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Crab Larvae and Algae Cultivation

Crab larvae [zoeaSmall.jpg]
[Diatoms - crab larvae food, thalgthal.jpg]

There are two cultivation processes conducted by Sara Persselin and researchers at the Kodiak Laboratory - cultivating crab larvae, such as the individuals shown above left (click photo for full image), and cultivating the food that the larval crab eat. Pictured above right is the diatom Thalassiosira sp, cultured from seawater collected from Trident basin just outside the Kodiak Laboratory (see other colorful photos of local diatoms).

Throughout development, from early larval stages to young juveniles, crab cultivated at the Kodiak facilities are used in a variety of studies. Such studies include investigations on larval crab settlement behavior, cannibalism/predation, use of critical habitats, as well as comparative growth studies. Experimentation with different culture containers, as well as static and flow-through seawater systems, is also being conducted to ultimately design an optimal culture system that will provide researchers with larvae (at various stages) for the various research projects.

[millions of Tanner crab larvae, thzcult1.jpg=8KB]

[red king crab larvae; thgrglauco2.jpg=7KB]

[Chroomonas salina culture, thalgredcult.jpg=8KB]

The first step in the crab cultivation process is to collect larvae from individual female crabs that have been isolated from each other. The larvae are then cultivated in 20-liter containers. The picture at left shows them in a lab cold room maintained at an optimum temperature for growth studies of 8 degrees centigrade.

[volumetrically counted crab larvae, thalglarvol.jpg=7KB]Adult female crabs release variable numbers of larvae daily over a period of time. The photo on the right shows volumetrically the different daily densities of larvae released by three different crab over a five day period.

There are several larval or zoea stages, each one an evolution in body morphology until the organism begins to take on the characteristic shape of a young crab. The growth of juvenile and adult crabs is punctuated by a complex and fascinating process called molting. For example, after hatching out as a planktonic larva, a king crab larva molts 5 times, at 1-2 week intervals, before becoming a small crab. Different species of crab have different numbers of zoeal stages. After a few months, larvae molt to the megalops or glaucothoe (in red king crab) stage. The second photo on the left is of a red king crab glaucothoe, its 5th larval stage. At about 8 degrees centigrade, red king crab settle in approximately 64 days. No longer larvae, the juvenile crab settle to the bottom and hide: they are approximately 1.5 mm in length and weigh about 1.30 mg (Dr. Bradley Stevens, unpubl. data).

Microalgae, or phytoplanton, provide crab larvae with nutrients essential to their development and survival. Especially important are certain fatty acids that are not necessarily present in the zooplankton the larvae eat. Although algae on their own cannot fulfill the metabolic needs of red king crab and Tanner larvae, they do increase the survival of larvae in the laboratory when fed in combination with Artemia sp. (brine shrimp) nauplii.  Sara Persselin cultures various flagellated algae including this red brew shown on the left called Chroomonas salina as well as Isochrysis galbana and Dunaliella tertiolecta, and the diatoms Thalassiosira nordenskioeldii and T. aestivalisThalassiosira species are common in Alaskan waters while Isochrysis and Chroomonas are warm water species often used in aquaculture. These five algae were used for Tanner crab larvae survival trials in 2002 to determine which species of algae result in the highest survival rates of larvae.

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