A full-course meal for oyster larvae

April 1, 2021

This is part three of a series about how oysters are cultivated and grown at the University of Maryland Center for Environmental's Science's Horn Point Oyster Hatchery

Oyster larvae at University of Maryland Center for Environmental Science’s Horn Point Laboratory receive a gourmet selection of algae, both flagellates and diatoms, grown in house, to aid in their growth and survival in their early stages of life. Their specially curated diet by hatchery employees gives larvae the best possible chance at growing into its final oyster form and Horn Point’s Hatchery has it down to a science.

The process begins as fertilized eggs are placed into large tanks where they will mature for the next 21 days. By the time oyster eggs are placed in the tanks, the growing of their food has already been weeks in the making.

At the algae lab in Horn Point’s Hatchery, Julie Trommatter,  a faculty research assistant, starts with small samples of algae called stock cultures that eventually make their way into large growth chambers in a greenhouse.

“There is a lot of manual labor that goes into growing the algae, but once we have the algae growing in tanks in our greenhouse, there are automated systems to help monitor algae growth and determine when that algae can be dispensed into the larvae holding tanks” said Trommatter.

Decades ago, oyster larvae were hand fed, with hatchery scientists having to manually dump food into the larvae tanks. Now technological advances, like the automated feeding system developed  for the Horn Point Hatchery, help streamline the feeding process. 

Growing algae at the hatchery ensures the oyster larvae can always be fed. While growing algae might not seem to be a challenge, there is a delicate balance that must be maintained to keep it healthy and thriving. From start to finish, a single algae culture’s  growth process takes about 8 weeks in whole, requiring a decent amount of planning into when larvae will need what kind of algae. A recently renovated greenhouse at the hatchery helps ensure that the algae get plenty of sunlight during the day and use of Kessil-brand LED growing lights keeps growth high at night An in-house air conditioner helps to keep the greenhouse cool and the algae content at optimal conditions.

The first feeding installment for newly developing oyster larvae is a mix of tiny algae, such as Isochrysis sp and diatoms such as Thalassiosira pseudonana or Chaetoceros calcitrans. Having a mix of algae  adds nutritional variability.The blend of algae grown at the oyster hatchery provides a healthy balance of omegas, lipids, and proteins  which are absorbed for development and growth It is not only a matter of type of algae, but how much. Scientists in the hatchery track larval growth to make sure they are not only getting a diverse diet, but sufficient food.

“If you were to be on a diet and you only ate one food throughout your entire development, how happy and healthy would you really be if those were the only nutrients you were getting?,” said hatchery manager Stephanie Alexander, “By giving them a variety of food, we are covering the entire spectrum of nutrients need to encourage good growth and healthy larvae.”

After the initial diet of tiny algae, as larvae grow, their food grows with them.

After about six days the larvae’s diet includes slightly larger algae (Chaetoceros muelleri), and an even larger algae at nine days (Tetraselmis chui). Tetraselmis wasn’t always part of their diet but after evidence showed that it’s high lipid content aided in larvae’s transition from being free swimming to setting and becoming spat, it was added to the mix. After its integration, setting efficiencies increased..

Larval growth is monitored throughout their early life stages to ensure that their development is on track and that their diet is supporting that growth.

But how do you measure such a small organism, that is only microns long?

Every couple days, hatchery workers have to drain the larvae growing tanks, called mass larval tanks or MLTs, to change the water—a process which takes several hours. Scientists take this opportunity to take a sample, count larvae to track survival rates and obtain a range of sizes that give larvae feeders an idea of what course in the menu they are in. Toward the last week of the larval period, hatchery workers are constantly checking on the size of soon to be mature larvae to ensure they are not yet setting in the swimming tanks.

At 14 days old, there is a mix of mature and immature larvae in the tank so different sized sieves are used to sort and determine larval size. These observations help inform hatchery staff as to whether the larvae have reached the stage where they are ready to leave the nest or not. If larvae are determined to be mature enough, they are removed from the swimming culture and introduced into setting tanks where they can continue their journey to become a fully-grown oyster.  Smaller larvae are returned to the larval tank where they will continue to grow until they are mature as well. 

Once they attach to a shell and are moved to a natural environment, their diet changes completely as they now acquire food through their surroundings as compared to in the hatchery, where their specially curated diet is completely provided for them. 

If you’re interested in how the larvae got to this stage, check out our previous pieces on oyster broodstock and spawning to get the whole story.