Date of Award

Spring 2018

Project Type


Program or Major

Biological Sciences

Degree Name

Master of Science

First Advisor

Larry G Harris

Second Advisor

Jennifer A Dijkstra

Third Advisor

Charles W Walker


Introduced species have become established in multiple systems around the globe where they are both predators and have been recognized as prey by native species. In the Gulf of Maine, introduced tunicates (Botrylloides violaceus, Diplosoma listeranium, and Didemnum vexillum) have become established in fouling and natural rocky benthic systems. In recent years, many native species such as Mitrella lunata and Stronglyocentrotus droebachiensis have recognized and begun to consume these introduced species. One such species is the native blood star, Henricia sanguinolenta. H. sanguinolenta, is a generalist sponge predator, but it has started to consume these invasive tunicate species as a result of declines in its native food source. Although tunicates appear to be an inferior food source when compared to native sponges, they are present in high abundances, specifically during the summer and fall periods. These studies recorded how the growth and reproduction of sea stars has been affected by these invasive tunicate species.

A series of experiments was designed to examine changes in seasonal prey consumption of the blood star and effects that invasive colonial tunicates have on its growth and reproduction. Monthly monitoring of in situ blood star diet revealed they feed opportunistically on colonial tunicates. When tunicate abundance was low, they supplemented their diet with detritus, jingle shells (Anomia simplex), and barnacles (Semibalanus balanoides). It is apparent that sea stars are primarily foraging on invasive tunicate species, and they were not observed consuming their native sponge prey species.

Growth and reproduction of H. sanguinolenta on invasive tunicates and native sponge (Haliclona oculata) diets were assessed through laboratory studies. Sea stars were fed one of 6 treatment diets consisting of a combination of the native sponge, H. oculata, and the invasive tunicates, B. violaceus, and D. listeranium. Their growth was monitored over several months and their final body, gonad, and pyloric caeca mass were recorded. Sea stars grew best when fed a diet of sponges, and lost weight on a diet of tunicates. Less weight was lost on a diet of D. listeranium than was on a diet of B. violaceus. In addition to measuring growth, reproduction was also assessed by weighing gonad and pyloric caecal mass. Gonadal masses across all treatments were statistically similar. However, individuals that fed on D. listeranium had higher pyloric caecal masses than those that fed on B. violaceus. These results suggest that sponges are a higher quality food source than tunicates, and that D. listeranium is a superior food source than B. violaceus.

Prey choice experiments designed to test H. sanguinolenta’s preference for specific colonial tunicates or sponges was assessed through a flume study. Animals were presented with a combination of sponge and tunicate species and their movement and feeding behavior was recorded. Animals appeared to prefer H. oculata and D. listeranium over B. violaceus. However, they did not show a preference when given the choice between H. oculata and D. listeranium. This was surprising given that their growth rates are higher on a diet of sponge than on tunicates. Individuals for this study were collected from ecosystems with few to no sponges, so it is likely that they have been conditioned on this tunicate diet.

Generally, when a native predator eats a diet of primarily invasive prey, this leads to declines in their health and reproduction. However, there have been significant increases in the population of H. sanguinolenta since the introduction of these invasive tunicate species. H. sanguinolenta is a generalist predator that switches its diet to forage optimally in order to maintain high populations. In the field, tunicate abundance is high, which provides an endless quantity of a low-quality food source and sea star populations are increasing as a result of feeding on a variety of tunicate species.

These studies provide support for the idea that the impacts of invasive species are nuanced. While tunicate species have negatively impacted the Gulf of Maine by competitively excluding some native benthic invertebrates, such as sponges, they have also provided a food source for sea stars. As waters in the Gulf of Maine continue to warm, and invasive species move northwards, it is likely that H. sanguinolenta will continue to adapt to its changing ecosystem.