The vast majority of soft corals rely greatly on zooxanthellae for their nutrition. Some such as those in the Xeniidae family, rely almost exclusively on their zooxanthellae since they lack nemato-cysts and have a reduced digestive system (see Polyp Anatomy). However, recent studies have shown zooxanthellae may not be able to meet the total nutritional needs of other soft corals. Fabri-cius and Klumpp (1995) found that twelve of the most common photosynthetic soft coral species investigated on the Great Barrier Reef could not meet their carbon requirements by photosynthesis alone. This brings up the question of just where do they get their carbon from? Many octocorals are known as polytrophic feeders, meaning that they are capable of obtaining nutrition from more than one source (Williams, 1993). Possible sources may be one or

all of the following: the direct absorption of nutrients, the ingestion of zooplankton and/or phytoplankton, or the ingestion of "marine snow' and its attached bacteria and organic material.

Members of the Xeniidae family are known to absorb organic and inorganic nutrients directly from the water column and it is possible that other soft corals have similar abilities (Schlichter, 1982a and b). In the case of Heteroxenia fuscescens, photosynthetically produced excess amino acids are actually released directly into the environment, thereby enriching the water (Schlichter and Liebezeit, 1991). If other such sources exist it is possible that combined they could provide enough to allow other corals to absorb and utilize these substances as an important nutrition source.

Given that octocoral polyps have few, small stinging cells (or none at all and that their pinnules offer a large surface area, they are generally classified as suspension feeders, straining fine particles from the passing water. As such their feeding efficiency is affected by the rate of current flow, polyp and colony flexibility, and orientation. Several studies have shown that feeding efficiency generally increases up to a maximum velocity and then drops off at velocities beyond that (Best, 1988; Sponaugle and LaBarbera, 1991; Dai and Lin, 1993; Fabricius, et al, 1995b).

However, the flexion of the polyps and colony can act together to increase the range of current velocities over which suspension feeding is successful (Sponaugle, 1991). The polyps themselves can actually modulate the flow around them, to enhance prey capture. In a study of the effects of flow on particle capture in the asymbiotic temperate octocoral Alcyonium siderium, Patterson (1991) found that at low flows (2.7 cm/s) tentacles on the upstream side of the polyps capture the most prey. At interme-

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The COMPLETE guide to Aquariums

The COMPLETE guide to Aquariums

The word aquarium originates from the ancient Latin language, aqua meaning water and the suffix rium meaning place or building. Aquariums are beautiful and look good anywhere! Home aquariums are becoming more and more popular, it is a hobby that many people are flocking too and fish shops are on the rise. Fish are generally easy to keep although do they need quite a bit of attention. Puppies and kittens were the typical pet but now fish are becoming more and more frequent in house holds. In recent years fish shops have noticed a great increase in the rise of people wanting to purchase aquariums and fish, the boom has been great for local shops as the fish industry hasnt been such a great industry before now.

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