The Effect of Photobacterium damselae Availability on the Behavior of Choanocytes in the Marine Sponge Amphimedon queenslandica

Abstract

Sponges are aquatic filter feeding animals that use internal chambers comprised of choanoflagellate-like cells called choanocytes to pump and filter water. In this study I investigated the behavior of choanocytes in juvenile Amphimedon queenslandica, a coral reef demosponge, when they were fed Photobacterium damselae subsp. damselae compared to when they were maintained in 0.22μm filtered seawater. Prior to these treatments I fluorescently-labelled choanocytes with Cm-DiI, which allowed tracking of the movement and behavior of these cells over 24 h. I found that choanocytes moved out of the chambers significantly more in unfed sponges, suggesting that choanocyte chambers and the aquiferous system are constantly being remodeled in unfed sponges. When fed however, the choanocytes were more prone to staying in their respective chamber, presumably collecting P. damselae from the water. Furthermore, when the juvenile sponges were exposed to P. damselae the choanocyte chambers grew in size, becoming 20% larger than the unfed juveniles. Lastly, there were significantly more cells being shed into the water column when the juveniles were fed. This suggests that under natural conditions, when the aquiferous system is fully functioning, replacement and shedding of choanocytes occur. I conclude that the aquiferous system of A. queenslandica is very dynamic, where the choanocytes have the ability to leave and join other pre-existing chambers or form entirely new chambers. However, when the choanocytes are exposed to food they are less likely to leave their chamber, resulting in development and growth of the choanocyte chamber.

Popular Abstract

Investigating the Behavior of the Sponge Feeding Cell, the Choanocyte

Sponges are one of the earliest evolving animals still thriving today. Their success comes from simplicity. Sponges lack many of the things that we normally associate with animals. They don’t have a nervous system, they don’t have a gut and their morphology is unstructured. But look below the simple unassuming exterior and you find an advanced network of canals and chambers which together, has an extreme ability to remove nutrients from the water. So efficient, in fact, is this system of canals and chambers that the water that leaves the sponge is close to sterile.

The efficient filtering of the sponges provides vital ecosystem services for many oceans around the world. Perhaps mostly in coral reefs, surrounded by crystal-clear nutrient poor water. The sponge is able to extract nutrients from the water that are accessible to few other organisms. They then use these nutrients to build more cells, which later dies off and are shed back into the water column. This organic matter is then accessible to a wide variety of organisms on the reef, thus completing this nutrient cycle. The cell that sponges use for feeding is called a choanocyte. These cells can pick up nutrients from the passing water and deliver the food to other cells in the sponge. It’s also these choanocytes that often clog or die that are being shed into the water column in large numbers.

For my master thesis I investigated these intriguing choanocytes in closer detail. I went over to Brisbane Australia to work on a sponge called Amphimedon queenslandica. I went to a small island on the Great Barrier Reef called Heron Island to collect some adult sponges and bring them back to the lab. I then simulated low tide by increasing the water temperature by a few degrees, which induces the sponges to release their larvae from the brood chambers. I then collected and raised the larvae for approximately five days until they became juveniles. Then, to be able to investigate the behavior of the choanocytes, I labelled the choanocytes with a red fluorescent dye (Cm-DiI). Later, the juveniles were separated into two different groups, one was given food, in the form of the marine bacterium Photobacterium damselae, and the other group was not given any food. In order to draw conclusions about the behavior of the choanocytes I monitored the movement and behavior of the red labelled choanocytes using an inverted fluorescent microscope. By 24h I stopped the experiment, and used a confocal microscope, which allowed more detailed images of the choanocytes.

This study found that the choanocytes in A. queenslandica are capable of moving around the sponge, most likely in search of food because when food is present the choanocytes move a lot less, instead growing and expanding the established chambers. However, when food is absent the chambers constantly dissolve and re-form. This study also found further evidence that sponges are recycling nutrients back into the water by shedding choanocytes. These choanocytes are shed most likely because they have clogged or expended their all their energy.

Researching the biology of choanocytes further and the shedding of sponge cells could further inform us on the importance of sponges in marine ecosystems. This could then be used to implement suitable conservation methods. Furthermore, choanocytes have been an important element in understanding the evolution of multicellularity, due to the similar morphology choanocytes have with other unicellular organisms. By performing transcriptomic research on the choanocytes in the presence and absence of food, and comparing the results with a suitable unicellular organism, could provide answers on the evolution of multicellularity.

Supervisors: Bernard & Sandie Degnan, Johan Hollander
Master’s thesis Aquatic ecology 45hp, 2018
Department of Biology, Lund University

• author: Jönsson, Mathias
• supervisor: Johan Hollander; Bernard Degnan; Sandie Degnan
• organization: Degree Projects in Biology
• course: BION01 20171
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8937975
• date added to LUP: 2018-03-23 14:47:06
• date last changed: 2018-03-23 14:47:06