And then in 2018, only in 2018, the
second species Hoilungia was discovered. 51 00:04:04.549 --> 00:04:08.939 Another big step in the 52 00:04:09.339 --> 00:04:12.975 biological description of Placozoans was 53 00:04:13.175 --> 00:04:15.487 the publication of the genome. 54 00:04:15.987 --> 00:04:22.007 In 2008, we published the Trichoplax adhaerens genome and we found that 55 00:04:22.307 --> 00:04:24.650 this genome represents 56 00:04:25.150 --> 00:04:30.708 all gene families from higher animals in a simplified version. 57 00:04:31.008 --> 00:04:37.340 So there's fewer genes, and there's less variation between the genes, 58 00:04:37.540 --> 00:04:43.049 just a more compact genome, but all the major gene families people are interested in 59 00:04:43.349 --> 00:04:47.454 from higher animals are already present in Placozoans, and what that means for example 60 00:04:47.754 --> 00:04:50.102 for cancer research, we're going to see later. 61 00:04:50.393 --> 00:04:53.367 Okay, I'm here to show you the food source for placozoans now. 62 00:04:53.667 --> 00:04:57.760 To find the food of placozoans or even placozoans, all we do 63 00:04:58.440 --> 00:05:04.206 we just grab a rock from the water and look if it's dirty. This one is kind of dirty 64 00:05:04.506 --> 00:05:09.519 and it has a biofilm on it, so this means chances are there's placozoans feeding 65 00:05:09.819 --> 00:05:11.710 on the biofilm on this rock. 66 00:05:12.340 --> 00:05:17.179 Any rock will work, I mean independent of the colour as long as there's some dirt 67 00:05:17.479 --> 00:05:21.984 some biofilm on it, we have to look for. The other way to do it or collect placozoans 68 00:05:22.184 --> 00:05:27.712 is we come here with microscopy glass slides, place the slides somewhere here 69 00:05:28.012 --> 00:05:32.877 in the water, wait a few days, come back, pull them in and hope there will be 70 00:05:33.177 --> 00:05:35.177 placozoans in the biofilm on it. 71 00:05:35.677 --> 00:05:42.434 The depths we collect can be anything from here 10cm to 10 metres. 72 00:05:42.734 --> 00:05:46.934 The deepest depth we found placozoans yet was 50 metres, but that doesn't mean 73 00:05:47.234 --> 00:05:50.171 they don't occur deeper as long as there's 74 00:05:50.471 --> 00:05:54.228 algae biofilm growing, you just have to look for them. 75 00:05:55.297 --> 00:05:59.697 How do placozoans feed? I mean, I just told you they don't have any organs 76 00:05:59.976 --> 00:06:05.525 to do this, but they must feed, and they do it by extracorporeal feeding cavities. 77 00:06:05.725 --> 00:06:11.037 What is that? Well they are sitting on this substrate like a flat hairy plate 78 00:06:11.337 --> 00:06:14.699 and now they need a feeding cavity. How do you form a feeding cavity? 79 00:06:14.930 --> 00:06:19.312 Very simple! You just lift up your body in the center like this and now you have 80 00:06:19.712 --> 00:06:25.282 I'll let you look into it - an external feeding cavity. All you have to do now 81 00:06:25.482 --> 00:06:31.335 is secrete the digestive enzymes into the feeding cavity, digest everything in this 82 00:06:31.535 --> 00:06:36.938 cavity and then just suck it back in, and that's the feeding they do, 83 00:06:37.538 --> 00:06:44.474 and everything that is below the body will be digested, so the biofilm will be digested 84 00:06:45.330 --> 00:06:51.265 could be protists, it can be algae, whatever is there and will be used as food. 85 00:06:52.465 --> 00:06:56.875 So this video just shows you the extra -corporeal digestion in some detail 86 00:06:57.175 --> 00:06:58.675 in a video clip. 87 00:06:58.960 --> 00:07:03.142 And basically what the animal does, it crawls over the substrate 88 00:07:03.442 --> 00:07:08.801 hoping to find a place where there's lots of biofilm to digest and it stops here 89 00:07:09.101 --> 00:07:13.807 and does what I just explained to you before it forms this extracorporeal cavity 90 00:07:14.107 --> 00:07:18.676 by lifting up the center of the body and you see the result of digestion here in red 91 00:07:18.976 --> 00:07:22.305 by the algae releasing red particles. 92 00:07:22.605 --> 00:07:27.164 Okay, we just saw what placozoans or Trichoplax are feeding on. 93 00:07:27.464 --> 00:07:29.149 They feed on biofilm. 94 00:07:29.449 --> 00:07:32.500 But now the question is who feeds on Trichoplax or placozoans? 95 00:07:32.800 --> 00:07:37.465 Well, usually no other animal would eat placozoans because they have shiny spheres 96 00:07:37.665 --> 00:07:43.930 which prevents others to eat placozoans, but there's at least one gastropod known 97 00:07:44.230 --> 00:07:47.233 Rhodope placozophagus 98 00:07:47.780 --> 00:07:53.652 and this guy seems to only feed on placozoans. 99 00:07:53.952 --> 00:07:56.767 So we have seen that placozoans are very simple animals. 100 00:07:57.067 --> 00:07:59.583 They have no nervous system, no organs, no nothing. 101 00:07:59.836 --> 00:08:04.561 But still, we know that they can smell. We know that they can see, and 102 00:08:04.861 --> 00:08:09.315 we know they can show coordinated behaviour in response to light. 103 00:08:09.615 --> 00:08:15.347 And on top of it, we know they can sense gravity. But what that means, 104 00:08:15.647 --> 00:08:17.830 we're going to come back to later. 105 00:08:18.320 --> 00:08:22.005 So how do placozoans reproduce? Well the most basic, most simple way to 106 00:08:22.305 --> 00:08:24.920 reproduce is by binary fission. That means one animal 107 00:08:25.530 --> 00:08:32.459 cuts its body in half, divides into two, one makes two. This is binary fission. 108 00:08:32.659 --> 00:08:36.670 And it's called vegetative reproduction, the standard mode. 109 00:08:36.970 --> 00:08:40.944 There's another mode not that often seen in the lab, that you have a mother animal 110 00:08:41.244 --> 00:08:45.433 and this mother animal forms lots of little daughter animals, swarmers, 111 00:08:45.933 --> 00:08:50.655 and these swarmers get released into the environment, and the mother stays. 112 00:08:50.855 --> 00:08:55.331 By evolutionary theory, we should not only have vegetative reproduction in the animal 113 00:08:55.631 --> 00:08:59.145 We should ought to have sexual repro -duction, and we know that placozoans 114 00:08:59.345 --> 00:09:00.770 do have sexual reproduction. 115 00:09:01.190 --> 00:09:06.651 And from genetic evidence, we know that two animals, different haplotypes 116 00:09:06.951 --> 00:09:11.659 different genomes get together and perform some kind of sexual reproduction. 117 00:09:11.959 --> 00:09:17.846 Unfortunately, in the lab we cannot follow it to the end because the larval stages 118 00:09:18.146 --> 00:09:24.632 or the first embryonic stages they die after the 128 cell stage. 119 00:09:24.932 --> 00:09:29.357 So we never see a real larva, we never complete the sexual life cycle, we just 120 00:09:29.557 --> 00:09:35.841 know from field observations or from the genetics of field samplings that 121 00:09:36.041 --> 00:09:37.997 there must be sexual reproduction. 122 00:09:38.263 --> 00:09:42.159 If we want to work with placozoans as a model system, if we were to do any work 123 00:09:42.459 --> 00:09:45.905 with placozoans in the lab, well we have to be able to culture them 124 00:09:46.305 --> 00:09:51.305 So we culture them in the lab because they easily reproduce by 125 00:09:51.605 --> 00:09:56.707 vegetative reproduction, by binary fission, as we just have seen, and the conditions 126 00:09:57.007 --> 00:09:59.710 are quite simple: just take normal 127 00:10:00.110 --> 00:10:04.616 water from the environment normal sea water and they will grow. Just add some 128 00:10:04.816 --> 00:10:11.708 algae as a food source, well it has to be algae that settle on the substrate 129 00:10:12.008 --> 00:10:16.470 that settle to the floor of the aquarium on rocks or slides. 130 00:10:17.230 --> 00:10:21.683 And usually if the conditions are fine, normal temperatures anywhere between 131 00:10:21.983 --> 00:10:28.936 18 and 25 degrees, normal salinities between 30 and 35/mL will just do the job. 132 00:10:29.379 --> 00:10:34.454 So in this video we have just learned some basic biology about placozoans, 133 00:10:34.654 --> 00:10:40.114 which is quite exciting, but even more exciting is to use this knowledge and go on 134 00:10:40.414 --> 00:10:45.166 and ask questions about the evolution of placozoans and about whether placozoans 135 00:10:45.466 --> 00:10:47.473 might be the mother of all other animals, 136 00:10:47.710 --> 00:10:51.971 or whether placozoans might be able to help us fight cancer, so very exciting 137 00:10:52.271 --> 00:10:53.550 stuff coming up.