SALINITY AS A REFUGE FROM PREDATION IN A NUDIBRANCH-HYDROID RELATIONSHIP WITHIN THE GREAT BAY ESTUARY SYSTEM

TABLE OF CONTENTS

DEDICATION.................................................................................................................. iii

ACKNOWLEDGEMENTS.............................................................................................. iv

TABLE OF CONTENTS................................................................................................... v

LIST OF TABLES............................................................................................................. vii

LIST OF FIGURES.......................................................................................................... viii

ABSTRACT....................................................................................................................... ix

INTRODUCTION............................................................................................................. 1

METHODS......................................................................................................................... 7

Fouling Panel Study............................................................................................. 7

Field Surveys....................................................................................................... 10

Cordylophora Colony Growth............................................................................ 10

Cordylophora Field Trial....................................................................................... 13

Tenellia Growth and Reproduction.................................................................. 14

RESULTS.......................................................................................................................... 18

Fouling Panel Study........................................................................................... 18

Field Surveys....................................................................................................... 28

Cordylophora Colony Growth............................................................................ 30

Cordylophora Field Trial....................................................................................... 34

Tenellia Growth and Reproduction.................................................................. 35

DISCUSSION................................................................................................................... 41

Population Distributions and Dynamics......................................................... 41

Species Interactions............................................................................................ 43

Effects of Salinity on Cordylophora Growth..................................................... 46

Effects of Salinity on Tenellia Growth and Reproduction............................. 48

The Effects of Salinity on the Cordylophora–Tenellia Relationship................ 52

The Cordylophora–Tenellia Relationship within the Great Bay Estuary System.................................................................................................................. 55

CONCLUSIONS.............................................................................................................. 61

LITERATURE CITED...................................................................................................... 63

LIST OF TABLES

Table 1. Retrieval and replacement schedule for fouling panels.............................. 9

Table 2. Two-factor design for the Tenellia adspersa survival and fecundity experiment..................................................................................................... 16

Table 3. Pearson correlation coefficients of the percent cover of hydroids with the number of mussels at Stations 1 and 2 for each month................... 24

Table 4. Pearson correlation coefficients of the percent cover of hydroids with the number of barnacles at Station 3 for each month............................. 25

Table 5. Observations of Cordylophora lacustris colonies.......................................... 30

Table 6. Size and growth rates of Cordylophora lacustris colonies at 4 different salinities.......................................................................................................... 32

Table 7. Repeated measures ANOVA for the effect of salinity on the number of polyps of a Cordylophora lacustris colony................................................... 32

Table 8. Repeated measures ANOVA for the effect of salinity on the length of the stolon of a Cordylophora lacustris colony............................................. 33

Table 9. Results from the Cordylophora lacustris field experiment.......................... 34

Table 10. Two factor ANOVA for the effect of salinity on the number spawn masses produced by an individual Tenellia adspersa................................ 37

Table 11. Two factor ANOVA for the effect of salinity on the number eggs produced by an individual Tenellia adspersa.............................................. 37

Table 12. Observed fecundity measures for Tenellia adspersa with respect to developmental salinity and adult salinity................................................. 39

Table 13. Two factor ANOVA for the effect of salinity on the life span of Tenellia adspersa........................................................................................................... 40

Table 14. Comparision of mean fecundity measures for Tenellia adspersa from this study with previously reported values when fed Cordylophora lacustris........................................................................................................... 49

LIST OF FIGURES

Figure 1. Map of the Great Bay Estuary system showing stations for the fouling panel study...................................................................................................... 7

Figure 2. Wooden array with fouling panels.............................................................. 8

Figure 3. Salinity and temperature at each station for the fouling panel study... 19

Figure 4. Distribution of colonial invertebrates at each station during the five months of the fouling panel study............................................................. 21

Figure 5. Distribution of solitary invertebrates at each station during the five months of the fouling panel study............................................................. 22

Figure 6. Weekly recruitment of hydroids at each station during the fouling panel study.................................................................................................... 23

Figure 7. Abundances of Tubularia spp. and its predators at Station 1 in July..... 25

Figure 8. Abundances of campanularid hydroids, Tenellia adspersa, and T. adspersa spawn masses at Stations 1, 2, and 3 in June............................................ 27

Figure 9. Map of the Great Bay Estuary system showing where Cordylophora lacustris and Tenellia adspersa were found in 1993 to 1998...................... 28

Figure 10. Growth of Cordylophora lacustris at four salinities as measured by the number of polyps (top) and the length of the stolon (bottom)............ 31

Figure 11. Effect of salinity on the maximum height of Cordylophora lacustris..... 33

Figure 12. Box plots showing the number of veligers and juvenile Tenellia adspersa surviving development at 6, 12, 18, and 24‰........................... 36

Figure 13. Effects of salinity on the fecundity of Tenellia adspersa.......................... 38

Figure 14. Life span of Tenellia adspersa at different salinities................................. 40

Figure 15. Salinity of the Great Bay Estuary system at low and high tide in May.......................................................................................................................... 59

Figure 16. Salinity of the Great Bay Estuary system at low and high tide in July.......................................................................................................................... 60

ABSTRACT

SALINITY AS A REFUGE FROM PREDATION IN A NUDIBRANCH-HYDROID RELATIONSHIP WITHIN THE GREAT BAY ESTUARY SYSTEM

David J. Blezard

University of New Hampshire, May 1999

Hydroids are important early colonists in fouling communities. They are among the first occupiers of space and have been shown to affect the course of succession in the community by facilitating the recruitment of some later colonists while inhibiting others. Predators, especially aeolid nudibranchs, rapidly recruit to hydroid colonies and can provide sufficient predation pressure to remove hydroids from the community potentially altering the successional process. Data from a study of the recruitment and interactions of early fouling community organisms within the Great Bay Estuary system, NH, USA, illustrate these principles. In some cases, hydroid colonies are not removed and occupy space for longer periods. Understanding the interactions between nudibranch predators and hydroid prey is an important component in understanding the development of fouling communities.

The hydroid Cordylophora lacustris is a common species in the upper reaches of the Great Bay Estuary system. Colonies of C. lacustris can reach large sizes effectively occupying space, and populations persist in the same locations from year to year. The nudibranch Tenellia adspersa is the primary predator of C. lacustris. T. adspersa has very short generation times (21 days) and life spans (36 days) and can produce large numbers of offspring. In laboratory conditions, the nudibranch populations overwhelm the hydroid prey.

Salinity is an essential factor in the relationship between these two species as revealed by studies of their growth and reproduction at 6, 12, 18, and 24‰ salinities. C. lacustris grows at all of these conditions but increases most rapidly at 6‰. Adult T. adspersa can survive in this salinity range but show increased stress and reduced fecundity at or below 12‰. Fecundity is highest at 24‰. Development of T. adspersa fails at both 6‰ and 12‰, and survival of metamorphosis is reduced at 18‰ compared to 24‰. Low salinity environments are a refuge from predation for the hydroid because the nudibranch population cannot increase in size to overwhelm the prey population. The salinity tolerances of these two species along with the seasonal variations in salinity may explain the natural distributions of C. lacustris and T. adspersa.

INTRODUCTION

Predation is a major factor in determining the distribution and size of populations of organisms. Prey species are a resource for predators leading to increases in the predator population while predatory acts that consume all or part of a prey organism lead to reductions in the prey population. This basic interaction between predator and prey species has been the focus of numerous studies and models to understand the ecological consequences of predation (Lotka, 1925; Voltera, 1926; reviewed in May, 1981a; Crawley, 1992).

In reality, the situation in which a predator and prey interact is much more complex than the strict isolation enforced by mathematical models. Other species can compete with and predate on both the predator and prey, and physical factors in the environment can either augment or depress the rate of predation (see Chesson, 1978; May, 1981b; Malcolm, 1992). One special circumstance is when the physical environment provides a means for some or all of the prey population to hide from or otherwise avoid predators. In such a situation, the prey have a refuge. If a refuge exists, it can lead to the persistence of a prey population that otherwise would be overwhelmed by its predators (see Crawley, 1992).

Beyond the direct determination of populations of the two species, predation is a highly significant factor in determining the development and the structure of a community. As succession takes place, early colonists may be preyed upon by later arrivals and removed from the system, or established predators may prevent new species from colonizing an area by removing the new arrivals before they become established (e.g., Lubchenco and Menge, 1978; Ayling, 1981; Day and Osman, 1981; Harris and Irons, 1982; Harris, 1987). These processes can have further consequences to the course of succession. Early colonists can facilitate, inhibit, or tolerate later arrivals (Connell and Slayter, 1977). For both the facilitation and inhibition models, the presence of a predator could reduce or even reverse the effect; a species that would have been inhibited by earlier arrivals might now be able to recruit into an area or vice versa. An established predator may also indirectly benefit other non-prey species by preventing a superior competitor from out competing them (Paine, 1966).

Both facilitation and inhibition interactions have been shown to take place in marine benthic and fouling communities (Dayton, 1971; Standing, 1976; Dean and Hurd, 1980; Harris et al., 1984). In these communities, hydroids are often early colonists of open space but may be ephemeral with other species such as mussels, barnacles, and tunicates replacing them over time (Clark, 1975; Harris and Irons, 1982; Lovely, 1995). Early hydroid colonies have been shown to facilitate the arrival of mussels and tunicates (Standing, 1976; Dean and Hurd, 1980; Dean, 1981; Okamura 1986) and to inhibit the recruitment of barnacles (Standing, 1976; Harris and Irons; 1982).

Aeolid nudibranchs are common predators on hydroid colonies (Clark, 1975; Todd, 1981; 1983). Predation by nudibranchs is a significant factor in determining the fate of a population of hydroids. Predation can create open patches within the colony, or, at high predator densities, the nudibranchs can completely consume the colony removing it from the community (Lambert, 1985; Harris, 1987). Thus, high levels of predation by nudibranchs could change the inhibition and facilitation effects of early hydroid colonists on later successional species.

The hydroid, Cordylophora lacustris Allman, 1853, is a gymnoblastic colonial hydroid of the family Clavidae. The colonies grow attached to hard substrates and consist of branching stolons with periodic uprights bearing one to several feeding polyps. Individual colonies are often small, less than 100 cm², but can grow to cover more than a square meter of substrate (personal observation). The uprights with the polyps reach a maximum height of approximately 5 cm (Pollock, 1998). Under appropriate conditions, the uprights also bear gonophores for sexual reproduction.

C. lacustris is unique among hydroids in its ability to tolerate extremely low salinities including fresh water. In fact, C. lacustris has been located in freshwater systems in New York, Ohio, Texas, and Florida (Pennak, 1978; Hubschman and Kishler, 1972; Streever, 1992; Kelly and Franks, 1995). Laboratory studies using artificial seawater show that C. lacustris colonies grow most rapidly in conditions with a Cl^- concentration of 0.07 M (equivalent to 4.5‰ based on 0.545 M Cl^- in 35‰ seawater (Kalle, 1971)) but that the hydroid can grow in conditions ranging from nearly 0‰ to 35‰ (Fulton, 1962). Growth rate of the related (and possibly synonymous (Pollack, 1998; SERC, 1998)) C. caspia is highest at 16.7‰. C. caspia grew in conditions ranging from fresh water to 30‰ although abnormalities were noted at salinities over 24‰ (Kinne, 1956; 1971). Natural populations of Cordylophora spp. are reported only in brackish or fresh water (Calder, 1976; Jormalainen et al., 1994). Calder (1976) reported salinities between 0‰ and 7‰ as the range in which the hydroid was found in a South Carolina estuary. Within the Great Bay Estuary system of Maine and New Hampshire, C. lacustris is only previously reported from low salinity areas (Crocker, 1972).

The nudibranch, Tenellia adspersa (Nordmann, 1845), is a major predator of Cordylophora lacustris. T. adspersa is a small (5-7 mm) aeolid nudibranch of the family Cuthonidae. Like C. lacustris, T. adspersa is unique among nudibranchs in its tolerance of estuarine conditions. Harris et al. (1980) cultured Tenellia fuscata (=Tenellia adspersa) in a variety of salinities and temperatures, finding that the nudibranch did best at 20°C and 30‰ but that the nudibranch could survive and reproduce at 10‰. Adult nudibranchs survived and produced eggs at a much reduced rate at 5‰, but these eggs did not develop. Conspecific T. pallida was able to reproduce at both 20‰ and 12‰ (Rasmussen, 1944).

Tenellia adspersa is also distinct among nudibranchs in that it is a generalist, feeding on a wide variety of hydroid species in addition to C. lacustris. A literature review by McDonald and Nybakken (1997) lists over 20 distinct species as prey items for T. adspersa including Bougainvillia spp., Campanularia spp., Gonothyraea lovenii, Ectopleura dumortieri, Eudendrium spp., Obelia spp., and Tubularia spp. as well as C. lacustris. This same review, however, lists only three North American species of nudibranchs (Eubranchus exiguus, E. pallidus, and Facelina bostoniensis) other than T. adspersa as feeding on C. lacustris, and each of these three is commonly associated with other prey (Pollack, 1998). While T. adspersa will feed on a variety of hydroids, it appears that C. lacustris is a preferred prey item and that T. adspersa is the main predator for C. lacustris. T. adspersa is often described in association with C. lacustris (Moore, 1964; Clark, 1975). In addition, a study by Chester (1996) found that T. adspersa grew more rapidly and reached maturity at an earlier date when raised on a diet of C. lacustris compared with either Obelia commiseralis or Hydractinia echinata.

When T. adspersa is maintained in a laboratory situation, the nudibranchs grow and reproduce rapidly enough to overwhelm any size C. lacustris colonies (personal observation). When fed on C. lacustris, T. adspersa has a short life cycle and generation time and exhibits very high fecundity. Harris et al. (1980) found a generation time of 16 to 20 days with a life span of 28 to 30 days. Fecundity ranged from 682 to 2687 eggs per individual. Other results are similar with a generation times of 17 days, life span of 24 days, and fecundity of 1301 eggs per individual (Chester, 1996). Although C. lacustris does respond to low levels of predation by T. adspersa by changing the growth form of the colony, a change that presumably helps defend the colony (Gaulin et al., 1986), the rate at which T. adspersa reproduces means that the nudibranch population will increase very rapidly and consume any conceivably sized colony within a few weeks time.

In order for C. lacustris populations to avoid extinction, one or more factors must exist that limit the predation by T. adspersa in the natural environment. Salinity is a likely candidate as the two species appear to have differing tolerances for salinities. Based on past studies, C. lacustris can tolerate, and is often found, at very low salinities within an estuary whereas T. adspersa grows best at higher salinities. Both species, however, appear to be able to survive over a wide range of conditions.

Low salinity may function as a refuge for Cordylophora lacustris from predation by Tenellia adspersa. This study examines this hypothesis with evidence from natural populations of both species within the Great Bay Estuary system and from laboratory studies of the two species under directly comparable conditions. It also presents data about the interactions between hydroids, nudibranchs, and other species in early fouling communities.

METHODS

Fouling Panel Study

In June through October 1993, a study was conducted to examine the recruitment and interactions of early fouling community organisms at different locations within the Great Bay Estuary system, NH, USA (Figure 1). Acrylic panels measuring 13 cm x 8 cm were suspended in wooden arrays with four replicates per location (Figure 2).

Figure 1. Map of the Great Bay Estuary system showing stations for the fouling panel study.

The arrays were anchored such that the panels would always remain submerged at the lowest tide. The vertical panels were allowed to rotate freely with the current so that both surfaces of the panel received equal exposure to current while reducing the effects of sedimentation. The top of the wooden array provided shade to the panels to reduce the growth of algae.

Figure 2. Wooden array with fouling panels.

Each array’s four panels were numbered 1 through 4 corresponding to the number of weeks that the panels were exposed before retreieval. At low tide each week, the appropriate set of panels were removed, sealed in individual plastic bags, and replaced with fresh panels. Table 1 shows the retrieval and replacement schedule. Salinity and temperature measurements were taken as the panels were changed.

Table 1. Retrieval and replacement schedule for fouling panels

Panel	Week
	0	1	2	3	4	5	6	7	8
1	S	C	C	C	C	C	C	C	C
2	S		C		C		C		C
3	S			R	S			R	S
4	S				C				C
S = Set out new panel. C = Change panel (retrieve and replace). R = Retrieve panel only.

The retrieved panels were examined in the lab. All organisms present were identified under a dissecting microscope to the lowest possible taxon using keys to local fauna (Frazer, 1944; Smith, 1964; Crocker, 1972). As measures of abundance, numbers of individuals or colonies were counted, and the percent cover of non-motile species was measured using a grid of 100 random points.

The design of the replacement schedule for the panels allowed examination of both temporal variations in recruitment and early successional changes in the community structure. By comparing panels that have been exposed for the same duration (across a row of Table 1), one can examine the changes in the abundance of species throughout the season. Panels that were put in place on the same date and were removed one per week for the four-week period (down a diagonal of Table 1) illustrate the interactions and succession of the community members.

Statistical analysis of the data was performed using StatView (SAS Institute, Inc., Cary, NC). Pearson correlation coefficients were calculated to examine the relationships between the abundances of hydroids and other organisms. All percent cover data were normalized using an arcsine transformation before use in statistical calculations (Sokal and Rohlf, 1981).

Field Surveys

As the fouling panel study results (see page 18) contained no information about the distribution of C. lacustris and only reflect the conditions at the four study sites, a series of field surveys was begun in May 1994. The goal of the surveys was to examine other locations with floating docks, pilings, or other suitable substrate in the Great Bay Estuary system that were likely to have either C. lacustris or T. adspersa. When either organism was found, salinity and temperature of the water were measured. Any locations where either species had been found were checked at two-week intervals throughout the summer. Informal surveys continued through the summer of 1998.

Cordylophora Colony Growth

Fragments from several colonies of the hydroid, Cordylophora lacustris, were collected in July, 1997 from the Bellamy River, Dover, NH within 100 m downstream of the dam that separates the estuarine and fresh water portions of the river. These pieces of colonies were placed into an aquarium as the start of a laboratory culture. The hydroids in the stock culture were fed daily with one-day-old Artemia salina nauplii hatched from resting cysts (Aquatic Lifeline, Inc., Lehi, UT) at approximately 15‰.

The aquarium was maintained at 15‰ salinity and approximately 20˚C. To obtain water at the desired salinity, unfiltered seawater collected from the Piscataqua River at the state fishing pier in Portsmouth, NH was combined with the available well-drawn tap water. These same water sources were used throughout all laboratory cultures and experiments.

To produce a sufficient number of colonies in the stock culture, pieces of hydroid colony with one to a few polyps were secured to a glass microscope slide with monofilament line. The slides were suspended from wooden hangers resting on the top of the aquarium walls. Within a few days, the colony produced new stolon tissue that attached to the glass slide. These culture techniques are similar to those used previously to culture hydroids (Crowell, 1953; Fulton, 1960; 1962; Gaulin et al., 1986).

To assess the effects of salinity on the growth rate of C. lacustris colonies, 31 colonies were trimmed back to the point that only two uprights, each with a single polyp, connected by an unbranched length of stolon remained. These colony slides were returned to the stock tank for five days, after which time, all of the colonies had recovered from the trimming procedure and had grown new tissue at each end of the cut stolon. Twenty-four of these colonies were randomly selected and assigned into four groups of six replicates. Each group represented one of the treatment salinities of 6, 12, 18, and 24‰. These salinities were selected because they represented a range of conditions over which both C. lacustris and T. adspersa were likely to be able to survive based upon past studies (Kinne, 1956; 1971; Fulton, 1962; Harris et al., 1980; Chester, 1996).

The monofilament line was removed from around the glass slide before each slide was placed into its own separate dish, hydroid colony facing up. Each dish was filled with approximately 250 ml of water at a salinity of 15‰. After two hours, one-half of the water in each dish was removed and replaced with a like volume of water at the appropriate treatment salinity. After another hour, the procedure was repeated. After an additional hour, the entire volume of water was siphoned off and was replaced with water at the treatment salinity. The purpose of these steps was to reduce or increase the salinity gradually to protect the colony from damage due to a sudden change in water chemistry.

To maintain these experimental colonies, each dish was supplied with an air tube to slowly aerate and circulate the water. The colonies were fed daily with concentrated Artemia salina nauplii. The nauplii were concentrated by placing them in an elongated dish with a light at one end for several minutes then siphoning off the ones that swam to the light. This technique provided a high amount of food with a minimum of water or unhatched cysts to pollute the dishes. The dishes were kept at a temperature of approximately 20˚C. Water in the dishes was changed once each week to remove waste and uneaten food.

The growth of the colonies was monitored by photographing each colony every two days using a 35 mm camera equipped with a macro lens set at a ratio of 1:3. The color negative film was developed into Kodak Photo CD. The images on the compact disc were examined using both NIH Image 1.61 and Graphic Converter 3.3.1 on a Macintosh computer system. From the images, both the number of polyps per colony and the total length of the colony’s stolon were measured. For length measurements taken with NIH Image, the long edge of the glass slide provided a convenient size reference.

The colonies were maintained and monitored for a total of 20 days. On the last day, in addition to the routine photographing, each colony was examined under a dissecting microscope. The total number of polyps was counted as a check of the accuracy of the numbers that would later be obtained from the digital images. Comparison of the two sets of data revealed that careful counts made from the images are a very accurate measure of the true number of polyps with an mean difference of 1.5 polyps between the counts and no difference in 12 of the 24 cases. Also on the final day, the longest upright from each colony was removed at its base and measured as an additional indicator of colony size.

The remaining seven C. lacustris that had been trimmed dow