borges et al. 2014). Such resilience can be attributed to the specialized anatomy of the organism. The shipworm has a specialized body structure morphologically distinct from their bivalve relatives (Figure 1). Most notably, particularly in reference to other bivalves, the shipworm presents Figure 1: Physical Anatomy of the Shipworm a greatly reduced hinge and shell https://poi-australia.com.au/teredo-navalis-termites-of-the-sea-ship-worm/ (Turner 1966). This notched shell, found at the most posterior portion of the animal, is used as a rasp to bore into wooden substrates rather than a supportive structure (Dame 2012). Also located Hoberty 8 at this posterior end is a truncated foot and a well-developed system of adductor muscles (Turner 1966). These muscular structures work to aid in wood boring and anchors the organism within its burrow (Turner 1966). Moving along the anatomy of the shipworm, the body cavity of the animal is made of a largely closed mantle that houses the important internal anatomy. This cavity holds the majority of the feeding structures of the shipworm such as the gills and digestive organs (Turner 1966). The anterior portion of the shipworm boasts two relatively short siphons that can be extended outside of the wooden burrow. Interestingly, the shipworm lives their lives in these wooden burrows. In this intricate dynamic, the wood acts as both a food source and a habitat for the organism (MacIntosh et al. 2014). As such, the shipworm is forced to balance its feeding and other life processes with the continual self-destruction of habitat. With these characteristics, the family Teredinidae