Category Archives: Marine Invertebrates

Collector urchins and their collections

According to my marine invertebrates book, collector urchins (Tripneustes gratilla) gather algae, shells, and other material on their spines. Why they do this is not entirely clear. It might be for disguise, protection from the light, or even to store food. These urchins are quite common in the shallows and their adornments are many and varied.

Posted in response to Becky’s October Squares challenge theme of ‘Past Squares – Spiky.’ See more responses here.

Watching an octopus

A day octopus in the waters off Hawaii

The easiest way to spot an octopus is to see it swimming (top photo). They’re not large creatures but they’re quite distinctive when they swim.

If they’re not swimming, one thing to look for is certain fish, such as goatfishes and jacks, just hanging around in a spot for no apparent reason. When these fish are hunting alone, they tend to be more active in probing the rocks and trying to disturb prey. But when they’re hunting with an octopus, they seem more content to let the octopus do the work and snapping up whatever emerges. I’ve found that goatfishes are particularly helpful as an octopus indicator.

A while back, there were videos online of an octopus apparently punching a goatfish. I wasn’t surprised by this. The octopuses I’ve seen don’t seem best pleased by the presence of goatfishes. Part of this might be down to feeling that the goatfishes are not pulling their weight in the hunt. But another factor might be that if goatfishes give away their position, for the octopus that can be fatal.

Octopus is a popular food in Hawaii and has long been so. If I’ve learned to look for goatfish as an indicator of their presence, then no doubt spear fishermen have too. These days, if I see an octopus when anyone’s spear fishing nearby, I don’t do anything to draw attention to it.

A day octopus in the waters off Hawaii

Signs: Beware of spiny urchins

The beach in front of Four Seasons Resort Hualalai
A sign at the beach in front of Four Seasons Resort Hualalai

I was amused by this sign on the beach in front of the Four Seasons Resort Hualalai. Spiny urchins in the rocks are like white lines on the highway. But of course, a visitor is not necessarily aware of this and the Four Seasons wouldn’t want any of its guests impaling themselves on spiky marine life, especially if they were likely to complain about it later.

This sign wasn’t far from another one that basically said you’d die if you set foot on the beach (here). Perhaps this was why the very lovely beach was deserted when I was there.

Morning swim

A spotted eagle ray swims in the waters off Hawaii
Spotted eagle ray swimming
A spotted eagle ray hunts in the waters off Hawaii
Spotted eagle ray hunting

On a morning swim with my wife a couple of days ago, we were lucky enough to see a spotted eagle ray cruising around looking for breakfast. It stopped often, to probe the sand and rocks for food, and was successful at least once, since it emerged from its efforts chewing and swallowing. This eagle ray looked a bit battered, with damage to its tail fins and a chunk missing from its right wing, but it didn’t seem to be affected by this at all.

As we continued swimming, I saw the ray heading the same way. For a while it followed us, got ahead, then we followed it. On the way we saw a couple of flowery flounders, a couple of day octopuses, a crowned jellyfish as roughed up as the ray, and an oriental flying gurnard. It’s not a great photo of that, but it’s the first one I’ve seen here.

Near the spot where we planned to turn around and head back, I passed over a hole in the rocks and, glancing down, saw the distinctive shape and colors of a green turtle. I think it must have chosen this spot to take a rest, but my appearance startled it and it clambered out of the hole and swam away.

  • A Green turtle resting in a hole in the waters off Hawaii
  • A Green turtle emerges from a hole in the waters off Hawaii
  • A Green turtle swims in the waters off Hawaii

Shortly after that, the turtle encountered the eagle ray. The two of them crossed paths a couple of times before going their separate ways.

A Green turtle and spotted Eagle Ray in the waters off Hawaii

Cushion stars

A cushion star in the waters off Hawaii
A red cushion star in the waters off Hawaii

Cushion stars (Culcita novaeguineae) look just like their name implies – small but comfy-looking cushions that would not look out of place on your couch. However, these cushions would move around on their little tube feet, which might be a bit disconcerting.

They come in several colors including red, yellow, and tan and there’s often quite a bit of variety in their markings. Cushion stars feed on living coral, which they eat by pushing their stomach out and consuming the coral where it sits.

Seven-eleven crab

A Seven-eleven crab off Hawaii

I’ve only seen one or two seven-eleven crabs (Carpilius maculatus) before, and not for several years. My wife had never seen one. One reason for this is that the crabs are active at night. But we saw this one a few days ago, scuttling over the rocks and sand. It was quite early in the morning, so the crab was probably headed for its home.

These crabs are quite large and have the distinctive markings that give it its name. There are the two spots alongside each eye and three across the center of the shell. Four more spots are less obvious on the back edge of the shell.

In Hawaii, these crabs are called ‘alakuma. Legend has it that a god caught an ‘alakuma, but the crab struck back and drew blood. This happened a second time, before the god finally snagged the crab. But the crab’s descendants continue to bear the markings of the god’s bloody fingerprints.