Setbacks and Science

The long spined black sea urchin, Diadema antillarum.

It’s finally time to get around to the reason I am here in Honduras; the research. The research I will be doing is the biggest part of the research project I have to do in 4^th year in UCD in order to get my degree in Zoology. My working title is “The effects of extreme temperature on predator avoidance response of Diadema antillarum: possible roles for reef material and complexity”. It is a mouthful I know! The aim is to test if and how the response to a simulated predator changes in the sea urchins of the species Diadema antillarum. I will get back to how we actually are doing this in a minute, but first let me just explain why these urchins are so important. D. antillarum is a key grazer on Caribbean coral reefs as it removes algae from rocks and rubble to make space for coral larvae to settle on. Coral can be pretty picky on where it settles so paving the way for them is one of the most important functions of D. antillarum. This grazing activity also stops the macroalgae from growing too large and causing the suffocation of already growing corals. I like to think of these sea urchins as the Cows of the Caribbean! Sure, there are other species of both urchins and fish that graze on algae, but by far the most biomass of algae is removed by D. antillarum. That is, when they’re there... In the early 1980’s the D. antillarum populations across the Caribbean underwent a mass mortality event, where some populations were reduced up to 99%. It is widely believed that a species specific, waterborne pathogen was the culprit that sent its evil spawn around the Caribbean on ocean currents and ballast water of ships to make sure the overall mortality of the long spined sea urchin reached ca. 98%. There were some potential suspects, but the true identity of the pathogen was never confirmed. And presently, the populations of D. antillarum are struggling to get back to pre-mass mortality numbers. There are many proposed reasons to why the populations are not bouncing back as expected, including hypotheses suggesting the populations crashed to such low numbers that the way these urchins reproduce is no longer effective. We are focusing on another view, whereby the spawning is successful, but for some reason or another, the juveniles have high mortality rates. 

A view from above with a few individuals of D. antillarum

So back to the project then! We began our project by wanting to do a little pilot study to determine whether or not we should acclimatise our urchins in the new higher temperatures for 6h before trials, or keep them in ambient temperature until doing the testing. We are fortunate that we have the new lab I mentioned in my last post, but I was a little hasty in saying that it was truly functional... We got the seawater hose working and in our excitement we went and collected 10 sea urchins for our pilot study with the aim of acclimatising 5 in elevated temperature and keeping 5 in the ambient water temperature. We filled out the tanks and put in heaters and filters in advance so that the tanks would be ready by the time we brought in the urchins. Me and Max were jumping around like excited little girls, high fiving as we put the urchins in their tanks and went off to have lunch, satisfied with our set up. Fast forward a couple of hours and it all started to go downhill... The urchins we set up in the heated tank had gone from creepy crawlies wiggling their spines to lethargic limpets barely reacting to touch. We thought this had to be the heat, so we moved them to a tank of ambient water and were happy to see them perk up a little. However, despite changing the urchins to non-heated tanks their activity seemed to be going down by the hour. We had no way of bringing them back to the reef that late at night, meaning they would just have to wait until the first boat going out in the morning. Unfortunately, when we got to the lab the next day, we were greeted with a smell of death... All 10 of our urchins had kicked the bucket, as well as one of the lionfish that was in one of the other tanks for another project. Even the urchins that were never heated were dead! Even though we were devastated, these urchins thankfully did not die in vain; there is a girl coming to do genetics studies on these urchins and now she doesn’t have to battle her conscience in order to kill them – we accidentally did it for her.

Natural reef material with low complexity.

The fact that we lost so many urchins in the first place puzzled Max most of all as he has never in previous years had a fatality. The urchins are pretty hardy critters, so the only explanation we came to was that there had to be something in the water. Therefore we set out to do another test, this time to see if the water was to blame. We got one urchin for water from the reef (brought with a jerry canister), one for water carried from the beach, and one unfortunate individual for water from the hose. We monitored the urchins pretty much on an hourly basis, and sure enough after a few hours the urchin in the hose water, which at this point was already called Antal’s toxic water, was deteriorating. To try and avoid any more fatalities we decided the urchin was not doing well enough 

Natural reef material with high complexity: LEGO.

and moved it to a tank with “good water”, hoping it would make the night to be brought back to the reef in the morning. This time they made it. All three were brought back to the reef and we had another answer: we could use the water from the beach which was good news, in a way. It just means that we will be really fit by the end of this project as each canister is about 25 litres, and we need a minimum of 17 canisters per day... that means hauling approximately 425 kg of water each day we do trials, which will probably be 6 days a week.......... Just the thought of it makes me tired! At this point we needed to test again for the acclimatisation. We did it with just 3 urchins per temperature this time, and with water collected from the shore. We stopped the acclimatisation half way through as the heated urchins started to deteriorate and decided acclimatising them to the medium and extreme temperatures would not be feasible as they would most likely die as a result. While the Banco Capiro reef system where these urchins were from hosts one of the highest post-mass mortality population densities in deep water, we couldn’t afford killing any more of them!

On a happier note - moving away from the mass mortality event of 2016 - we have now finally got up and running with the actual trials! On top of testing the effects of water temperature on the predator avoidance response (PAR) of the urchins, we have some reef material in the tanks, both artificial in the form of breeze blocks, and natural rubble from the reef itself. We are also trying to determine if the complexity of the reef system has anything to do with the extent of PAR in the urchins, so we will have trials with low complexity and high complexity for both reef materials. This means that for high complexity we will provide the urchins with a hiding hole; easily achieved with a breeze block, more like playing with Lego’s for the natural reef

Trial in progress with high complexity artificial reef.

material. We also managed to combine my design with the one from Andrew, another dissertation student coming in tomorrow. He is testing for differences in PAR between having no complexity (so no reef material in the tanks) and having any kind of complexity, in the different temperatures. This added another 3 “treatments” to the design - that is combinations of the different factors we are testing for - giving us a total of 15 different combinations to test! Fun times ahead hehe.. We measure the PAR as a percentage of long spines “wiggled” in response to a simulated predator presence. Our “predator” is a big flat scary board that casts a shadow on the whole tank when placed on top. These sea urchins have very primitive sensory systems, but have evolved to be something that can only be described as one big spiny eyeball. While their “eyesight” is nothing like ours, their entire body is able to detect changes in light intensity, and as bottom dwellers, anything casting a shadow is a potential predator. Wiggling their long spines (some up to 30cm long with toxins!) is a way of shoo’ing away anything that might want to take a bite! These wiggles are initiated by the shadow, as each spine is moved by muscles which contract when the light intensity decreases. So far we have done 9 trials, and I am to set up the lab and have it ready for some more for Wednesday evening. Max has gone off to welcome new volunteers in the city of San Pedro Sula and will be back late Wednesday evening. However, the plan is to continue the trials, with or without Max here, so Andrew, you’re in for some hard science on arrival!! 

Me and Manny

Helen and Max

But it is not all just hard work! We managed to sneak out for a kayak around the mangrove lagoon behind our house. After double-, triple- and quadruple checking there were no croc's in this particular lagoon (at least no one's ever seen one...) I really enjoyed the physical effort of kayaking. Seeing the crabs scurrying up the mangrove roots was weird and the smell of the anoxic water was atrocious, but the experience was a new one. Again. 

The lagoon behind our house...