Category Archives: Marine Invertebrates

Tahitian prawns

A Tahitian prawn in a stream in Hawaii
A Tahitian prawn in a stream in Hawaii

Tahitian prawns were introduced to Hawaii back in the 1950s and are now found on all the islands thanks, in part, to the fact that their life cycle includes a stage in the open ocean. The prawns have become a popular food here though, as with most introduced species, there’s a downside. They prey on native species in the streams they inhabit.

One such stream runs through Hawai’i Tropical Bioreserve & Garden, which is where I saw these Tahitian prawns. This stream is also one of several on the island where the prawns have been virtually wiped out on occasion. That’s because, while it’s legal to catch these prawns, it’s not legal to do so by dumping insecticide in the stream. Not only does this kill all the prawns, but it wipes out pretty much everything else that’s alive in there. And then there’s the small matter that these prawns are harvested for human consumption. Would you like insecticide with that?

Catching people in the act has proved difficult, but last year authorities did apprehend one man who had killed 6,200 prawns using this method. Earlier this year, he was fined $633,840 for his actions and the hope is that the big fine will discourage others. Mind you, the perpetrator looked like someone who would have trouble raising $840, let alone the other $633,000.

Signs: Jellyfish present

A sign warning about Jellyfish at Spencer Beach Park, Hawaii

This is the first time I’ve seen signs like this one at Spencer Beach Park. I didn’t see any jellyfish on the beach so perhaps the signs were a warning for those getting in the water. There are often jellyfish in the water, but not in such numbers as to be a problem.

This park is popular with families with small kids so perhaps the authorities were being extra cautious with the signs.

Abstracts: Rock-boring urchin art

Channels made by rock-boring urchins in Hawaii

Yesterday I posted a photo of a crown-of-thorns star, with a few rock-boring urchins in it. Today’s post shows one example of the channels left by these urchins. This image initially looked like a brain to me, but then I began to see more of a tree in it.

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

Crown-of-Thorns star

A crown-of-thorns star and rock borer urchins

I saw this crown-of-thorns star working its way across the rocks in shallow water. Those spiky spines are venomous so this is not something to handle. Also visible in this photo are some equally spiky, though not poisonous, rock-boring urchins

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

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