<div class="fish">
  <div class="head"></div>
  <div class="notHead">
    <div class="torso"></div>
    <div class="torso"></div>
    <div class="torso"></div>
    <div class="torso"></div>
    <div class="torso"></div>
    <div class="torso"></div>
    <div class="torso">
      <div class="pelvic"></div>
    </div>
    <div class="tail">
      <div>
        <div>
          <div>
            <div>
              <div>
                <div>
                  <div>
                    <div>
                      <div class="dorsal">
                        <div class="upper"></div>
                        <div class="lower"></div>
                      </div>
                    </div>
                  </div>
                </div>
              </div>
            </div>
          </div>
        </div>
      </div>
    </div>
  </div>
</div>

body {
  margin: 0;
  box-sizing: border-box;
  font-size: .6vh;
  display: grid;
  height: 100vh;
  background-image: radial-gradient(at 0 0,#330,#001818);
  background-size: 100vw 100vh;
  perspective: 150em;
  overflow: hidden;
}

body * {
  transform-style: preserve-3d;
}

.fish {
  place-self: center;
  display: flex;
  margin-right: 105em;
  animation: swim 5s linear infinite;
  position: absolute;
}

@keyframes swim {
  0% {
    translate: 100vw
  }
  100% {
    translate: -100vw;
  }
}

.head, .torso, .tail {
  position: relative;
}

.pelvic, :is(.tail, .dorsal) div, :where(.head, .torso, .tail, .tail div)::after, :where(.head, .torso, .tail, .tail div:not(.dorsal))::before {
  position: absolute;
}

.head {
  width: 69em;
  height: 69em;
  background-image: conic-gradient(from -105deg at 20% 93%,#000, #0000 15deg), radial-gradient(at 38% 50%, #333 0%, #222 60%, #010101);
  box-shadow: inset 4em -2em 3em #fff2;
  border-radius: 30em 43em 30em 5em;
  transform-origin: 9% 104%;
  scale: 1.33 1;
  transform: skewY(-10deg) rotate(45deg);
  z-index: 1;
  filter: sepia() saturate(3);
}

.head::before {
  content: '';
  inset: 29em 28em;
  position: absolute;
  background-image: radial-gradient(#000 35%, #555 35%);
  box-shadow: 0 0 5em 1em #000, inset 0 0 2em #000;
  border-radius: 50%;
  translate: -20em 10em;
}

.head::after {
  content: '';
  width: 40em;
  height: 15em;
  background-image: repeating-conic-gradient(at 0 100%,#000 .5%, #333 2%);
  border-radius: 100% 0;
  box-shadow: 3em -3em 5em #000;
  top: 61%;
  left: 94%;
}

.notHead {
  display: flex;
}

.torso, .tail {
  width: 10em;
  height: 100em;
  margin-left: -1px;
}

:where(.torso, .tail, .tail div:not(.dorsal, .dorsal div))::before {
  content: '';
  inset: 0;
  background-image: radial-gradient(#0003,#000f 80%), repeating-linear-gradient(57deg, #0006 50%, #000b 50% 53%, #0006 70% 100%), repeating-linear-gradient(123deg, #aaa 50%, #555 50% 53%, #aaa 70% 100%);
  background-size: 320em 100em, 10em 15em, 10em 15em;
  background-position: calc((var(--nth) - 1)*-10em - 120em) 0, 0 0, 0 0;
  clip-path: polygon(0 calc(20%*(1 - sin(19deg*(var(--nth) - 1)))), 100% calc(20%*(1 - sin(19deg*(var(--nth) - 0)))), 100% calc(100% - 5%*(1 - sin(19deg*(var(--nth) - 0)))), 0 calc(100% - 5%*(1 - sin(19deg*(var(--nth) - 1)))));
  filter: sepia() saturate(3);
}

:where(.torso:nth-child(n+6), .tail, .tail div)::after {
  content: '';
  inset: -15% 0 10% 0;
  background-image: linear-gradient(123deg, #000 0%, #111 30% 50%, #000 50%, #111 80% 100%);
  background-size: 10em 15em;
  background-position-x: 4em;
  clip-path: polygon(0 calc(20%*(1 - sin(19deg*(var(--nth) - 1)))), 100% calc(20%*(1 - sin(19deg*(var(--nth) - 0)))), 100% 100%, 0 100%);
  z-index: -1;
  filter: sepia() saturate(3);
}

.tail div {
  inset: 0 -99% 0 99%;
}

.tail, .tail div {
  transform-origin: 0 0;
  animation: wag 1s ease-in-out alternate infinite;
}

@keyframes wag {
  0% {
    rotate: y 3deg;
  }
  100% {
    rotate: y -3deg;
  }
}

.torso:nth-child(1) {
  --nth: 1;
}

.torso:nth-child(2) {
  --nth: 2;
}

.torso:nth-child(3) {
  --nth: 3;
}

.torso:nth-child(4) {
  --nth: 4;
}

.torso:nth-child(5) {
  --nth: 5;
}

.torso:nth-child(6) {
  --nth: 6;
}

.torso:nth-child(7) {
  --nth: 7;
}

.pelvic {
  width: 40em;
  height: 25em;
  top: 99%;
  border-radius: 0 10em 10em 0;
  background-image: repeating-conic-gradient(at 0 0,#000 2%, #222 5%);
  filter: sepia() saturate(3) hue-rotate(-30deg) brightness(.66);
  mask-image: conic-gradient(from 90deg at 0 0, black 40deg, transparent 45deg);
  -webkit-mask-image: conic-gradient(from 90deg at 0 0, black 40deg, transparent 45deg);
  transform-origin: 0 0;
  rotate: -10deg;
}

.tail {
  --nth: 8;
}

.tail div {
  --nth: 9;
}

.tail div div {
  --nth: 10;
}

.tail div div div {
  --nth: 11;
}

.tail div div div div {
  --nth: 12;
}

.tail div div div div div   {
  --nth: 13;
}

.tail div div div div div div  {
  --nth: 14;
}

.tail div div div div div div div  {
  --nth: 15;
}

.tail div div div div div div div div  {
  --nth: 16;
}

.tail div div div div div div:after {
  content: none;
}

.torso:nth-child(6)::after {
  mask-image: conic-gradient(at 100% 0%,#000 215deg, #0000 135deg);
  -webkit-mask-image: conic-gradient(at 100% 0%,#000 215deg, #0000 135deg);
}

.tail > div > div > div > div > div::after {
  mask-image: conic-gradient(from 205deg at 100% 40%,#000 180deg, #0000 180deg);
  -webkit-mask-image: conic-gradient(from 205deg at 100% 40%,#000 180deg, #0000 180deg);
}

.dorsal {
  margin-left: -5em;
  perspective: 150em;
  perspective-origin: 0 63em;
  transform-style: flat;
}

.dorsal div {
  background-image: repeating-conic-gradient(at -50% 0%,#000 .5deg, #111 2deg 4deg);
  width: 50em;
  left: 0;
  aspect-ratio: 2;
  top: 63em;
  border-radius: 0 61% 0 72%;
  transform-origin: 0 100%;
  box-shadow: -1em 0 1em #000;
  filter: sepia() saturate(3) hue-rotate(-30deg) brightness(.66);
  mask-image: linear-gradient(90deg, #000, #000a);
  -webkit-mask-image: linear-gradient(90deg, #000, #000a);
  transform-origin: 0 0;
  scale: 1 2;
}

.dorsal .upper {
  transform: rotateX(137deg);
}

.dorsal .lower {
  transform: rotateX(41deg);
}

@media (orientation: portrait) {

  body {
    font-size: .6vw;
    width: 100vw;
  }

  .fish {
    place-self: auto;
  }

  @keyframes swim {
    0% {
      rotate: 90deg;
      translate: 0 100vh;
    }
    100% {
      rotate: 90deg;
      translate: 0 -150vh;
    }
  }

}

/*

View in Firefox if you can - though animation gets wonky towards the end. In Chrome it runs smoothly, but it doesn't do trigonometry (the curves on the fish's back and belly). I'll redo it in SCSS but not today, I am so tired...

*/