By Robyn Norman, GEMM Lab summer 2018 intern, OSU undergraduate
Although the average human may think all zooplankton are the same, to a whale, not all zooplankton are created equal. Just like us, different whales tend to favor different types of food over others. Thus, creating a meal perfect for each individual preference. Using a plankton net off the side of our kayak, each day we take different samples, hoping to figure out more about prey and what species the whales, we see, like best. These samples are then transported back to the lab for analysis and identification. After almost a year of identifying zooplankton and countless hours of looking through the microscope you would think I would have seen everything these tiny organisms have to offer. Identifying mysid shrimp and other zooplankton to species level can be extremely difficult and time consuming, but equally rewarding. Many zooplankton studies often stop counting at 300 or 400 organisms, however in one very long day in July, I counted over 2,000 individuals. Zooplankton tend to be more difficult to work with due to their small size, fragility, and large quantity.
A sample that looks quick and easy can turn into a never-ending search for the smallest of mysids. Most of the mysids that I have sorted can be as small as 5 mm in length. Being difficult to identify is an understatement. Figure 1 shows a sample in the beginning stages of analysis, with a wide range of mysids and other zooplankton. Different species of mysid shrimp generally have the same body shape, structure, size, eyes and everything else you can think of. The only way to easily tell them apart is by their telson, which is a unique structure of their tail. Their telsons cannot be seen with the naked eye and it can also be hard to find with a microscope if you do not know exactly what you are looking for.
Throughout my time identifying these tiny creatures I have found 9 different species of mysid from this gray whale foraging ecology project in Port Orford from the 2017 summer. But in 2018 three mysid species have been particularly abundant, Holmesimysis sculpta, Neomysis rayii, and Neomysis mercedis.
H. sculpta has a unique telson with about 18 lateral spines that stop as they reach the end of the telson (Figure 2). The end of the telson has 4 large spines that slightly curve to make a fork or scoop-like shape. From my own observations I have also noticed that H. sculpta has darker coloring throughout their bodies and are often heavily pregnant (or at least during the month of August). Neomysis rayii and Neomysis mercedis have been extremely difficult to identify and work with. While N. rayii can grow up to 65 mm, they can also often be the same small size as N. mercedis. The telsons of these two species are very similar, making them too similar to compare and differentiate. However, N. rayii can grow substantially bigger than N. mercedis, making the bigger shrimp easier to identify. Unfortunately, the small N. rayii still give birth to even smaller mysid babies, which can be confused as large N. mercedis. Identifying them in a timely manner is almost impossible. After a long discussion, we decided it would be easier to group these two species of Neomysis together and then sub-group by size. Our three categories were 1-10 mm, 11-15 mm, 16+ mm. According to the literature, N. mercedis are typically 11-15 mm meaning that anything over this size should be a N. rayii (McLaughlin 1980).
While mysids comprise the majority of our samples, they are not the only zooplankton that I see. Amphipods are often caught along with the shrimp. Gammarids look like the terrestrial potato bug and can grow larger than some species of mysid (Fig. 3).
As well as, Caprellidae (Fig. 4) that remind me of little tiny aliens as they have large claws compared to their body size, making it hard to get them out of our plankton net. These impressive creatures are surprisingly hardy and can withstand long times in the freezer or being poked with tweezers under a microscope without dying.
In 2017, there was a high abundance of amphipods found in both of our study sites, Mill Rocks and Tichenor Cove. Mill Rocks surprisingly had 4 times the number of amphipods than Tichenor Cove. This result could be one of the possible reasons gray whales were observed more in Mill Rocks last year. Mill Rocks also has a substantial amount of kelp, a popular place for mysid swarms and amphipods. The occurrence of mysids at each of these sites was almost equal, whereas amphipods were almost exclusively found at Mill Rocks. Mill Rocks also had a higher average number of organisms than Tichenor Cove per samples, potentially creating better feeding grounds for gray whales here in Port Orford.
Analyzing the 2018 data I can already see some differences between the two years. In 2018 the main species of mysid that we are finding in both sites are Neomysis sp. and Holmesimysis sculpta, whereas in 2017 Alienacanthomysis macropsis, a species of mysid identified by their long eye stalks and blunt telson, made up the majority of samples from Tichenor Cove. There has also been a large decrease in amphipods from both locations compared to last year. Two samples from Mill Rocks in 2017 had over 300 amphipods, however this year less than 100 have been counted in total. All these differences in zooplankton prey availability may influence whale behavior and movement patterns. Further data analysis aims to uncover this possibility.
The past 6 weeks working as part of the 2018 gray whale foraging ecology research team in Port Orford have been nothing short of amazing. We have seen over 50 whales, identified hundreds of zooplankton, and have spent almost every morning on the water in the kayak. An experience like this is a once in a lifetime opportunity that we were fortunate to be a part of. For the past few years, I have been creating videos to document important and exciting times in my life. I have put together a short video that highlights the amazing things we did every day in Port Orford, as well as the creatures that live just below the surface. I hope you enjoy our Gray Whale Foraging Ecology 2018 video with music by Myd – The Sun.