The Rhombus in Nature

Fig. 1

In geometry class back in secondary school, there was one happy day, at least—the day we talked about the rhombus. The rhombus is a parallelogram in which the angles of the opposite sides are equal: diagonals drawn through the center of these angles will intersect each other at right angles in the center of the rhombus (see fig. 1). It is a beautiful shape with a stylish name that everyone started saying in amused wonder. Meanwhile, off the coast of Cuba or Tenerife or Okinawa, divers sometimes chance upon a mysterious human-sized blob of diaphanous pink gelatin composed of delicate loops of exquisite pink spheres.  What is the connection between these disparate stories?

Mysterious pink blob in the ocean (photo by David Fleetham)

The pink alien blobs floating in the tropical and semi tropical seas of the world are the work of Thysanoteuthis rhombus, a.k.a. the diamond squid (which completely sounds like a crime boss name). The species is actually quite large for an invertebrate and some individuals can grow up to a meter (3 feet) in length and mass up to 30 kilograms (66 pounds). Thysanoteuthis rhombus is named for its huge fins which run along the entirety of its mantle and give it the appearance of a rhombus. If you draw diagonals through the center of its angles they would probably intersect at right angles too (although you shouldn’t do this in the real world since T. rhombus is a tremendous swimmer with ten strong tentacle arms–including two extra-long club arms covered with extra-rows of tentacles for grabbing prey or fighting).

The Diamond Squid (Thysanoteuthis rhombus)

Diamondback squid jet through the warm parts of the oceans in pairs and tiny schools hunting for swift and intelligent fish. They in turn are hunted by some of nature’s most fearsome predators: cetaceans, sharks, and the Japanese.

The squid hunt near the surface at night, and retreat to middle depths during the day. Somewhat uncharacteristically, they have no bioluminescence. The large enigmatic pink blobs I mentioned are their eggs. Once the female is fertilized, she lays a vast helix of eggs which are embedded in a stickly translucent line. These egg clusters look like salps or siphonophores (or extraterrestrials) but they are actually thousands of diamond squid eggs. When they hatch, they become adorable larval squid which head off into the phytoplankton to hunt.

Closeup of the eggs of Thysanoteuthis rhombus

Hatchling Diamond Squid

A Mystery Nudibranch from a Mile Down

Biologists estimate that there are approximately 8.8 million species of eukaryotes (animals with complex cell structure) currently alive on Earth.  So far, humankind has only cataloged 1.9 million species and entire biomes remain largely unknown to us.

Unknown Order of Nudibranch Sea Slug swimming in the depths off Monterey (Image Credit: NOAA/Monterey Bay Aquarium Research Institute)

To illustrate this point, here is a photograph of a completely unknown genus of nudibranch mollusk photographed 1 mile beneath the surface of the ocean near Davidson seamount (which is an extinct underwater volcano just off the coast of Monterey).  I wish I could tell you more about the strange mollusk, but this photograph, taken from a robotic deep sea submersible in 2002 is pretty much all that humankind knows about this species.  The mission photographed a huge number of other gelatinous creatures in the middle depths of the ocean, and in fact caused scientists to rethink the importance of such animals in the oceanic ecosystem. The Monterey Bay Aquarium Research Institute (MBARI) worked on the mission with the National Oceanic and Atmospheric Administration. Their website explains the robotic study by paraphrasing Bruce Robinson, an ecologist who pioneered the use of robot submersibles:

One of the most important discoveries has been the realization that gelatinous animals are important as grazers and predators that comprise a large percentage of the open ocean animal biomass. Robison estimates that gelatinous animals make up about 40 percent of the biomass in the deep sea water column.

Nudibranch mollusks are largely thought of as colorful predators of the tropical reef, so it is a big deal if they (together with other floating mollusks, cnidarians, and siphonophores) constitute such a substantial percentage of the biomass of the largest portion of the ocean.  As an unscientific postscript I think the delicate translucent nudibranch is very beautiful with its alien and ghostlike (and, yes, gelatinous) features.

The same mollusk (Image Credit: NOAA/Monterey Bay Aquarium Research Institute)

Sea Angels

Clione or “sea angels” swimming in a Tokyo aquarium (Photo by REUTERS/Kimimasa Mayama)

In terms of taxonomical diversity the gastropods are second most diverse class of animals on Earth (outnumbered only by the teeming class Insecta of the other great invertebrate phylum Arthropoda).  This means that there are some deeply strange arthropods out there. While we traditionally think of gastropods as snails and slugs there are odd subcategories of these creatures, like the subject of today’s post, sea angels (of the clade Gymnosomata).

A “Sea Angel” (Clione limacina)

Sea angels consist of six different families of pelagic marine opisthobranch gastropod molluscs.  Gastropods are named for their famous foot (the name means “stomach-foot”–a misnomer since gastropods all have true stomachs elsewhere) however the name is even more inappropriate for sea angels.  In these free-smimming predators, the gastropod foot, so familiar to us as seen on snails, has evolved into a pair of delicate wings for swimming through the water. Sea angels are very small: the largest species only reach 5 cm (2 inches) in length and most varieties are much more miniscule.  They prey on other tiny creatures swimming among the plankton—particularly other smaller slower species of gelatinous mollusks.

A hunting sea angel (photo by Alexander Semenov)

Adult sea angels lack any sort of shell—which they discard when they metamorphose into adulthood.  Their feeding apparatuses can be strangely complicated—pseudoarms and tentacles which recall their cousins the cephalopods. Sea angels are numerous in the oceans but some scientists are concerned that the acidification of the world’s oceans will cause substantial problems for the tiny translucent gastropods.

Sea Angel (Platybrachium antarcticum)