2006 Young Naturalist Awards - The Effect of Hatchery Cell Size on Growth of Juvenile Blue Crabs, Callinectes sapidus Rathbun

Justin The Effect of Hatchery Cell Size on Growth of Juvenile Blue Crabs, Callinectes sapidus Rathbun

Continued...

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Hypotheses and Objectives

Previously, an experiment that was conducted on redclaw crayfish showed that cell size proportionally influenced crayfish size (Aflalo, 2002). Since the blue crab is also a crustacean, it was expected that The Effect of Hatchery Cell Size on wet weight, carapace width, and molt frequency (time between molts) would be significant for blue crabs and that the development of a curve modeling the ratio between crab size and cell size would be possible.

Methods

Table 1 (top) and Figure 1 (bottom) (Click to view)

Setup and Materials
First, to construct the cages, PVC board was cut with a table saw and pieced together to the cell dimensions shown in Table 1, with the adjacent pieces interlocking. Three sets of the layout shown in Figure 1 were constructed, each placed in one tank connected to the sump.

Next, the 90-gallon tanks were rinsed with tap water, cleaned with LiquinoX soap or 12% ammonia bleach, and rinsed out with tap water again. Then the system was filled with 12% bleach at 200 parts per million (approximately 2.07 liters of 12% bleach for the 1,300-liter system), and the bleached system was run for 24 hours. Finally, the system was drained and fresh water was run through the system for 24 hours and drained.

The Biofilter

After the system was cleaned, the tanks were assembled. With PVC pipes, every tank's drain was run into the sump. Two medium air stones were then run into each tank using plastic hose. Biofilter, two medium aeration stones, two large aeration stones, an ultraviolet filter, and the pump were all combined to create the sump, a fourth 90-gallon tank in the system. Then the inflow to each tank was set up using PVC piping and plastic hose, creating a circuit of filtered water through every tank. The physics of water equalization is used in the circuit design: the pump gives water flow into the tank and as the water level rises above that of the drain, gravity drains the overflow into the sump. The system was filled with salt water to 27 ppt, two heaters were placed in the sump with a temperature regulator set at 23°C, and the dead Biofilter was replaced with living Biofilter. Finally, the pump and air were turned on.

Next, a grid floor was propped up on PVC pipes in the tank so that the depth in the cells was about 5 cm. On top of the grid floor, a mesh floor was placed so that the crabs couldn't escape through the bottom, but air could still flow through. Then the assembled cages were placed on top of the mesh floor, and grid ceilings were placed over each block of cages. To seal the cages, extra four-inch PVC pipes and sand-filled 16-ounce plastic bottles were placed on top of the ceiling.

To set up the C6 (meaning "crab stage 6" or "6 molts into crab form since reaching megalopa form") juvenile crabs in the cells, carapace widths and wet weights were measured, the genders noted, and the crabs individually and randomly assigned to and placed in cells.

Table 2: Cell Depth Setup Parameters

PVC Stand (Shallow)	21 1/2 cm Diameter 8 cm Height	34
Grid Floor	10 cm Length 10 cm Width 1 cm Height	150
Mesh Bottoms	21 1/2 cm diamete	102

First, the cells and cell stands, mesh bottoms, and grid floors were cut to the parameters described in Table 2 (see above) using pipe cutters, a utility knife, and scissors, respectively. The mesh bottoms were then hot-glued onto the bottoms of every cell.

After this, standpipes were installed in each tank to insure uniform water levels throughout the tanks. The tank water level equalizes at the mouth of the standpipe and stays there constantly, eliminating the water level variability experienced in the cell area experiment. In addition, the tanks themselves were leveled and placed at the same height for the benefit of the standpipes' operation and to further insure uniform water depth throughout and among the tanks. Once the water levels were finalized, the stands were cut so that the floor depth was 12 cm, the largest depth tested.

Figure 2 and Figure 3 (Click to view)

Next, the 90-gallon tanks were cleaned and prepared using the same method as in the area experiment, but the temperature was increased to 26°C to induce molting. Within each tank, depths were randomly assigned to each position and each tank reserved 11 positions for one depth, 11 positions for another depth, and 12 positions for the third depth, shown in Figure 2. For the deep cells, the cells were simply placed directly onto the overall grid. As shown in Figure 3, for the medium-depth cells, 5-cm-high stands and then 1-cm grid floors were placed beneath the actual cells. And for the shallow cells, 8-cm-high stands and then 1-cm grid floors were placed beneath the actual cells.

2-month old juvenile crabs. Photo courtesy of Center of Marine Biotechnology, University of Maryland Biotechnology Institute.

To set up the C4 juvenile crabs in the cells, carapace widths were measured with the aid of a microscope, wet weights were measured on a scale, and the crabs were individually and randomly assigned to and placed in cells. The different depths accomplished a weighted data collection for the smaller depths because the 11-cm depth tested for a significant effect in general and the closer 3-cm and 6-cm depths gave a more precise view of the significance.

Experimentation
The duration of this project was two to three months based upon the collected data and crab health. Trial 1 lasted from November 14, 2004, to December 9, 2004; Trial 2 lasted from December 20, 2004, to March 22, 2005. Crabs were alternately fed diced squid or Zeigler 8mm shrimp pellets ad-libitum daily. Each day, if any crabs molted the previous day, the new wet weight and carapace widths were measured. New molts that day were noted, and any dirty cells were cleaned.

Justin feeding the crabs

At the end of the experiment, when all data was collected, the final carapace width and wet weight of every crab was measured. Then the crabs were donated to another scientist for use in a separate study.

This project began on August 22, 2005, and ended on October 17, 2005. The same procedure as that of the area experiment was used. Every day, crabs were fed Zeigler 2mm shrimp pellets ad-libitum, and one tank was measured in case any molts were missed

Analysis
Cell Area The data was analyzed for a significant effect of cell area upon juvenile blue crab growth with the Analysis of Covariance (ANCOVA) procedure. The Shapiro-Wilk test was used to determine if the data was properly distributed for valid analysis. When it was found that wet weight, carapace width, and molt frequency were not usable, the logarithm of each was taken to normalize the data. Molt increment, or percent increase in carapace width, was normal, so the raw percents were analyzed. The logarithm of wet weight, the logarithm of molt frequency, and the molt increment were analyzed with the initial wet weight as the covariant while the logarithm of carapace width was analyzed with the initial carapace width as the covariant.

Cell Size This data was analyzed for a significant effect of cell depth upon juvenile blue crab growth with the Mixed Linear Model procedure. Specifically, the carapace widths and wet weights for each molt, molt increment after each molt, molt frequency, and total molts were analyzed for significant differences. The initial carapace width was used as the covariant for all data involving carapace width and initial wet weight was used as the covariant for the remaining data. For post hoc analysis, Tukey's LSD Post Hoc One-Way ANOVA was used.

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