Brittle Stars and Fireworms

What Are They?

    Of the class Polychaeta (phylum Annelida), including the species Hermodice and Eurythoe. Fireworms sting if touched. The body of H. carunculata,, is covered with fine, white, bristles that break if touched; they become imbedded in human skin and produce a substance that is irritating.  The bearded fireworm is usually found on reefs, under stones in rocky areas of the sea, and on some mud bottoms.  It is encountered throughout the tropical western Atlantic and pacific. It can be found near ocean reefs and at most 150m deep.  The fireworm primarily eats detritus but also can eat coral and anemones.
Brittle Stars

    Two thousand living species of marine invertebrates comprise the class Ophiuroidea (phylum Echinodermata). They have five thin and often forked and spiny arms and radiate off the the small disk-shaped body.  They can easily "drop" and arm as a defense mechanism.  Brittle stars are mainly scavengers and plankton feeders, but sometimes trap sizable animals.  Brittle stars are common members of reef communities, where they hide under rocks and even within other living organisms.

What were we testing?
The relationship between fireworms and brittle stars was tested.  Brittle stars and fireworms tend to be found together more than apart, we wanted to answer whether this was coincidental or not.  We also wanted to look at substrate and rock type so we could conjecture as to the reason they are found together.
                            It is not by coincidence that brittle stars and fireworm tend to occur together.
                        Null Hypothesis
                            Brittle stars and fireworms only occur together randomly and by coincidence.
To perform this study 51 rocks were turned over within a 5 meter lane parallel to the ocean (every rock in the transect that could be flipped over was flipped over). A straight line was walked in zone three of the inter tidal zone.  The zone was determined by finding the correct indicator species.  The rock type (basalt/coquina), number of brittle stars, number of  fireworms and substrate type (rocky/sandy) were recorded.  This was performed at Station Beach, Puerto Penasco, Mexico on September 9th, 2007 from about 9 PM to 10 PM.   The weather was about 75-85 degrees fahrenheit with little or no wind.  The only material used was a tape measure, which indicated the 5 meter path used.
Below are the final chi squared values, these chi squared values show that our results are significant and that the occurance of fireworms and brittle stars together are not coincidental.

BS Present BS not Present
FW Present 20 4 24
FW Not Present 8 19 27

28 23 51

Expected BS Expected BS no Expected
FW Expected 13.1764706 10.8235294 24
FW Not Expected 14.8235294 12.1764706 27

28 23 51

Chi Squared

3.53361345 4.30179028 7.83540373

3.14098973 3.82381358 6.96480331

x^2= 14.800207

p= 0.0019

df= 3
Rock Number B/C Fireworm Brittle Star Substrate
1 b 0 0 rocky
2 b 1 1 sandy
3 b 0 2 sandy
4 b 1 2 sandy
5 b 0 5 sandy
6 b 0 1 sandy
7 b 1 5 sandy
8 b 2 1 sandy
9 b 1 2 sandy
10 b 1 1 sandy
11 b 2 1 sandy
12 b 6 1 sandy
13 b 0 0 sandy
14 b 0 0 rocky
15 b 1 0 rocky
16 b 1 4 sandy
17 b 0 2 rocky
18 b 0 1 rocky
19 b 0 0 rocky
20 b 0 1 rocky
21 b 0 0 rocky
22 b 3 3 rocky
23 b 2 1 rocky
24 b 3 0 rocky
25 b 1 2 rocky
26 b 0 0 rocky
27 b 1 4 sandy
28 b 3 2 sandy
29 b 0 0 rocky
30 b 2 1 rocky
31 b 0 0 sandy
32 b 0 0 rocky
33 b 2 0 rocky
34 b 2 2 sandy
35 b 1 2 sandy
36 b 2 2 sandy
37 b 0 1 sandy
38 b 0 0 rocky
39 b 0 0 rocky
40 b 1 1 sandy
41 b 2 6 rocky
42 b 0 0 sandy
43 b 0 0 rocky
44 b 0 0 rocky
45 b 0 0 rocky
46 b 0 0 sandy
47 b 1 0 rocky
48 b 0 3 sandy
49 b 0 0 rocky
50 b 0 0 rocky
51 b 0 0 sandy
Above is the raw data of the 51 rocks turned over for the experiment, this data includes number of fireworms and brittle stars, rock type and substrate type.


    The focus of this experiment was to see if brittle stars and fire worms would be found under the same rocks in Rocky Point Mexico. During the course of this experiment 50 basalt rocks were overturned in order to examine the marine life underneath. This data is recorded in the table above, showing if brittle stars and/or fire worms were found and the number of each. Also, the substrate conditions (sandy or rocky) and rock type (basalt or coquina) were observed and recorded, regardless if brittle stars or fire worms were found. A series of chi squared tests were done in order to determine the significance of the findings. The first test that was done was to establish the significance of the association between brittle stars and fire worms. The numbers of brittle stars present and fire worms present together with when they were not present gave a p value of 0.0019.  The data suggests that there is a significant association between the fire worm and brittle star species in Rocky Point Mexico.
    Many ophiuroids are scavengers or detritivores that may also prey on small crustaceans or worms, and may be capable of suspension feeding, using the mucus coating on their arms to trap plankton and bacteria. Fire worms are omnivorous and will scavenge both dead plants and animals. They use their mouth parts to scrape and squeeze bits of food into their mouths. The association could be that their food sources are similar and can be found in similar places. The fire worms and brittle stars are found significantly more in the sandy substrate.  Which could indicate they both prefer the same substrate.
    This topic needs more study, an extensive search of academic journals revealed no studies on brittle stars and fireworms together.  A study on the possible mutalisitic relationship could shed light on why these two animals are found together. 
Perhaps it would reveal a true mutualism or even just a habitat/substrate preference of both animals.

*Caption for graph: This pie graph represents the occurances of brittle stars, fireworms or both found under the 51 rocks turned over.  This shows that a majority of rocks turned over had both fireworms and brittle stars. 

"fireworm." Encyclopædia Britannica . 2007. Encyclopædia Britannica Online. 2 Dec. 2007  <>.
"brittle star." Encyclopædia Britannica . 2007. Encyclopædia Britannica Online. 2 Dec. 2007  <>.

Web Page Created by Alex Tuttle,, for Ecology 450: Marine Discovery.