Deep-Sea Brittle Stars Occur in Lateral Bands!!

This week's blog is based on a new paper (here) by Tim O'Hara (Museum Victoria), Ashley Rowden (the New Zealand Institute of Water and Atmosphere) and Nicholas Bax from the Commonwealth Scientiifc and Industrial Research Organisation in Hobart, Australia.

This paper has been making the rounds in the popular media (here) and the blogosphere (and here)...
First, a little bit of background so that everyone can appreciate the story with the same starting point..

Faunas & What is Biogeography?
At some basic level, I think that most people realize that most organisms, including plants and animals, do not occur all around the world at equal levels.

For example- If you are an aquarium hobbyist, you've probably noticed that some fish are "cold" salt whereas others are tropical or "hot" salt. That's because the animals that live at those different temperatures are originally from different specific locations in the world.

You can often find complete "sets" of particular organisms also known as faunas (or floras for plants) specific to these regions.

Thus, the fauna (i.e., the set or assemblage) of animals and plants you find in the cold/temperate water kelp forest (on the left) will be VERY different from those which you find in the tropical-hot water reef habitat on the right.Scientists take this basic definition one step further. There is an active field of study known as biogeography, which seeks to determine the significance of where organisms live.

This includes the evolution/character, structure, and history of how particular groups associated with specific regions came to be distributed in those regions. How do these regions transition into others? What dictates those distributions? These are big questions in biogeography.

Among the scientists who often work in biogeography are specialists called taxonomists who can identify the many different types of organisms present in each specific region. And based on this, they can characterize different biogeographic areas.

So, what does this have to do with the paper again??

Brittle stars are a good model organism for understanding how life is distributed on the sea bottom because they are abundant in the deep-sea and are consistently EVERYWHERE. They live in mud, under rocks, in sponges...some brittle stars even live on EACH other!Because brittle stars are important components of this region..and especially in the deep-sea, their distribution may hold clues to the biogeography, evolution and distribution of all organisms in this area which is ultimately important for marine resource planning and etc.

Enter the considerable talents of Tim O' Hara at the Museum Victoria who is one of the world's foremost experts on brittle stars!
He developed a large data set derived from 295 research expeditions, across an equator to pole sector of the Indian, Pacific and Southern oceans. This literally means identifying thousands of specimens from scientific expeditions, museums, universities and other places where brittle stars are found. The study area covers 1/8 of the globe using data from 24 museums!

Multivariate analyses (i.e., statistical) of this dataset were performed (also by O'Hara and etc.) and they found the following...

1. Brittle star faunas in different depth zones is different.
This includes:

1. A "shelf" zone, from 0-250 meters.
2. A bathyal zone from 250-2000 meters.

(data for below 2000 m apparently was too poorly sampled to yield results) This shows the depth vertically...
This figure (Fig. 1 in their paper) does a much nicer job of showing it on the map... (note the keys to different colors in the lower left hand corner).

(Figure 1 from O'Hara et al., 2011)

Brittle stars from shelf vs. those in bathyal regions are VERY different from one another-Except in the Antarctic where cold water basically forms a homogeneous environment all throughout the shelf-bathyal region.

So far so good. This goes along with what's been known.

Historically, its been thought that the geographical faunas of brittle stars would all be clustered based on ocean basin.. In other words.. different brittle star faunas for different discrete regions in the Indian and Pacific ocean basins. it turns out this was NOT the case...

2. Bathyal (deep-sea) brittle star faunas were observed to occur in latitudinal (i.e., "lateral") BANDS across the Indo-Pacific region!!

(Fig. 2 from O'Hara et al., 2011)

RED for Tropical species, such as this Acanthophiothrix purpurea

(photograph by Julian Finn, Museum Victoria)

GREEN for temperate species, such as this Conocladus australis

(photograph by Julian Finn, Museum Victoria)

and BLUE for POLAR species...such as this Ophiosteira sp.Although the three faunas appear very distinct, they don't show a very clear transition.

The geographical boundaries between the groups was rarely distinct. Many of the species ranges overlap and intergrade (you can see the colors in Fig. 2 blending together in many places).

So, the boundaries were transitional and not sharply, distinct disjunctions between biogeographical regions as was historically assumed.

Unfortunately, there are no clear clues as to what may be the cause of the overall "lateral band" pattern. An evolutionary hypothesis (i.e., a phylogeny) for brittle stars is unavailable and fossil brittle stars of the appropriate age aren't really common..

The authors speculate on possible reasons in this area are distributed in this manner.

1. Water temperature?
2. Primary Productivity? (i.e., food)
3. Oceanographic reasons-current flows, etc. that might affect settlement of the larvae

As much work as this represents, this likely represents only the FIRST step towards understanding the distribution and biogeography of animals such as these in the deep-sea.

Remember also that brittle stars are the MOST diverse group within the Echinodermata, boasting over ~2159 species! How will additional discoveries of fossils?? Or new species? Or developing evolutionary histories?? affect these patterns?

Understanding distributions like this aren't just academic questions, they are also used to help management plans of the ocean for deep-sea mining of oil and gas. Possibly to help develop a network of marine protected areas.

All from the humble brittle star...