A strong relationship exists between the feeding morphology of different fish species and their feeding habits. In the present study, the average standardized jaw length, gill raker length, and intestine length were investigated in two species: Yellow Perch (Perca flavescens) and Bluegill Herring (Alosa aestivalis). The differences in length of these structures across species were analyzed based on the type and energy/nutrient content of their preferred diets. In order to eliminate variation based on age and outside factors, the raw data was standardized based on the appropriate equation for the structure. Significant relationships were identified using a single-factor ANOVA test. The results suggest that long, thin gill rakers are associated with filter-feeding species while short, wide gill rakers are characteristic of species that feed on benthic macroorganisms. The findings also indicate that as the digestibility of the diet decrease, the length of the intestine increases in order to obtain sufficient amounts of nutrients and energy. This study is important in understanding the evolution of particular feeding morphologies in response to diet and feeding habits. The results can be used in the management of both wild and farm-raised perch and herring fisheries in order to support the most efficient growth of the populations.
Introduction
The purpose of the experiment was to compare the feeding habits of Yellow Perch (Perca flavescens) and Blueback Herring (Alosa aestivalis) based off their feeding morphologies. The morphologic characters investigated were jaw length, gill raker length, and total intestine length. Jaw length was chosen as a parameter for study because it is a good indicator for the size of prey that a particular fish species consumes. When the jaw length is shorter the gape size is smaller which often correlates with smaller prey items. Fish with longer jaw lengths and larger gape sizes feed on both larger and smaller prey items (Saikia, 2016). Gill rakers are also important appendages for feeding as they can filter out plankton from the water column for filter-feeding species. Long, thin gill rakers are characteristic of straining type species and short, wide gill rakers are associated with species that eat macroscopic organisms. The digestive tract is the main structure responsible for the breakdown of food and uptake of nutrients making it a great indicator of feeding habits in fish. Maintaining digestive tissues costs a lot of energy resulting in the intestine length often being the minimum length required for sufficient nutrient and energy uptake. The intestine length reveals the balance between maximizing nutrient and energy uptake with the energy cost associated with tissue maintenance (Wagner et al., 2009).
Bluegill herring is a marine species that inhabits the waters off the eastern coast of the United States. Their diet consists of minute organisms such as phytoplankton, zooplankton, and small crustaceans. When they are young they feed mainly on phytoplankton and move onto zooplankton as they mature. While foraging, herrings swim with their mouths open, trapping any planktons suspended in the water (Mullen et al., 1986). These plankton travel over the herrings’ gill rakers where they are filtered. Based on their diet, it is expected that the blueback herring will have long, thin gill rakers. Longer gill rakers are needed to filter plankton out of the water and therefore should correlate with herring species.
On the other hand, Yellow Perch is a freshwater species that inhabits soft-bottom lakes, streams, and ponds. They feed on benthic organisms such as isopods, crustaceans, insect larvae, and gastropods until eventually moving onto small fish. They have subterminal mouths and swallow prey whole. Since perch feed on macroscopic organisms, it is expected that they will have short, wide gill rakers in contrast to the herring. A longer intestine length than herring is expected based on the digestibility of their different diets. The diet of perch has a low digestibility rate and therefore requires more time and a larger surface area in order to intake an equal nutrient proportion. Herring should, in contrast, have a shorter intestine length due to the easy digestibility of a planktonic diet (Ke et al., 2008). Since the diet of perch consists of larger prey items than the diet of perch it is expected that the perch will have longer lower jaw lengths and therefore larger gape sizes.
Methods
The specimen used for this study were preserved perch and fresh, locally caught, store bought herring. Twenty-six of each species were sampled in order to obtain data that representative of the entire population. Since the fishes were chosen randomly, there was no control over the age or gender of the specimen which alters the ratios of the different measurements. In order to minimize the effects of these factors on the averages, the measurements for each individual were standardized. The three parameters studied were jaw length, gill raker length, and total intestine length. The jaw lengths from each individual were standardized using the following equation.
The gill raker length was standardized using the following equation.
The mean gill raker length was found by taking the average of three different gill rakers. The standardized total intestine length was calculated by the following equation.
In order to compare the average standardized characters between the two species, p-values were calculated using a single-factor ANOVA test.
Results
Figure 1- Comparison between the average jaw length of perch versus herring. Error bars represent calculated standard deviations from the means.
Herring appear to have a longer jaw length than perch as indicated by Figure 1. However, there was no significant difference (p=0.061) in jaw length between two species.
Figure 2- Comparison between the average length of gill rakers found in perch versus herring. Error bars represent calculated standard deviation from the mean.
Figure 2 shows that herring have significantly longer (p=0.0049) gill rakers than perch. The average standardized gill raker length of herring was 0.1039 larger than perch.
Figure 3- Differences between the average intestine lengths of perch versus herring. Error bars represent calculated standard deviation from the mean.
The total intestine length for perch was significantly longer (p=) than for herring as seen in Figure 3. The average standardized perch total intestine length was 0.3276 larger than herring
Discussion
The analysis on average standard jaw lengths between perch and herring showed that there was no significant difference in lengths, indicating that they possess similar sized jaws. One difference between the mouths of the two species was the presence of teeth. Yellow Perch possess small teeth that slant backward while Bluegill Herring are absent of any teeth. It has been suggested that these teeth are used for straining small food particles from the water passing through. Rather than using teeth to strain out food particles, herring depend primarily on their gill rakers for filtration.
Examination of the gill rakers in each species disclosed the presence of long, thin, closely-positioned gill rakers in herring and short, wide, spaced out gill rakers in perch. These findings correlate with the expected gill raker morphology based on their feeding habits. Herring require significantly longer gill rakers than perch because of the size of their prey. Not only do long, thin, close-together gill rakers effectively filter out planktonic particles, but also provide adequate protection for the gill filaments. The wide, short, spread out gill rakers that are characteristic of perch enable them to retain insect larvae while expelling inorganic particles (Almeida et al., 2013). These findings are consistent the results of Almeida et al. (2013) where they analyzed the various gill raker morphology observed in different species based off of their feeding habits.
The significant difference found between the intestine lengths of Yellow Perch and Bluegill Herring can be attributed to their diet. Adult Bluegill Herring filter feed primarily on zooplankton which have a high energy content and are easy to digest. Yellow Perch feed on benthic organisms such as isopods, crustaceans, insect larvae, and gastropods which are more difficult to digest. Since perch eat their food whole, the entire organism eaten passes through the digestive tract rather than just the desired portion. Benthic macroorganisms have many more parts that are difficult to break down than zooplankton does and therefore have a longer residence time in the digestive tract. The longer residence time correlates with the longer intestines length.
The analysis of average standard intestine lengths supports that as the digestibility of the diet decrease, the length of the intestine increases in order to obtain sufficient amounts of nutrients and energy. Previous studies have indicated that intestine length decreases as the concentration of zooplankton in the diet increases (Ke et al., 2008). Our findings of a shorter intestine length associated with a planktonic diet compared to a macroscopic diet is consistent with these previous studies. On the other hand, this result contrasts with the results of the study done on cichlids by Wagner et al. (2009) where they found that intestine length increased as the trophic level of the food source decreased.
Variation in the jaw, gill rakers and intestine lengths can in part be due to the different ages and environmental conditions within the sample groups. It can also be due to the efficiency and functionality of other intestinal components such as surface area, villus area, and volume (Wagner et al., 2009). Perch that are under one year of age display swift changes in body morphology with changes in environmental conditions (Hjelm et al., 2001). Since the perch were collected in various locations and preserved, both environmental conditions and age can factor into the range of lengths observed. The herring were locally caught and store bought, decreasing some of the variation due to environmental conditions (as they were collected from similar sites) and eliminated very young individuals (due to fishing laws) from the sample. However, herring do feed on phytoplankton when they are young, which requires a longer intestine length, and their intestines become shorter as they transition to a zooplankton diet. Therefore, the variation in intestine length in herring may reflect the different ages and life histories within the sample. Some of this variation was able to be accounted for through data standardization, however, cannot express every effective factor.
This results of this study are important because both perch and herring are important fisheries. In order to maintain steady and healthy populations, it is crucial to understand their feeding habits. These results can be used in fisheries management to make sure that the food sources of these species are sustained to support the greatest and quickest growth within the wild populations. It is also relevant to farmed perch and herring as a means to ensure the most efficient growth and healthiest conditions. Further lines of research could include the effects of age and environmental factors on the same parameters in perch and herring. This would provide information for constructing a more efficient system in farmed fish. By uncovering the ideal environmental conditions and diet to maximize growth and development of the most energy efficient system, an age based system could be created and implemented that transitions the fish to the best-suited tank throughout their lives.