Jackson Wyers
Nathan D. Johnson
Field Trip Report
MARB 435
Identifying and Collecting the Zooplankton of Galveston
Introduction
Zooplankton are heterotrophic organisms that are suspended in the water column at a
variety of depths. The most common zooplankton caught mesozooplankton which includes
groups such as copepods. The term mesozooplankton is a size defining term – meaning, it
encompasses zooplankton from the 200µm range to 2mm range. These animals are important
because they serve as a vital food source to animals higher up in the food chain. On January 28th
2013 a field trip was conducted to collect plankton from the Galveston ship channel. Besides
using different collection methods, the main purpose of this experiment was to be able to identify
the local species of zooplankton down to species. Identification of planktonic species is
important because it reveals a vast amount of information about other marine life in the area – ie:
larger marine animals that feed of these mesozooplankton. Water measurements were also taken
to check the environmental parameters that the zooplankton were living in.
Materials and Methods
The location that was selected for this specific experiment was a fishing pier that extends
approximately 20 feet out into the ship channel. Before collection, YSI data was recorded for the
current water conditions. To collect specimens, two different setups were used. One plankton
tow net was weighted so that it would sink; the other net was not weighted so that it would skim
the surface of the water. The mesh net that was weighted was held so that it stayed
approximately three feet just underneath the surface. Students were divided up into two groups to
work the two different nets. Each net was slowly walked down the pier and back – for
approximately 30 seconds each time – and then the collection was emptied into a collection
bucket – which was prefilled with a little bit of seawater, but not too much so that the samples
were in a concentrated manner. The net was then handed off to the next peer in the group and the
process was repeated for both groups. Overall, the process was conducted about five to six times
with students emptying the nets into the bucket each time. Once the process was done, the
equipment and buckets were collected and returned to the lab for analysis. Samples of both deep
and shallow water were collected in a small dish and observed under a microscope. Dissecting
scopes were used for a broad identification of the organism. For a closer examination, a few
drops of Protoslo solution were added to induce a more docile state to the animal; the organism
was then pipetted out and onto a slide for further examination under a compound microscope.
Results
Of the zooplankton identified, the most common was the copepod Acartia tonsa which
was found at both depths. The species that were observed from the deeper water overall were as
follows according to Table 1:
Table 1. Shown below are the species caught and identified from both tows.
Deep Water Tow (~3ft.)
Surface Tow
Chaetoceros lorenzienus
Mnemiopsis sp
Acartia tonsa
Acartia tonsa
Seba ekepuu
Balanus venustus
Temora turbinata
Barnacle cyprid
Balanus, Caligas
Pselicnema
Beroe ovata
Whenever an organism could not be identified with the dissecting microscope, it was collected
onto a slide and then examined with a light microscope for further detail and magnification. The
water readings that were recorded were as follows according to Table 2:
Table 2: YSI data for location of expeiment.
Water Temperature ºC
pH
Salinity
15.9
7.29
29.8
Dissolved O2 (ppm)
90.3
One of the larger species that was caught was Beroe ovate. This animal was a little more
than 1mm in length on average. Figure 1 may be referenced for a visualization of physical
characteristics.
Discussion
The species that were observed and reported would most likely coincide with the
environment at the time. Considering this region of Northern America, the water is fairly cold at
the moment. According to Jonathan Bird (1999) – author of Plankton: Ocean Drifters – a
combination of winter disturbances in the weather and the disappearance of the thermocline are
what allow for more nutrients to be present at the surface of the water. This is an interesting
point to consider when looking at the data received. With this notion, one might expect there to
be a higher abundance of the zooplankton on or near the surface instead of several feet below.
However, the data would reveal just the opposite; there was a greater presence of zooplankton
from the deeper water sample. Perhaps the water just is not cold enough and this hypothesis
therefore is not valid. This is a good possibility when comparing Galveston to other climates
around the globe. A more likely explanation is
that a higher abundance of zooplankton were
found deeper because the experiment was run
around mid-afternoon; and because of diel vertical
migration patterns, there would be less plankton
on the surface as compared to other depths.
It was observed that the copepod Acartia
tonsa was by far the most abundant species
recorded from the samples. This was an expected
result since copepods often represent up to 80% of
the catch. Another common species was the Barnacle
Figure 1: Scientific drawing of species Beroe ovata
cyprid. This was also a common organism. One possibility is that the Barnacle populations may
have been thriving if more nutrients have been available due to winter disturbances. This species
was particularly hard to view under the dissecting microscope because of its speed. To view this
plankton with precision, a slide was prepared from the sample that was under the dissecting
microscope, and was put under a compound scope. It was not a surprise to find Mnemiopsis sp.
since several specimens of Beroe ovata were identified. B. ovata is a predator of Mnemiopsis sp.
so it was expected to find this particular organism.
Taxon Description
Kingdom: Animalia
Phylum: Ctenophora
Class: Nuda
Order: Beroida
Family: Beroidae
Genus: Beroe
Species: Ovata
Beroe ovata is a marine zooplankton that is considered nekton because of its ability to
move freely in the water column. Their habitat is highly diverse with wide ranges of salinity
measures and regions. They are typically pelagic ranging organisms but can also be found in
near-shore environments ("Ecosystems where Beroe ovata occurs", 2010). B. ovata is bi-radially
symmetrical and have a tapered oval shape with the oral end being the widest part. They are a
transparent light color with a hint of pink (Finenko, et al., 2003; Shiganova, et al., 2001). Little is
known about the reproductive strategies of this species however they are classified as
hermaphrodites and each organism will discharge the egg or sperm into the water where they
will float freely. Some other reproductive qualities include year-round breeding times and group
spawning (Mills, 2001). These qualities are a synapomorphie of all ctenophores. The preferred
prey choice of B. ovata is mostly other ctenophore organisms. A very common feeding choice is
that of Mnemiopsis leidyi. Beroe ovata will expand the oral cavity to create negative pressure
which will inhale the target prey (Matsumoto & Harbison, 1991; Swanberg, 1974). Some natural
significance of these organisms is their ability to control the populations of unwanted animals
because of their status as a predator. They do not present any harm to humans due to the lack of
stinging tentacles. All in all, the organism has little to no effect on humans (Kube, et al., 2007).
References
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Bird, J. (1999). Plankton: Ocean drifters. Retrieved from
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