Rationale

The four tank designs talked about below were all created for the purpose of raising horseshoe crabs in a lab environment. These designs focus on the main structure and functionality of the tank, but all must properly integrate with the water flow system Mr. Trimble is creating. All tanks are expected to perform at a certain level and through the process laid out here the flaws and strong points of each design will be analyzed and used to determine the best tank. For these designs to work they will need to be beneficial to the survival rate of the crabs as well as the study of the species for human purposes. Efficiency will be achieved when excessive amount of water or additional nutrients will not be necessary. By raising a significant number of crabs without a high amount of waste of water or other resources we will know the tank works well and efficiently. As per the design of the project, our tank must work off of a closed system of water flow to eliminate the potential for contamination, but there must also be aspects of the tank which will cause constant flow of the water within the system while also taking into account the potential for things like water leakage or over filtration. The system will be considered a failure if there is no benefit to either the horseshoe crabs or the scientists studying them while they are in captivity.

Design 1: Cylindrical Tank
Being the first design I created, this tank is naturally the simplest design of the four. There is a very basic connection to the water filtration system in the sides of the tank and a large observation window in the front of the tank which would be used to watch the activity of the crabs throughout the day. This tank appropriately takes into account the need for constant submergence by the young horseshoe crabs. The simplicity of the design is easy to replicate, the only complications coming from the transition between solid fiberglass and plastic/acrylic to make the observation window. Since this design is one of the larger tanks, the cost of the fiberglass along with the piece of plastic or  acrylic would have to be taken into account along with any tools needed to created the desired shape and connections to ensure a stable and water tight product. If created properly the horseshoe crabs will benefit from having a stable and controlled environment where they will be taken care of during one of their more vulnerable life stages. The scientists will benefit from a proper product by being able to watch the horseshoe crabs in their daily life.

Due to the shape of the tank there is certainly a level of stability not achieved in a shape with more joints and corners. Having a cylindrical shape means having only one seam at the bottom of the tank and along the edges of the observation window, which can become completely water tight with the proper adhesive. Though maintaining structural integrity under the pressure of water is necessary in this project, that is one of the only aspects this tank has that greatly benefits both the horseshoe crabs and the users. The tank only barely resembles the natural environment of the horseshoe crabs and may not properly prepare them for release post-captivity.

Design 2: Shore Tank
With a better understanding of the natural environment of the horseshoe crabs, my next tank design was created. This design was the first design to take into account the natural slope and other aspects of the environment of the juvenile horseshoe crab. There is a section of dry, open sand at the top of the tank onto which the crabs can crawl if desired. This mimicked shore area would also allow my team to imitate the tides of the area where the horseshoe crabs will eventually be released. Again, this tank has a very basic connection to the water flow system, but this is something that could be easily altered as the most important parts of this design are the structural aspects like the sloping bottom. By including the sloped bottom of the tank, the horseshoe crabs become more accessible because with a shallower depth the scientists who will be working with the crabs will be able to reach in to grab the juveniles without submerging their entire arm. Due to the simple box shape the walls of the tank take on the design is easy to reproduce, the only confounding factor being the odd slope of the bottom (this is dealt with in Solution 3: Sloped Tank). The costs considered with this tank will need to be simply the materials (acrylic and adhesive) and tools (such as saws) needed for creating the tank. Overall, this tank will be a better option for raising the horseshoe crabs than the previous design would have allowed, because there is a better representation of the natural environment as well as a more ergonomic design for the scientists who will be using the tank for studying the species.

I find with this design that there is a better representation of the natural environment of the horseshoe crabs which makes this design a better choice than the previous tank. However, over time and after creating further designs I have found some flaws in this design. First off, the large portion of dry sand at the top of the tank, which is a focal point of this design, would be virtually useless during the age range the crabs will be in captivity. Generally, horseshoe crabs only make use of the intertidal zone when they are mating and laying eggs which is something the young crabs will not be doing. Even though the crabs will not make use of the dry sand, the availability makes the possibility of tide simulation feasible. Another obstacle faced with this tank as well as in the next three designs is the tendency of sand to settle; this will be a problem with the sloped bottom of the tanks where we expect the sand at the top of the slope to seek the lower parts of the slope.

Design 3: Sloped Tank
As a continuation and alteration of the previous solution, my third design came into being. This tank is a combination of the previous tank and the tanks which they are currently using in NOAA, as this is where the tanks will be used. There is a central drainage pipe where the excess water will flow through into the filters to then be put back into the tanks. I continued with the idea of the sloped bottom in this tank, however I altered the idea so the slope is less dramatic and easier to create and then reproduce. The slope will again allow for those using the tank for scientific purposes to have easy access to the crabs without needing to fully submerge their arm. This design, however, has the entire area of the tank floor submerged instead of keeping a section dry. By including this change there will be more area for the crabs to live in while keeping the benefit of a better simulated habitat.

Since this design was created as an alteration of the previous design, the changes made allow the tank to better fit into the NOAA environment while maintaining a simpler and more easily reproduced shape. The tank still has a good simulation of the natural environment while also incorporating a simpler and more reliable system for water flow. The potential problem with the water drainage in this tank is the possibility of horseshoe crabs (if they were to swim) getting caught in the drain. The drainage system also may not fully cause the water towards the bottom of the tank to filter and drain.

Design 4: Compartmentalized Tank
The final solution that has been made for this project is a tank which could be looked at as a combination of designs one and three. The basic shape of the first tank design was kept, but modified to better fit our needs. Center drainage and sloped bottom of design three was kept and combined with the shape of the first tank. Each compartment of this tank is meant to hold 20 of the 100 horseshoe crabs and the tank as a whole will hold all 100. This tank will save lab space as well as provide the horseshoe crabs with the best simulated habitat we can create. In the center drainage pipe there is one large slit where we want the water level to be maintained along with a series of small holes which will allow the water towards the bottom of the tank to trickle out and be filtered. As a safety, the top of the pipe will be open in case of overflow. On top of reach partition, small pipe holders are in place to hold the pipes of the water flow system. We tried to keep the design as simple as possible while also taking into account all the needs of the tank to work properly and also the needs of the horseshoe crabs.

This tank is the most designed tank with the most thought put into the structure. There are certain aspects of this design, such as the sloped bottom and the trapezoidal shape of the walls which add a bit of difficulty to the design. However, these things are necessary for the proper function of the tank. By using this tank, the space within the lab will be saved or better utilized and the partitions within this tank allow for a few different variable to be tested among the separate sections of crabs.

	Design One	Design Two	Design Three	Design Four
Has space to raise 100 crabs	4	3	4	5
Has potential to raise survival rate	4	4	4	4
Is beneficial to studying the species	2	3	4	4
Replicates the natural environment	1	3	4	4
Is stable under pressure	4	3	3	4
Properly connects with water systems	1	3	3	4
Able to survive constant exposure to salt water and live crabs	3	3	4	4
Water/leak proof	2	3	4	4
Is replicateable	3	2	4	3
Total	25	28	34	36

*on a scale of 1-5

After taking into account each specific need the tank needs to fulfill and comparing the tanks to each other, I have found that the compartmentalized tank will work the best in raising the young horseshoe crabs by mimicking their environment properly, but the design is also ergonomic for human use in a lab. Design four best allows for comfortable living conditions for the crabs, while also giving the NOAA scientists a tank which they may use to study the species and possible help further save them. This tank, though maybe the hardest to construct is thoroughly thought out and "safeties" within the design have been created to ensure constant, up to par function of the tank. This is, therefor, the best option for us to move forward with.

(to see the tank designs follow this link: http://habitatforhorseshoecrabs.blogspot.com/2013/09/alternative-solutions.html)