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Sea Hare - One of the Fastest Growing Utilitarian Additions to Clear Water Marine Aquaculture Systems

16 May 2016

Sea Hares (Dolabella auricularia) may not be familiar to anyone except a handful of medical researchers and marine aquarium enthusiasts. They get their name from the two rabbit ear like tentacles, known as rhinophores, on the top of their head that are used for smell.

These slug like animals are nocturnal, poisonous, plant eating machines, with an internal shell and no spines or thick skin. Instead, sea hares use poisonous secretions from their skin and a can shoot a cloud of purple ink to deter predators. They are however, preyed upon by lobsters and crabs, but an attempt by humans to eat sea hares can cause sickness. Sea hare eggs are consumed raw and cooked as a delicacy.

Sea hares feed on a variety of algae and are found in shallow coastal waters of the Pacific and Indian Oceans with juveniles inhabiting deeper water up to 18 meters. Being hermaphrodites, they form mating circles for reproduction and then lay from 80 up to 500 million eggs that will hatch in 10-12 days after maturing to a brown color a few days before. Free floating larvae then settle on macro algae after being adrift for 30 days.

There are a few uses for this odd looking animal other than consuming its eggs, or being used as a maintenance "algae cleaner" for Reef Aquarists. Sea hares possess a very simple nervous system that is used as a model in biomedical research.

A similar role is filled by this animal for studies in neurophysiology and human studies in hormonal development with an interesting compound from the animal also being investigated as an anticancer agent (

A growth trial with a sea hare found near the islands of Pacific Panama

A sea hare of 89 g was collected from a ball of old coral rubble at Seca Islands, Panama on 16-Aug-15. After a brief acclimation it was stocked into a zero water exchange tank inside a greenhouse, located 600 meters above sea level near Boquete, Panama.

This animal was identified as (Dolabella auricularia) by its flattened disk like shape at the posterior end with an exhalent siphon in the middle, and an enclosed mantle cavity which the animal draws water into via an opening in the middle of its back (Figure 1). D. auricularia is mottled brown/grey with dark blotches and at first glance looks like it may have been cut in half at some point in its life. It also has a wide, smooth strip along its ventral side aiding in sliding along as it is a busy feeder during most of the night.

Figure 1. Pictures of sea hare at stocking weighing 89 g

The tank housing the sea hare was operated as a low feed input (6 g/m3/day), full strength clear water system containing coral and 80 g of small reef fish, along with a few hermit crabs and snails.

It was equipped with large foam filters run by a air pump of 20W. The foam filters performed the function of solids removal, nitrification/denitrification and aeration/mixing. As with all clear water systems without water exchange containing any measureable nitrogen and phosphorous, the tank had large amounts of hair algae growing on the sides and bottom of the tank.

The sea hare was removed from the tank periodically, weighed and photos taken, with the first sampling data occurring on 12-September. It was found to have gained an amazing 4.6 g/day for 26 days with green fecal pellets on the tank bottom and greatly reduced hair algae to show for its efforts. Like many aquatic animals kept in captivity the color was more pale due to lesser diversity in the diet, but the animal appeared very healthy (Figure 1).

By the Oct-11 sampling date, growth of this animal was halved as its source of food had apparently run out. By 9-November-15 with the feed rate of fish increasing slightly, growth had increased again to 3.4 g/day for 27 days and the sea hare increased in weight to 368 g. By 29-November with a bare tank evident, the sea hare at this point lost 1.7 g/day for 20 days and was moved to another similar set up and run, adjacent tank.

Growth resumed at 2.7 g/day for a total of 39 days and at this point weighing nearly a pound at 438 g. With hair algae cropped back to almost nothing, this animal was moved yet again to take part in a coral growth experiment in a tank outside of the greenhouse. Once again growth resumed at 2.1 g/day for 89 days at the sampling date of 15-April, where it weighed in at a hefty 624 g.

An important fact to consider is that all this growth was from a "food" that is a pest and considered a waste in a clearwater system, a nuisance to get rid of by marine hobbyists and researchers alike with the only other option available being tedious, physical removal or expensive protein skimmers, and other N and P removal devices. When comparing the growth of another reported "fastest growing aquatic animal", information is scant.

However, in a document found on line ( one researcher reported that the common grass carp is likely the fastest growing fish in the world as it can grow from a juvenile size of 20 g to a size of 2.25 kg in 6 months and then 1 kg per month for the next 6 months reaching 8.5 kg in one year, ostensibly on plant material alone, likely considered "waste".

A mathematical model assembled and a growth curve established showed that the grass carp would likely grow at an maximum average of 3.3 per cent increase per day from a size of 80 to 228 g in 3 weeks.

The best growth period of the sea hare was the first 26 days after stocking, increasing at a average rate of 3.6 per cent per day in body weight from 80 to 208 g. So how did this animal wind up accumulating more biomass as daily growth than what was actually added as feed per day?

Most feed pellets are generally 10 per cent water as moisture while the sea hare is about 90 per cent water. As nitrogen retention of feed in wild fish is low at about 30 per cent, most of the N is wasted and released into the surrounding water. Even lesser amounts of phosphorous are retained.

Adequate carbon was provided by CO2 from respiration of fish for algae growth and N was provided at a rate of 0.20 g of N per day (5*0.36*0.16*0.7, or 5 g of feed per day* per cent protein*N in protein*N wasted in feed) and the increase of N in the sea hare at 0.14 g(4.6*0.2*0.16, or, 4.6 g of gain of biomass*20 per cent protein*16 per cent N in protein) so there is obvious ample N in the wasted feed to be acquired by the algae to feed the sea hare.

Likely very little denitrification took place and a single pass of N retained in the sea hare was only a small percentage and N was continually recycled through algal growth. Also, there was ample algae before the sea hare was actually put in the tank which explains the high growth rate of the first sampling period.

A benefit of having a sea hare besides cropping back algae, is that any N retained in the sea hare is non volatile and cannot add to deteriorating water quality as long as the animal is kept alive. This is different from algae and bacteria which continually degrade and are recycled in a zero water exchange system. The incredible value of keeping clearwater systems free of algae and the super high growth rate, of an animal produced from a nuisance waste product make the sea hare very interesting indeed.

Dr Bill McGraw

Dr Bill McGraw 

Dr Bill McGraw specializes in zero water exchange systems which are disease free and have a positive impact on the environment.

Originally from Pennsylvania, Dr McGraw has two degrees and a post doctorate fellowship at HBOI in Florida.


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