Growth of X. muta was studied at permanent 16 m-diameter circular transects on Conch Reef [24′56.996N; 80′27.223W] and Pickles Reef [24′59.272N; 80′24.660W], Key Largo, Florida, as part of a long-term monitoring program begun in 1997. On Conch Reef, 3 transects were established at 15, 20 and 30 m depth, and at Pickles Reef (~6 km from Conch Reef), 3 transects were established at 15 m depth. Because of bottom-time limitations of SCUBA-diving at depth, transects at 20 and 30 m at Conch Reef were set-up using NOAA’s underwater habitat “Aquarius” as a base of operations. Subsequent twice-yearly monitoring was undertaken by SCUBA diving from surface vessels using compressed air or nitrox. Within each transect, each sponge was mapped and given a unique stainless steel tag attached with a plastic cable-tie to a masonry nail driven into the limestone substratum next to the base of the sponge. Sponges were surveyed and each was photographed from above and in profile with a digital camera in the spring and fall of each year from 2000 to 2007. Due to inclement weather and limitations presented in the field, every sponge was not always photographed during each field season. A slate was photographed in each digital image, held parallel to the central axis of each sponge in side images and parallel to the plane of the top of the osculum in top images, displaying the unique tag number of each sponge and a 16 cm scale marker. For the purposes of this study, the spongocoel is defined as the central cavity of X. muta.

A cross-sectional approach was employed to study growth of X. muta. Size-increment data were obtained from digital images taken in May 2001 and December 2005, a period spanning 4.5 years. These timepoints were chosen because they included the largest number of measurable sponges with initial and final photographs compared to other field seasons. A total of 104 sponges, representing a broad range of sizes present in the population, were selected for study. Sponges from both Conch and Pickles reefs and from all three depths at Conch Reef were measured, but only sponges with characteristic “barrel” morphologies that had not experienced any obvious damage or tissue loss were included.

Measurements of sponges from digital images were made with UTHSCA Image Tool software. Measurements of sponge height and base diameter were taken from side images and two perpendicular measurements of osculum diameter were taken from top images (to ±1.0 mm). For the two osculum diameter measurements, the first diameter was chosen as the longest possible diameter, with the second perpendicular to the first. Volume was calculated by applying Eq. 1 for a frustum of a cone (Fig. 1).

A paired-sample t-test was used to test for differences between volume estimates from digital image measurements and from field measurements, uncorrected for spongocoel volume, for data from November 2006. As digital image estimates assume a solid geometry and do not account for the volume of the spongocoel, Eq. 3 was used to obtain spongocoel volume estimates for sponges measured with digital image analysis. Spongocoel estimates were then subtracted from original sponge volume estimates to correct for spongocoel volume. Specific growth rates were calculated by applying Eq. 5. Spearman rank correlation was used to examine the relationship between initial volume and specific growth rate estimates for digital image data, corrected for spongocoel volume.

A Walford (1946) plot was constructed with log_e final volume on the ordinate and log_e initial volume on the abscissa. Differences in growth between depths were tested with a one-way ANCOVA with initial volume as the covariate and depth (10, 20 and 30 m) as a fixed factor. Differences in growth between Pickles and Conch reefs at 15 m depth were similarly tested with a one-way ANCOVA. Following Trussell (1997), OLS regressions were used because techniques for model II ANCOVA are unavailable (Sokal and Rohlf 1995).

To examine the relationship between size and morphology, scaling relationships between height, base diameter, and osculum diameter were determined. Relationships between final and initial sizes of each dimension were similarly determined. The geometric mean of initial and final measurements was used in regressions between differing dimensions. Measurements were log₁₀-transformed and reduced-major-axis (RMA) regression was used because of measurement error in the independent variable (Ricker 1973). To test actual slope against an isometric slope of β = 1, the T statistic and degrees of freedom were calculated with equations from McArdle (1988).

Five commonly used growth functions were fit to size-increment data to determine which model best described growth of X. muta: the general von Bertalanffy growth formula (gVBGF) (von Bertalanffy 1938; Beverton and Holt 1957; Pauly 1981), specialized von Bertalanffy growth formula (sVBGF) (Richards 1959; Pauly 1981), Gompertz (Gompertz 1825; Winsor 1932), Richards (Richards 1959; Ebert 1980), and Tanaka (Tanaka 1982) growth functions. Growth equations were obtained from Brey (2001) and Tanaka (1982).

For brevity, only the Tanaka growth function will be detailed below, as it best describes indeterminate growth. For the following equations, S ₁ is the initial size (cm) and S ₂ is the final size (cm) after time interval dt (4.5 years); S _t is a continuous function of t (years) and S ₀ is size at time zero.

The Tanaka function (Tanaka 1982):

(6)

Or

(7)

is sigmoid with an initial period of slow growth, and a period of exponential growth followed by growth that slows but never reaches an asymptote. The biological meanings associated with the parameters are: a = related to maximum growth rate (≈1/a ^0.5); c = age at which growth is maximum; d = shifts size at which growth is maximum; f = measure of the rate of change of the growth rate (Tanaka 1982, 1988). The influence of each parameter on the shape of the curve is demonstrated in Ebert et al. (1999) and Ebert (1999).

The cube root of volume estimates, corrected for spongocoel volume, was used as an average linear size to model growth of X. muta. The difference equations of the models were fitted to final and initial linear sizes on a Walford plot by nonlinear regression (NLIN procedure SAS).

The sum of squared error (SSE), coefficient of determination and Akaike (1973) information criterion (AIC) were used to evaluate model fit. The AIC selects the most parsimonious candidate model by evaluating the trade-off between model bias that results from too few parameters versus variance that results from too many (Burnham and Anderson 2002). Therefore, AIC values can be used to evaluate any differences between the 2- and 3-parameter models that may otherwise be neglected through use of the SSE as the sole fitting criteria. In model selection, the model with the lowest SEE and AIC is sought. Because sample size, n, was small with respect to the number of model parameters, K, (n/K < 40), the second-order correction, AIC _c was used (Burnham and Anderson 2002). The model selection criteria AIC differences, Δ _i , give a measure of each model relative to the best model; and Akaike weights, w _i , are normalized relative likelihood values which give the probability that a given model is the best of the candidate models (Burnham and Anderson 2002).

Parameter estimates were used to produce size-at-age curves using the integrated forms of the growth functions. Growth functions were then cubed to obtain volume-at-age plots. In contrast to the four determinate models, Tanaka parameters described growth for 4.5 year intervals because the time interval between initial and final sizes is not included in the difference equation for this model. To determine parameters for yearly growth, nonlinear regression was performed to refit the curve. The parameter t ₀ was determined by solving each function at t = 0. The size of the smallest sponge measured in the dataset was used as size at t = 0 for all curves. Analyses were conducted with SPSS (vers. 14.0.0 for Windows, SPSS, Inc., Chicago, IL, USA) and SAS (vers. 9.1.3 for Windows, SAS Institute Inc., Cary, NC, USA) statistical software.

Estimates of volume of individual X. muta from field measurements did not significantly differ from estimates using digital images (paired t-test; P = 0.26, t ₃₁ = −1.16). Initial volume estimates from digital image analysis, corrected for spongocoel volume, ranged from 24.05 to 80,281.67 cm³. There was a significant negative correlation between specific growth rate and initial volume (r _s  = −0.798, P ≤ 0.01). All sponges had positive growth over the study interval and there was large variation in growth rates. The mean (±1 SD) specific growth rate was 0.52 ± 0.65 year⁻¹. Growth rates as fast as 4.04 year⁻¹ and as slow as 0.02 year⁻¹ were observed. In terms of volume, sponges gained an average of 1,955.37 ± 2,221.09 cm³ year⁻¹. ANCOVA analysis of log_e-transformed initial and final volume data indicated that there was no significant effect of depth (F _2,68 = 2.75, P = 0.07) or reef location (F _1,60 = 1.55, P = 0.22) on growth rate. Although not significant, mean growth was greater for sponges at 20 and 15 m depths compared to sponges at 30 m.

The initial volumes of sponges measured in the field ranged from 26.69 to 150,492.90 cm³. Specific growth rate during the summer was negatively correlated with initial volume (r _s  = −0.69, P ≤ 0.01), but no significant correlation was found between initial volume and specific growth rate during winter (r _s  = −0.105, P = 0.56). Growth over the entire year was significantly correlated with initial volume, with rates decreasing with increasing volume (r _s  = −0.67, P ≤ 0.01). Sponges grew significantly faster in summer compared to winter (paired t-test; P = 0.001, t ₃₂ = 3.65). Mean (±1 SD) specific growth rates during the summer and winter were 0.94 ± 1.20 and 0.14 ± 0.42 year⁻¹, respectively. The mean specific growth rate over the entire year was 1.15 ± 1.42 year⁻¹. In terms of volume, sponges gained an average of 4,195.53 ± 4,080.01 cm³ during summer compared to 1,472.40 ± 6,173.16 cm³ during winter. Growth was variable and three sponges experienced negative growth in summer while ten decreased in size over winter. When these sponges were omitted from analyses, positive growth rates remained significantly greater in the summer compared to winter (paired t-test; P = 0.003, t ₁₉ = 3.37).

Because X. muta has a variety of morphologies (i.e., low growth form with base diameter greater than height, high growth form with base diameter less than height), no one linear measurement best characterized sponge size. Moreover, the Tanaka growth model failed to converge when modeling volumetric growth. Therefore, the cube root of digital image volume estimates, corrected for spongocoel volume, was used to model growth of X. muta. This measurement of size is useful because it inherently accounts for the spongocoel correction and morphology of the sponge.

The gVBGF and Tanaka models were selected for further analysis, as the best of the candidate models, based on model fit. An evidence ratio, the ratio of Akaike weights, indicates that the gVBGF is only 1.07 times more likely to be the best model, given the set of candidate models. A Walford plot of final and initial sizes with the fitted models is presented in Fig. 3. Both models describe initial growth that gradually increases to a maximum and slowly attenuates with increasing size. Initial growth is slightly overestimated and the models start to diverge at larger size classes. The primary difference between the two models is the description of growth for the largest size classes. A plot of the two growth curves for both linear size and volume (Fig. 4a, b) shows the same patterns observed in Fig. 3. The gVBGF describes faster growth for larger individuals and approaches an asymptotic size of 6,612.11 cm. The Tanaka curve describes relatively constant growth rates for larger size classes. Extrapolation of the Tanaka curve to 3,000 years shows slowly attenuating growth that never reaches an asymptote (Fig. 4c). The age estimates obtained from the gVBGF and Tanaka curves for the largest sponge measured over the 4.5 year interval are 53 and 55 years, respectively.

Fig. 3 Walford plot of initial size (S _t ) and size 4.5 years later (S _t+4.5). Plotted sizes are the cube root of volume estimates. Fitted lines are the gVBGF and Tanaka function with parameters from Table 2

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Fig. 4 gVBGF and Tanaka growth curves from parameter estimates in Table 2. a Linear size as a function of time; b linear sizes from (a) cubed for volume as a function of time; c same curve as (b) with time expanded out to 3,000 years

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Specific growth rates of X. muta were found to decrease with increasing sponge volume. The same trend has been observed for many sponge species (Reiswig 1973; Hoppe 1988; Leys and Lauzon 1998; Garrabou and Zabala 2001). In contrast, Duckworth and Battershill (2001) found no differences in growth rates among the range of size classes studied. Wulff (1985) observed decreased growth rates with increasing size in only one of three rope-form species. It was hypothesized that relatively constant surface area to volume ratios of rope-form species allow for less influence of size on growth rates (Wulff 1985). For X. muta, physiological or mechanical limits may constrain growth in larger size classes while there may be a selective advantage for relatively faster growth of small sponges. The observed variability in growth rates is characteristic of sponges (Reiswig 1973; Dayton et al. 1974; Fell and Lewandrowski 1981; Ayling 1983; Wulff 1985; Hoppe 1988; Leys and Lauzon 1998; Duckworth and Battershill 2001; Garrabou and Zabala 2001) and other indeterminately growing species (Sebens 1987).

Growth rates did not differ for X. muta from three depths (15, 20, 30 m) on Conch Reef. In contrast, a common tube sponge, Callyspongia vaginalis, was found to grow significantly faster on the deeper slope of Conch Reef (25 m), where picoplankton concentrations were demonstrated to be higher, compared to shallower parts of the reef (14 m; Lesser 2006; Trussell et al. 2006), prompting these authors to conclude that food availability was the cause of the observed bathymetric growth pattern. Our study bracketed the same depth range on the same reef, but no significant effect of depth on growth of X. muta was observed, suggesting that differences in picoplankton abundance (Lesser 2006; Trussell et al. 2006) may not be important determinants of growth for all sponge species.

Cross-species comparisons of sponge growth rates are difficult to make, because specific growth rates decrease with increasing sponge size for X. muta (this study) and for other species (e.g., Reiswig 1973; Hoppe 1988), and average growth rates are influenced by the range of sizes measured. Comparisons are further complicated by the different methods used to measure growth rates. Nevertheless, Hoppe (1988) found average volume increases of 7.7–19.4% year⁻¹ for Neofibularia nolitangere, Ircinia strobilina, and Agelas clathrodes, all large sponges on the reefs around Curaçao. Reiswig (1973) found increases ranging from <5 to 60% year⁻¹ for Mycale sp., Verongia gigantea, and Tethya crypta on Jamaican reefs. The average specific growth rate of 52% year⁻¹ for X. muta reported in this study is comparable to these species, given the wide interspecific variation in reported growth rates.

In a study documenting regeneration of X. muta, Schmahl (1999) monitored the recovery of injured sponges following the grounding of a vessel on the Florida reef tract. Sponges were tracked for 13 months and linear growth in height was measured. Growth was variable, and 27% of the sponges did not grow. Sponges that grew increased an average of 1.94 cm in height, and growth rates were greatest during the peak of warm water temperatures. However, regeneration rates are known to differ from normal growth rates (Hoppe 1988) and involve separate processes (Simpson 1984). In another study that examined regeneration, Walters and Pawlik (2005) found X. muta capable of healing the area of a wound at a rate of 6.5% day⁻¹. In the present study, specimens of X. muta were specifically chosen to be free of physical injury so that normal rates of growth could be determined, and the average yearly rate of growth in height, 1.85 cm, suggests that normal growth rates may be less than regeneration rates reported by Schmahl (1999) and Walters and Pawlik (2005). Moreover, we observed substantial damage to other specimens of X. muta in our permanent transects caused by the movement of rope debris, and these exhibited very rapid subsequent regeneration; in one case, approximately twice the rate observed by Schmahl (1999).

Sponges grew significantly faster in summer compared to winter. This pattern is consistent with findings for smaller species (Elvin 1976; Fell and Lewandrowski 1981; Barthel 1986; Turon et al. 1998; Duckworth and Battershill 2001; Garrabou and Zabala 2001; Tanaka 2002). Johnson (1979) found the opposite pattern of growth for calcareous sponges, while others have found no seasonal growth (Ayling 1983; Hoppe 1988; Tanaka 2002). Based on historical average water temperatures on Conch Reef (Leichter and Miller 1999), faster growth rates of X. muta occurred in the summer months when the metabolic rate would be expected to increase.

Trade-offs of energy allocation to processes of reproduction and chemical defense may also influence temporal changes in growth rate. A trade-off between growth and chemical defense has been found for the sponge Crambe crambe in the Mediterranean (Turon et al. 1998), but a relationship between growth and reproduction has yet to be established. The chemical defense of X. muta is variable and is not correlated to sponge size (Chanas and Pawlik 1997); however, no study has examined temporal patterns of metabolite production in this species. Likewise, little is known about reproduction of X. muta. The age of first reproduction is unknown and little information exists on the timing of reproduction. Spawning has been observed in both spring and late summer (May 2004: Ritson-Williams et al. 2005; May 2005: McMurray, personal observation; and September 2007, O. Rutten, personal communication). Recruitment has also been observed in the spring and fall (Pawlik, unpublished data), suggesting that X. muta reproduces at least twice yearly. Growth rates are expected to decrease with increased reproduction if a trade-off in energy allocation between these two processes exists. Energy diverted from growth to gamete production may occur over several months, which could explain lower growth rates in the winter, prior to spawning in May. However, not all sponges in the population spawn during any one season (Pawlik, unpublished data), and some spawn in the fall, so there is not a clear trade-off between reproduction and growth.

Coral reef sponges may obtain food as picoplankton from the water column (Lesser 2006), or derive it from symbiotic algae (Frost and Williamson 1980; Hill 1996). Picoplankton availability as a function of depth may influence spatial differences in growth rates of some tube sponges (Lesser 2006; Trussell et al. 2006) but a similar relationship was not observed for X muta, and seasonal picoplankton abundance on Conch Reef would not be expected to vary sufficiently to explain seasonal differences in sponge growth. In a study of the growth of the sponge Halichondria panacea, Barthel (1986) found that temporal patterns were explained by water temperature and not correlated with food. Seasonal variation in light levels would be important if photoautotrophic cyanobacterial symbionts provided nutrition to X. muta, as has been demonstrated for other coral reef sponges (Wilkinson and Cheshire 1988). While X. muta does harbor cyanobacteria, available evidence suggests that they are commensals that do not benefit the host sponge (Thacker 2005; López-Legentil et al. 2008).

Field measurements revealed variable growth of X. muta, with shrinkage occurring for some sponges. It has been well documented that sponge growth is highly plastic (Reiswig 1973; Barthel 1986; Turon et al. 1998; Garrabou and Zabala 2001), and shrinkage rates are also known to be lower than rates of growth (Garrabou and Zabala 2001). Plasticity is a common characteristic of indeterminate growth (Sebens 1987). The patterns of seasonal growth and short-term shrinkage events highlight the influence that the temporal scale of measurements has on observed growth rates. Field measurements over half-year intervals revealed dynamic growth processes, while the image-analysis technique, used over a much longer time interval, integrated short-term processes and provided smoothed growth rates.

The spongocoel has been excluded in geometric volume approximations of sponge size in previous studies of large reef species (e.g., Reiswig 1973), and the present study is the first to address scaling of the spongocoel with sponge size. Spongocoel volume was positively allometric with increasing sponge size, necessitating the correction of geometric approximations for spongocoel volume across a broad range of sponge size-classes. In the present study, sponge volumes and growth rates would be overestimated without correcting for this relationship. It is unclear why larger sponges have a proportionally larger spongocoel, but two important functions are likely responsible: water pumping (Reiswig 1971) and reproduction (Ritson-Williams et al. 2005). The spongocoel functions as the excurrent site for water pumped through the sponge. The relationship between pumping rates and spongocoel volume is unknown; however pumping rates are known to increase proportionally with tissue volume (Reiswig 1975). Increased spongocoel volume may aid pumping rates as sponges grow larger, thus potentially reducing physiological problems associated with large size (Schmidt-Nielson 1974). The positive allometry between spongocoel volume and sponge volume also increases the ratio of surface area to sponge volume. The outer surface of a sponge is covered with ostia for water intake, and subsequently, food capture, waste removal, and gas exchange (Reiswig 1971). During reproduction, the spongocoel is the excurrent site for gametes during spawning events (Ritson-Williams et al. 2005). Reproductive output is expected to increase with sponge size and this may require an increasingly larger spongocoel, with a proportionally greater surface area, as sponges grow larger.

Scaling relationships between sponge dimensions characterize the form of sponge growth (Schmidt-Nielson 1974). The way in which an organism grows has many ecological implications (Werner and Gilliam 1984). For example, Hoppe (1988) found significantly faster vertical rates of growth compared to horizontal for the sponge Ircinia strobilina and attributed this difference as a strategy to avoid competition. In the present study, growth in height, base diameter, and osculum diameter were compared. Both horizontal measures of base and osculum diameter increased at the same rate as vertical growth (height). While a significant allometric relationship was not found between these dimensions, examination of the slopes of the regressions reveals a tendency for base diameter to increase slower than height and for osculum diameter to increase faster than height (Table 1). Osculum diameter was found to widen significantly faster than base diameter. As X. muta grows, its morphology changes from a frustum of a cone to a cylinder, or barrel-shape, with osculum diameter equal or larger than base diameter. There are, of course, exceptions to this generality and growth form may be influenced by an array of physical (e.g., water flow) and biological factors (e.g., damage by fish or turtle grazing).

Among all the growth models considered, the gVBGF and Tanaka growth models were found to provide the best fit to the data. The Walford plot of size-increment data (Fig. 3) shows that, indicative of indeterminate growth, all sponges were actively growing. Both curves fit the wide range of size classes measured for the current study. The primary difference in the fit of the two curves lies within the large size-classes: the gVBGF extrapolates growth beyond the largest measured sponge to approach an asymptote, while the Tanaka describes slowly attenuating growth. Much larger individuals of X. muta exist on Conch Reef, and throughout the Caribbean, than those included in this study (e.g., Nagelkerken et al. 2000). In fact, estimated volumes for the largest individuals exceed the asymptote of all asymptotic growth curves studied. The Tanaka curve does not have an asymptote and is therefore the most relevant model to describe growth of X. muta. The Tanaka function has previously been used to describe growth of other marine invertebrates, including sea urchins (Ebert et al. 1999; Rogers-Bennett et al. 2003) and molluscs (Tanaka 1988).

The growth curve determined for X. muta is conservative in several aspects. First, the volume used at time zero, 24.05 cm³, is an overestimation of true volume at settlement. Measuring growth of recruits is imposed by the limits of detection in the field. Recruits that are approximately 5 cm³ were frequently observed and larvae of X. muta have been reported to be 0.1–0.2 mm in length (Lindquist and Hay 1996). The growth of these earliest stages is presently unknown and may occur over a significant length of time. Because of their plastic nature, a decoupling of sponge size and age may exist in some cases. Sponges have the ability to lose large amounts of volume and regenerate (Henry and Hart 2005). When X. muta suffers partial mortality, a ring of multi-oscule remnants may regenerate at the base of the sponge. Through time, remnants have been observed to grow and fuse with reduction in the number of osculae as the sponge grows, although the resulting form is not typical. Should a regenerated sponge grow sufficiently to restore single-osculum morphology, the application of the Tanaka growth curve to establish its age would result in an underestimation.

Can the growth model derived herein be used to estimate the age of specimens of X. muta at other locations in the tropical and subtropical Western Atlantic? A definitive answer awaits additional research, but our data indicate little reason to believe otherwise. Physical and biological parameters are remarkably similar on coral reefs throughout the greater Caribbean (e.g., Goreau et al. 1993), which may partially explain the consistency of the flora and fauna over such a large biogeographic region. We detected no differences in growth of X. muta as a function of depth, despite differences in temperature and picoplankton availability along this cline (Lesser 2006; Trussell et al. 2006). Seasonal cycles of temperature are common to Caribbean reefs at all latitudes, with an offset of temperature between northern and southern reefs of only a few degrees (Goreau et al. 1993). Further, there is no reason to believe that growth rates of X. muta have changed over decadal to millennial scales; our recent transect data show population increases in X. muta over the past decade (McMurray and Pawlik, unpublished data), suggesting that sponges are generally healthy. Interestingly, it remains to be seen whether growth and age may reliably be predicted from our data for the closely related Indo-Pacific giant barrel sponge, Xestospongia testudinaria, which occur at similar sizes and densities on Australia’s Great Barrier Reef (Fromont and Bergquist 1994).

The worldwide decline of coral reef ecosystems has prompted many local restoration efforts (Jaap 2000; Precht 2006). Funding devoted to restoration can be substantial; for example, one 1994 ship grounding on a coral reef resulted in a $3.9 million remediation settlement (NOAA 1997). Efforts typically focus on reattachment of reef-building corals (e.g., Hudson et al. 2007), but based on our data, greater emphasis should be placed on large coral reef sponges. Future remediation settlements should take into consideration the importance of barrel sponges to habitat complexity, as well as their great age. In addition to damage from vessel groundings, X. muta may be affected by the cutting movements of chain, rope, or monofilament fishing line, particularly as these items are dragged during storm events (Schmahl 1999; Chiappone et al. 2002, 2005). After these events, detached large sponges are commonly found, still alive and intact, between reef spurs on sand or rubble where they slowly erode under the action of oscillating currents. We have recently developed new techniques for the reattachment of dislodged X. muta to the reef substratum (McMurray and Pawlik 2008).

Perhaps greater than the threat of physical damage affecting X. muta populations is the increasing evidence of disease affecting large coral reef sponges (Webster 2007). We recently described a syndrome “sponge orange band” (SOB), that usually results in the death of X. muta on which it is observed (Cowart et al. 2006). In fact, the largest and oldest sponge (~130 years) in our permanent transects succumbed to this syndrome within 6 weeks in July 2005 (Cowart et al. 2006). The same syndrome was likely responsible for the death of the very large sponge described from Curaçao by Nagelkerken et al. (2000), estimated at ~2,300 years old. We have observed that the number of sponges affected by this syndrome is highly patchy within sponge populations, and its intensity changes greatly from year to year. It is not clear if this syndrome is a new phenomenon, but if sponges in the population have an equal probability of contracting it in any given year, the likelihood that any individual sponge will survive to a great age would diminish. If diseases of sponges similar to SOB are becoming more prevalent (Webster 2007), we would predict profound changes in the age structure of long-lived coral reef sponges such as X. muta.