Behaviour

A common observation of all organisms is that they are only able to ingest a certain amount of organic or inorganic matter at a given time. The usual limiting factor is the size of the mouth or pore in which the matter is ingested. It is known that in order to develop its calcareous tube, Christmas tree worms have to process sand particles and other organic particulate matter in a specialized gland to develop the calcium carbonate building material. However, what is the size limitation of food or sand particle that Spirobranchus sp. can intake at a given time? In this observational experiment, the goal is to determine what size particle Spirobranchus sp. is capable of ingesting. As the method to obtain nutrition is conducted in a similar fashion, these observations can also be used to make inferences about their feeding behavior.  

Materials and methods:

Specimen collection

Ten specimens of Spirobranchus sp. were carefully extracted from intact pieces of coral using a hammer and chisel. To minimize the impact the experiment had on the worm’s well being, they were placed into a large bucket of freshly obtained sea water once removed from the coral boulder. These specimens were obtained from the reef crest off the southern side of Heron Island. Once collected, these specimens were transferred to a filtered sea water tank located in the Heron Island Research facility, owned and operated by the University of Queensland.

When required in the experiment, the specimen was placed into a plastic container in a sink and filled with fresh filtered sea water. This water was allowed to overflow out of the container for roughly one minute to reduce the amount of particles which may have been present within the coral host. The filtration of the sea water ensured that only the particles introduced into the system would influence the results. Once filtered and cleaned, the specimen was covered in at least 3cm of fresh filtered sea water. This process was repeated for each specimen tested.   

Sand collection, sorting and preparation

Handfuls of sand were collected from varying sites off Heron Island’s beach (from the foreshore, backshore, sand dune and hind dune) to ensure that sand of varying sizes were obtained and placed into a large plastic takeaway container. Approximately a teaspoon of a random assortment of these samples were put into a mortar and pestle and ground into a fine powder to increase the range of the sand granules tested in the experiment. To aid this grinding, 10mL of fresh filtered seawater was added into the pestle. All these samples were placed into a dryer oven in the plastic container for one hour at 60°C to allow for the samples used to become completely dehydrated. The dehydration process not only permitted sorting the sand, but also killed any microbial presence on the sand which may have altered the worm’s reaction to the particle.

Once the sand was dried, it was sorted into its relevant sizes by placing it into a mechanical sifter for two minutes. The mechanical sifter consisted of several layers of sieves which divided the sand into different sized elements: 2mm, 1mm, 500microns; 250 microns, 125 microns and 64 microns. Each sample size was collected into its own plastic container and clearly labeled to avoid future confusion.

With the samples divided, two scoops with the end of a probe of each of the six sized grain of sand samples were transferred into another separate plastic container and again labeled. Two full pipettes of fresh filtered sea water were added to these containers. This allowed for the smaller sized sediment to be introduced into the system using a pipette and allowed the sand particle to sink rather than float when dropped into the water above the Christmas tree worm.

Experimental procedure

A specimen was placed underneath a dissection microscope at approximately 45° from horizontal to better observe the reaction of the worm to the varying particle sizes. The angle of the worm also prevented gravity from influencing the uptake of certain particles. Once the Christmas tree worm had relaxed and extended from the calcareous tube, a small sample of the certain sized sand particles was gently dropped in the water around 1cm above the tip of the whorl from a pipette. Particles of 2mm diameter were introduced directly with a pair of forceps as it was unable to fit into the opening of the pipette. The distance the sample was inserted into the environment guaranteed the sample was introduced to the worm without seriously influencing its behavior.

Each sized sand particle was tested individually on the specimen. The reaction to the sand dropped was observed and recorded. Specifically, it was noted whether the specimen accepted or rejected the certain sized particle and which part of the prostomium was used to accomplish this. A particle was considered accepted if the Christmas tree worm transported the particle to the mouth via the tentacles pinnules and radioles. Rejection was noted if the worm moved the particle to the distal part of the radiole and dropped the sediment piece or if the worm retracted into the tube when interacting with the particle.    

This experiment was repeated on all the samples involved. For each specimen, to minimize bias in results, the samples of sand were introduced to the worm in different orders. As a control, each of the specimens was observed prior to exposure to the sand to monitor their normal behavior. Their behavior prior to experimentation was also recorded.

To record the reactions of the experiment, a camera designed for dissection microscopes called “Dinoscope” and its associated software was used. This allowed for direct recording of video from the microscopes perspective to be loaded directly onto a computer.   

Results:

Table 1. Did it accept/reject particle and which appendage did it use?

(A) = accept; (R) = reject

As shown in tables 1 above, all sediment sizes 500 microns and above was rejected. The reaction to the particles either one or two millimeters in diameter was for the worm to retract into its tube. There were some exceptions, especially in larger specimens, in disposing of 1mm sand particles using the radioles. All species accepted particles both 125microns and smaller, mostly using both the radioles and the pinnules to gather these pieces. Larger specimens were able to dispose of 250micron particles using their pinnules as well.

Most of the specimens undertook little activity in the container used to observe them prior to experimentation. They occasionally would make a flicking motion with their radioles, but rarely used the pinnules on them. This may have been because of either irritation by the temporary accommodation or been filtering smaller particulate matter present in the container.

It was observed that all the specimens studied possessed either 4 or 5 whorls on each tentacle. All the specimens possessed an operculum which appeared to be lined with algal growth, possibly of a Halimeda species. There was some variance in the diameter of the largest whorls amongst all the species, ranging from 1.4 to 1.9cm.

Discussion:

Except for one occasion, it is evident that the particle limitation for Spirobranchus sp. is approximately between 125microns and 249 microns. A specific amount cannot be identified with certainty as it is theoretically possible that the worm is able to uptake pieces of sand ranging from 125 to 249microns large. This is due to the lack of specificity in the mechanical sifter, as it is only able to distinguish between these certain ranges rather than specific sized particles. However, it is likely that the ability to intake larger particles of sand would be proportional to the size of the Christmas tree worm; larger worms being able to ingest and process larger particles.

The smaller sized particles induced more activity from the worm than any of the larger particles. It is likely it would prefer to uptake smaller size particles as the surface to volume ratio of the particle would be large. This would most likely increase the efficiency in processing these particles as the increase surface area would be better subjected to whatever processes occur.