THE BIOLOGY OF TOXOPNEUSTES ROSEUS IN RHODOLITH BEDS IN BAJA CALIFORNIA SUR, MEXICO A Thesis Presented to The Faculty of the Department of Biology San Jose State University In Partial Fulfillment of the Requirements for the Degree Master of Science by David Wayland James August 1998
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THE BIOLOGY OF TOXOPNEUSTES ROSEUS IN RHODOLITH BEDS
The diet of T. roseus is also unique as few other urchins specialize on a single
plant type (Kempf 1962; Ogden 1976; Vadas 1977; Larson et al. 1980; Ogden et
al. 1989). Some of the available algae may have been avoided because of
chemical defenses. Caulerpa sertularioides contains caulerpin and complex
terpenoids and is resistant to herbivory (Norris and Fenical 1982; Paul et al.
1987). Sargassum spp. may contain polyphenolics (Norris and Fenical 1982)
but are not avoided by all urchins (Ogden 1976; Shunula and Ndibalema
1986).
Urchins at Pardito, the rocky site without a rhodolith bed, appeared to
also select nongeniculate coralline algae in their diet. While Enteromorpha
intestinalis and Sphacelaria didichotoma made up a large part of their diet,
these algae grew on the coralline algae, and thus may represent incidental
consumption. Hawkins (1981) found that the absorption efficiency of
Diadema antillarurn was higher for nongeniculate coralline algae than
endolithic filamentous and epipelic algae. If this is also true for T. roseus,
then it may have been more advantageous for them to select the coralline
algae at Pardito.
Toxopneustes roseus contributed a large amount of carbonate sediment
as a result of their feeding on rhodoliths. Urchins at El Cardon and Diguet
produced 3.87 and 7.96 g of carbonate/individual/ day. Glynn (1988)
31
determined individuals in rubble covered with nongeniculate coralline
algae and dead pocilloporid coral generated 1.57 g carbonate/individual/ day.
On an individual basis, carbonate production at El Cardon and Diguet was
much higher than values reported for Caribbean urchins and parrotfish,
which are well known sediment producers on tropical reefs. On a daily
individual basis, Diadema antillarum created from 0.63-1.44 g of new
carbonate and the parrotfish Scarus croicensis produced 3.0 g new carbonate
(Ogden 1977; Scoffin et al. 1980). More carbonate was ingested daily but 20-
50% of this was reworked or "old" carbonate (Hunter 1977; Ogden 1977;
Scoffin et al. 1980). Due to higher densities, Diadema antillarum produced
more new carbonate/m2 /year (4.6-5.3 kg), while Sparisoma viride and S.
croicensis produced 0.03 and 0.49 kg new carbonate/m2 /year, respectively.
While Toxopneustes roseus in rhodolith beds did not generate as much
carbonate per area (an estimated 51-74 g carbonate/m2/year), their
contribution is still large over time.
Toxopneustes roseus feeding rates may exceed the growth rates of the
rhodoliths. The growth rates of the rhodoliths Lithothamnion corallinoides
and Phymatolithon calcareum in Ireland were 88 and 249 g
carbonate/m2 /year (Bosence 1980). Lithophyllum incrustans, a nongeniculate
coralline algae in south-west Wales, had a growth rate of 379 g
carbonate/m2 /year (Edyvean and Ford 1987). However, coralline algal growth
rates vary seasonally and are higher in warmer water, such as Baja
California (Adey and McKibbin 1970). Rhodoliths in Baja California may
grow faster than these reported values.
32
Their feeding and movement rates suggest it was unlikely that urchins
ate entire rhodoliths. By eating only branch tips, most of the thallus is left
intact. Rhodolith fragments (2-10 mm) were also dropped during feeding.
Such fragments are likely capable of vegetative regrowth as 1-2 mm pieces of
rhodolith with pigmentation on the entire surface were observed in the field.
Damage to apices of rhodoliths can alter branching patterns (Bosence 1983;
Foster et al. 1997). Branching and growth may therefore be altered by T.
roseus grazing, and the potential effect of urchin herbivory on rhodolith
shape should be considered when interpreting factors that may have shaped
fossil and modern rhodoliths.
Clear bands in urchin jaws were found to be formed during periods of
slow growth caused by food deprivation and opaque bands formed during
rapid growth periods (Pearse and Pearse 1975). The lack of clear bands in the
urchin jaws suggests that a diet of coralline algae fulfills dietary requirements.
Toxopneustes roseus may grow at a constant rate.
The jaws at Pardito were relatively larger than at Diguet, further
suggesting specialization on coralline algae as these plants were less abundant
and urchin density was higher at Pardito. These larger jaw sizes in areas of
less available algae and higher urchin density are consistent with those
previously reported (Ebert 1980; Black et al. 1982; Black et al. 1984; Edwards
and Ebert 1991; Levitan 1991). Larger jaws may also facilitate scraping
coralline algae off rocks.
33
The difference in relative jaw sizes between El Cardon and Diguet may
be related to the method of feeding on nongeniculate coralline algae. Urchins
at El Cardon had more rhodolith bits in their guts while individuals at Diguet
had more scraped pieces of coralline algae. Larger lanterns may have more
strength, and additional strength may be necessary to constantly bite off pieces
of rhodolith. The similarity in relative jaw sizes between El Cardon and
Pardito may also be due to large jaws being necessary at El Cardon.
Although the test diameter and jaw length slopes of El Cardon and
Pardito were not significantly different, the slope for El Cardon suggests that
as individuals at this site get larger, their jaws will be relatively smaller than
at Pardito. This is also indicated by the larger urchins at El Cardon having
smaller lanterns than at Pardito.
Movement and covering material did not appear to be affected by
predation. Predation was never observed at El Cardon, and tests were not
destroyed or transported out of the area for at least 8 months. Evidence of
predation would most likely have been seen if it had occurred. There were
very few potential predators at Diguet (several fish in the family Balistidae)
and only one test was found with a molluscan drill hole. Potential fish
predators were very abundant at Pardito (Scaridae, Balistidae and Labridae),
but individuals with very little covering were commonly exposed on rocks
34
there. Cracked tests were never seen but occasional tests with holes drilled
them were observed. Local fishermen reported that octopus occasionally prey
on T. roseus. The well developed globiferous pedicellaria of T. roseus contain
venom which may deter most predators (Halstead 1988).
Toxopneustes roseus was very mobile. However, while there is an
inverse relationship between food availability and movement for many
urchins (Mattison et al. 1977; Russo 1979; Harrold and Reed 1985; Andrew and
Stocker 1986; Laur et al. 1986), this relationship clearly does not apply forT.
roseus. Urchins were surrounded by food in rhodolith beds, were always on
rhodoliths, and often carrying them. A consequence of their large
movements is that individuals did not remain in one area, which may
prevent all of the rhodoliths in a small area from being severely grazed.
Individuals may occupy different areas of a rhodolith bed over time.
During one observation at El Cardon in March 1997, only one dead and two
live tagged urchins were seen in the study area. It is unlikely that these
missing urchins died in the vicinity as old tagged tests persisted in the area
at least 8 months. The missing individuals probably moved out of the area.
Urchin densities appeared the same as in November 1996 and were most
likely similar due to new urchins moving into the area.
Toxopneustes roseus may move between rhodolith beds. At Requeson,
a rhodolith bed 5 km away from El Cardon, extensive searching by many
divers found that T. roseus was almost entirely absent in l\!Iarch 1996. Adult
35
urchin densities were similar at El Cardon and Requeson by November 1996.
It was unlikely that individuals were buried deep enough to be unnoticeable;
at Requeson, fine, anoxic sediment is present from several centimeters below
the top surface of rhodoliths at the deep margin of the rhodolith bed to 15-20
centimeters below the surface in the middle of the bed (Foster et al. 1997).
Toxopneustes roseus is probably not capable of surviving buried in fine,
anoxic sediment. The nearest rhodolith bed was 0.4 km away, suggesting that
urchins at Requeson moved a great distance on sand to get there. Individuals
were seen moving across large stretches of sand at Manto de James.
The diel movement at Diguet may be related to surge. There was
generally more wind and surge during the day at this site (D. James, pers.
obs.). Surge has been shown to decrease movement (Lees and Carter 1972;
Ogden et al. 1973; Lissner 1980; Tertschnig 1989), and Dance (1987) observed
that movement was negatively correlated with wind speed. The amount of
sand may also influence the timing of movements. Laur et al. (1986) found
that urchins moved slower in sand. When surge was present1 T. roseus was
more prone to being tumbled. Aggregations ofT. roseus occurred at Diguet in
areas of the highest rhodolith densities. These rhodoliths may have provided
protection from surge when individuals were buried in the sand, covered
with rhodoliths.
Surge also influences how much material is carried. Urchins at Diguet
were exposed to the most surge during this study. Rhodoliths at Diguet were
36
significantly larger than rhodoliths in Bahia Concepcion and lateral fusion
branches increased as depth decreased (Foster et al. 1997). Bosence (1983)
found that densely branched, lateral growing rhodoliths occur in high energy
areas. The amount and weight of covering material held by individuals at
Diguet would have increased their total weight. This material may have
helped anchor them on and around the sand, which covered 59.5±3.0
(mean±SE) percent of the bottom around the collected urchins. Covering
material may serve as a stabilizing force, as Lees and Carter (1972) found that
Lytechinus anamesus increased their covering during surge exposure, and
reduced it when surge ended and urchin movement increased. Individuals
at El Cardon and Pardito were exposed to less water motion and had similar
covering material/body weight ratios. Urchins at Pardito held less material
that weighed more than material at El Cardon, which resulted in similar
ratios.
Flat, stable substrate may also reduce covering material. Individuals at
Pardito on rocks in the shallowest water often had the least amount of
covering material, perhaps because there was more rock for the podia to
attach to and secure the urchin.
Toxopneustes roseus caused extensive bioturbation of rhodoliths.
Their feeding activity moved and turned rhodoliths, and plants were also
moved about when urchins picked them up or grabbed nearby ones.
37
Bioturbation resulted when covering material changed seasonally, and
rhodoliths were probably dropped and replaced over shorter time intervals.
Movement activity also resulted in bioturbation. Individuals often
plowed visible trails through the rhodoliths and frequently dug themselves
into the bed, creating pits up to 10 em deep. This activity is similar to that of
T. roseus in Panama which bury themselves in rubble during the day, and
emerge at night, extensively mixing the uppermost 10 em of substrate (Glynn
and Wellington 1983). Movement over rhodoliths no doubt causes
rhodoliths to shift their position, and the extent of movement suggests this
occurs over large areas. Bioturbation by urchins may be more important in
rhodolith beds at greater depths as Steller and Foster (1995) found that
rhodolith movement from water motion declined with increasing depth in a
rhodolith bed in Bahia Concepcion, Mexico.
Bioturbation also affected mats of the green alga Caulerpa
sertularioides. Urchins moved underneath the alga, plowing through the
rhodoliths and attached rhizomes. Caulerpa sertularioides appeared to be
uprooted and individual urchins also removed pieces of the plant for
covering material. Williams et al. (1985) report that the growth rates and
biomass accumulation of C. sertularioides were negatively affected by
bioturbation.
Rhodolith beds have a diverse assemblage infauna, epifauna and
cryptofauna (Steller 1993). While T. roseus may not be critical to bed
38
formation, their bioturbation may help maintain the integrity of the diverse
rhodolith community and the persistence of the beds. These positive effects
may more than offset negative effects from high feeding rates at high
densities. However, feeding impacts appear to be localized due to their large
movements.
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Table 1. Density (mean #±SE/20m2; n=201) of Toxopneustes roseus and algal bottom cover (mean %±SE; n=90) at El Cardon in November 1996. Data are untransformed. Ip = standardized Morisita index of dispersion ( -1 = maximum uniformity; 0 = randomness; + 1 = maximum aggregation; 95% confidence limits above 0.5 and below -0.5). P = probability of being statistically different than a random distribution using Morisita's index of dispersion (Id) and a chi-square distribution. Algal cover data are from the area of the rhodolith bed where most of the urchins occurred (see text).
Deep Middle Shallow
Urchin Density
0.4±0.08 1.0±0.25 0.8±0.20
Ip
0.02 0.52 0.52
p
0.453 <0.001 <0.001
Live Rhodolith
0.22±0.03 0.31±0.04 0.45±0.03
Caulerpa
0.16±0.04 0.28±0.04 0.30±0.04
48
Table 2. Movement rates and distances of Toxopneustes roseus at El Cardon and Diguet (mean±SE). Data are untransformed. Distances are standardized to 24 hours. El Cardon: urchins used= 48; movement data n = 216. Diguet: urchins used = 47; movement data n = 159.
El Cardon Diguet
Day Night 24 Hour Day Night 24 Hour Rate, Rate, Distance, Rate, Rate, Distance, cm/hr cm/hr em cm/hr cm/hr em
Table 3. Covering material and Cover Weight/Body Weight (n=20; mean±SE) of Toxopneustes roseus at El Cardon in November 1996, Diguet, and Pardito.
Percent Cover
Cardon Diguet Pardido
68.1±5.5 92.3±4.9 38.1±4.0
Cover Weight/ Body Weight
Cardon Diguet Pardito
0.18±0.02 0.52±0.03 0.21±0.03
50
Table 4. Covering material of Toxopneustes roseus at El Cardon in November 1996, Diguet and Pardito (%, n=20). Caulerpa =C. sertularioides. At El Cardon, urchin covering material Other = Spyridia filamentosa, Sargassum herporhizum and worm tubes; all were rare; substrate Other = S. filamentosa. At Pardito, Other = dead rhodoliths, bryozoans, urchin test and worm tubes.
El Cardon
Covering Material Substrate Electivity Mean± SE Mean± SE Index, Ei
Live Rhodolith 12.3±2.6 38.5±4.3 -0.52 Dead Rhodolith 8.0±1.3 53.0±4.1 -0.74 Shell 28.5±4.3 4.5±1.7 0.73 Sponge 6.5±1.8 5.0±2.1 0.13 Caulerpa 12.0±2.4 32.0±4.8 -0.46 Other 1.0±0.7 5.5±1.5 -0.71
Diguet
Covering Material Substrate Electivity Mean± SE Mean± SE Index, Ei
Covering Material Substrate Electivity Mean± SE Mean± SE Index, Ei
Small Rock 10.7±2.9 0.0±0.0 1.00 Coral Rubble 2.8±1.0 2.5±1.2 Shell 21.3±2.0 1.0±0.7 0.91 Other 3.2±1.5 0.0±0.0 1.00
51
Figure 1. Location of study sites Baja California Sur, Mexico
12 March 1996
10
8
6
4
2
0
12
November 1996 10
8
6
4
2
60 65 70 75 80 85 90 95 100 105 110
Test Diameter, mm
Figure 2. Size-frequency distribution of Toxopneustes roseus at El Cardon in March 1996 and November 1996. March: n = 47; November: n = 144
52
53
6
4
2
o~--~~~~~~~~~~~~~~~~~~~~~~~
75 80 85 90 95 100 105 110 115 120
Test Diameter, mm
Figure 3. Size-frequency distribution of Toxopneustes roseus at Diguet in December 1996 (n = 102)
54
0 Cardon 27 Diguet
25
s 23 s ,..c::' D ~
gf 21
ClJ ~ !'?
"' 19
~ ~
17
15~--------L----------------F------------60
Figure 4. Jaw (demipyramid) length vs. test diameter of Toxopneustes roseus at Cardon, Diguet, and Pardito. Regression equations: Cardon= 9.83 + 0.12 9
Feeding preference experiments were done in the field at El Cardon
November 1996 and at Diguet in December 1996 (see Feeding Preferences).
Preference was based on the amount of rhodoliths eaten.
55
Three treatments were used for each replicate. Treatments were: a
control of rhodoliths and other algae alone (n=3), one urchin with rhodoliths
alone (n=3), and one urchin with rhodoliths and other algae alone (n=3).
Algae were cleaned of obvious debris and herbivores, blotted dry and
weighed. Approximately 32 g of rhodoliths and 11 g of Caulerpa
sertularioides were used at El Cardon and approximately 47 g of rhodoliths,
0.2 g of Berkeleya hyalina and 0.4 g of Enteromorpha intestinalis were used at
Diguet. The experiment was repeated three times at El Cardon and twice at
Diguet. Urchins used were approximately the same size and were pre-starved
for 24 hours in the containers with no algae. Individuals were left each
treatment for 48 hours and then the final weight of algae was determined.
Differences in rhodolith weights among treatments were determined
by ANOV As. Multiple comparisons were done with Tukey' s test.
56
RESULTS
Feeding Preferences
The mean difference in rhodoliths eaten between the rhodoliths only
and rhodoliths and Caulerpa sertularioides treatments was only 1.33 g over 48
hours at El Cardon (Appendix Table 1). There was a significant difference in
changes in rhodolith weights in feeding preference treatments (ANOV A:
F=16.888, df=2, n=27, p<O.OOl). The control was significantly different than
both treatments (multiple comparisons: p<0.001 for control vs. rhodoliths
only; p=0.002 for control vs. rhodoliths and C. sertularioides). The amount of
rhodoliths eaten was not significantly different between the two treatments
(multiple comparison: p=0.194). The ANOVA was only powerful enough to
detect a difference of 2.42 g. However, the actual difference of 1.33 g is small
compared to 7.74 (the amount eaten 48 hours based on fecal production).
The difference in the mean amounts of rhodoliths eaten may be an
artifact of the urchin's need to cover themselves. Most of the individuals
used much of the available algae in the containers as covering material. This
activity, as well as the artificial setting of a plastic tub, may have caused a
reduction in their feeding rate.
At Diguet, the mean difference in rhodoliths eaten between the
rhodoliths only and rhodoliths with other algae treatments was only 1.17 g
57
over 48 hours (Appendix Table 1). There was not a significant difference
between treatments (ANOV A: F=3.270, df=2, n=18, p=0.066). The ANOV A
was only powerful enough to detect a 1.45 g difference. Compared to 16.38 g
(the amount eaten in 48 hours based on fecal production), a difference of
1.17 g is quite small. The lack of a difference between the control and other
treatments was probably due to the loss of sand in the rhodoliths. Sand was
stuck in between rhodolith branches and was loosened by the slight
movements of the containers in the surge.
58
Appendix Table 1. Amount of rhodoliths eaten by Toxopneustes roseus at El Cardon and Diguet (mean±SE). Sample size: El Cardon= 9; Diguet = 6. Other algae = Caulerpa sertularioides (El Cardon); Berkeleya hyalina and Enteromorpha intestinalis (Diguet).