Length-Weight Relationships for 32 Species of Cryptobenthic Reef Fishes from the Red Sea

Cryptobenthic reef fshes (CRFs) are often neglected in reef biodiversity assessments, trophodynamic studies, and biomass models. Tis oversight is due to the challenges associated with recording them in traditional underwater visual surveys and the scarcity of literature detailing their life history, ecology, and body growth parameters. Given their pivotal role in the functioning and maintenance of coral reef ecosystems, addressing these information gaps for CRF species is of great importance. In this study, we have computed the length-weight relationships (LWRs) for 32 CRF species spanning seven families in the Red Sea. Tis marks the frst comprehensive report of LWR parameters for CRFs from this region, and for 31 of these species, it serves as their frst LWR data report. Te coefcient of determination ( r 2 ) ranged from 0.82 to 0.99, indicating a good ft for the LWRs. Half of the presented species belong to the Gobiidae family, including three undescribed species. In addition, we present LWRs for species from the families Blenniidae (5 spp.), Tripterygiidae (2 spp.), Apogonidae (4 spp.), Pseudochromidae (3 spp.), Plesiopidae (1 spp.), and Scorpaenidae (1 spp.). Tis research contributes invaluable insights into the growth patterns of CRFs not only in a global context but also beyond, as 50% of the recorded species are endemic to the region. Te data generated holds great signifcance for conducting functional diversity analyses, ecosystem assessments, and coral reef health monitoring. By capturing this critical information, this work provides foundational metrics to take signifcant strides toward the conservation of these essential coral reef fshes.


Introduction
Cryptobenthic reef fshes (CRF) signifcantly difer from larger and conspicuous reef fshes.Tey are difcult to observe because of their minute size (less than 5 cm in length), their cryptic behavior, and their association with the benthic habitats [1,2].As a result of these characteristics, traditional reef fsh studies exclude CRFs from their assessments, leaving a signifcant knowledge gap in the understanding of coral reef diversity and functioning.Te omission of CRFs from fsh surveys conceals up to 50% of fsh individuals and up to 40% of fsh species of coral reefs [3].Despite the increasing research and inclusion of CRFs over the last few decades, the knowledge around diversity, ecology, diet, habitat, movement, and life cycle, among others, is limited and absent for many species.Even with CRFs' high diversity and abundance across coral reefs, we still lack a substantial and comprehensive understanding of the ecological functions they provide in these ecosystems.Among all the potential ecosystem functioning roles CRFs can have, it is worth highlighting their role as abundant and constant protein sources for marine organisms of higher trophic levels [4].On the other hand, because of their size, their metabolic requirements and thresholds make them highly vulnerable to environmental stressors and habitat alterations [3,5,6], impacting communities, population dynamics, and individual physiological resilience.
Traditionally, length-weight relationships (LWRs) have been used to estimate fsh biomass based on easier-to-obtain fsh length data as opposed to weight data.It is a tool that has become essential for fsheries management, conservation of endangered species, and catch restrictions or regulations [7].In coral reef ecological studies, it is a tool that allows the integration of biomass calculations based on size estimations from in situ visual surveys [8] for both spatial and temporal comparisons.LWRs have also been used to compare ftness by calculating individual body condition factors [9] with contrasting environmental conditions [10,11] or for optimizing aquaculture conditions [12].Knowledge of CRF species LWRs is essential for integrating these species into functional diversity analysis, ecosystem functioning assessments, and coral reef health monitoring.Currently LWRs models for many fshes, especially CRFs, are based on models conducted at the family or subfamily level or from species with similar body shape.Our study is the second dedicated efort to estimate the length-weight relationships of cryptobenthic reef fshes (20 species from the Gulf of California, [13]) and the frst for the Red Sea.
Te aim of the study is to estimate the LWRs for CRF from direct measurements of length and weigh.A total of 32 species of cryptobenthic reef fshes were collected and measured from the Central Red Sea.Of these, 50% are endemic to the Red Sea.Published data on LWRs is lacking for all these species studied except for Asterropteryx semipunctata Rüppell 1830 (family Gobiidae); however, these specimens were all from the Indian Ocean with no Red Sea data [14].

Methods
Collections took place in July 2023 on the Saudi Arabian coast of the Central Red Sea (Figure 1).Collections were from three diferent reefs at an increasing distance from the shore.In each reef, six habitats: back reef (5 m), back crest (2 m), reef fat (1 m), exposed crest (2 m), shallow slope (5 m), and deep slope (15 m) were sampled.Knowing that CRF communities vary spatially in the Red Sea [15], sampling was designed with the purpose of covering a wide range of habitats within a reef.Tis also provided specimens from a range of environmental conditions that could infuence growth and body condition and could help ofset the fact that collections were only conducted during a single timepoint.For some species this might infuence the relationship by incorporating reproduction or seasonal infuences.Specimens were collected within 1 m 2 quadrats with the use of rotenone [16] and hand nets on scuba, immediately placed on ice, and transported to the laboratory.Each specimen was identifed to the species level, photographed in a photo tank, weighed with an accuracy of 0.001 g (fresh weight), and measured (total length) with a digital caliper with an accuracy of 0.1 mm.We selected species for which we collected at least 20 specimens, resulting in a fnal number of 32 species.Te samplings were done under the approved ethics protocol number 20IAU-CUC05 issued by the KAUST Institutional Animal Care and Use Committee.
We visualized the length-weight relations to remove outliers, likely due to errors during data recording and entry.After the data cleanup using Froese's equation (7), we calculated the LWRs of the selected species as follows: where W is the weight of the fsh in grams (g), L is the total length in centimeters (cm), a is the intercept in the y-axis, and b is the slope.Using nonlinear least squares implementing the R function nls [17], we adjusted the LWRs 2 Journal of Applied Ichthyology N corresponds to the sample size, CI to the confdence interval, and SE to standard error of the b parameter.Te maximum length corresponds to the total length published for each species in FishBase [20]  Journal of Applied Ichthyology models for each CRF species.We also included the mean Fulton's condition factor (K) for each species [18]: We plotted the LWR relationships in R statistics of the 32 CRF-selected species to visualize their growth (Figure 2).

Results and Discussion
A total of 2,671 specimens belonging to 32 species and seven families of CRFs were included in the analysis.Samples per species ranged from 21 (Ecsenius frontalis) to 511 (Trimma avidori) and covered a range of body lengths within each species (Supplementary Materials S1 and S2).Tis was supported by a r 2 above 0.82 for all species (Table 1).Te most represented family was Gobiidae, with 16 (50%) species, the most species-rich family on coral reefs and common across Red Sea reefs [15,19].Tis was followed by Blenniidae with fve, Apogonidae with four, Pseudochromidae, and Tripterygiidae with three species; a single species represented the families Plesiopidae and Scorpaenidae.Based on the broad sampling design, this is representative of CRF communities in the region.
Te coefcient of determination (r 2 ) for Length-Weight Relationships (LWRs) ranged from 0.82 (Eviota sp. 1 and Eviota guttata Lachner and Karnella 1978) to 0.99 (Alloblennius jugularis) and provides an indicator for the growth model, where values higher than 0.7 suggest the model is a good ft.Our results indicate that we can have confdence in all 32 CRF growth models.Te b parameter, which represents the growth type of the fsh (3 for isometric, <3 for negative allometric, >3 for positive allometric), spanned from 2.5 (Ecsenius nalolo and Hetereleotris sp.) to 3.5 (Eviota marerubrum).Species with b values below 3 tend to become slimmer as they grow in length, while those with b values exceeding 3 tend to gain weight as they grow in length, potentially indicating optimal growth conditions.Most species revealed b values below 3 (23 of 32 species, 72%), while both species of Tripterygiidae were above 3 (3.32-3.58).Te lower b values in the majority of the evaluated species could indicate physiological stress, food shortage, or it could be an attribute of fsh species with high metabolic rates and low tolerance to environmental fuctuations such as CRFs.Even though Fulton's K is most useful over broader spatial or temporal comparisons that were not the primary focus of this study, this data could be considered as a baseline for future studies collecting similar species.Interestingly, 29 out of 32 species of CRF showed K values above 1, which could indicate healthy growth conditions [7].
For the one species where data already exists (A.semipunctata), our study yielded a b value of 3.15 (95% CI: 3.08-3.22),which is higher than the previously reported value of 2.97 from Zanzibar (see [14]).However, it is important to note that their growth model was done within diferent sampling sites, leading to a wide range of b values spanning from 2.4 to 3.5.While our b value for the growth model of A. semipunctata falls within the range modeled by Mnemba et al. [14], it is essential to emphasize the importance of performing LWRs for Red Sea species within the Red Sea.Tis recommendation is based on recognizing potential disparities in growth patterns and physiology attributable to environmental factors specifc to the Red Sea (e.g., elevated temperatures and salinity, low productivity), even when external data sources are available.Moreover, some Red Sea fshes believed to belong to widespread cryptobenthic species may be undescribed species endemic to the Red Sea (e.g., [21,22]).
Modeled r 2 values for some Gobiidae species, like Eviota, E. guttata, Eviota sp. 1, Eviota oculopiperita, and Trimma avidori, were lower than the 0.85 even though sample sizes were high (n � 69-511).Instead of attributing this to measurement errors, we propose that natural diferences in habitats and environmental conditions may infuence these values.Notably, the LWRs for these species reveal high variation compared to other species.Tis suggests that understanding these variations requires considering the ecological context of each species (Figure 2).
From collected specimens, we modeled LWRs for 32 species of CRF from the Red Sea.Our study represents the frst signifcant efort to record these parameters directly from specimens in the Red Sea and for a widespread number of CRF species.Until now, LWRs were not accessible for most of these species (except for A. semipunctata), greatly enhancing the data availability for this fsh group, encompassing half of which are endemic to the region.Te data presented in this study should be useful to increase our knowledge of CRF species in the Red Sea and globally and can be applied to studies that aim to include (i) biomass estimates based on size frequency data, (ii) model biomass dynamics, and (iii) model trophodynamics of coral reefs with the inclusion CRF.Including CRF into fsh community analysis helps to provide a more holistic picture as this group includes almost 50% of the diversity and abundance of fshes found on coral reefs.

Figure 1 :
Figure 1: Map illustrating the location of the collections of CRFs in the Saudi Arabian coast of the central Red Sea.Map created by Ute Langner.

Figure 2 :
Figure 2: Length-weight relationship plots for 32 species of CRFs.Panel (a) includes 16 species from the family Gobiidae.Panel (b) includes 16 species from 6 families of CRFs found in the Red Sea.Te r 2 of the length-weight relationship is indicated for each species.Silhouettes indicate the typical body shape for each species.

Table 1 :
Growth parameters of 32 species of CRF species from the Red Sea.
. Values in parenthesis represent the standard error.Species endemic to the Red Sea is marked with a Y, and species endemic to the Red Sea, including the Gulf of Aden, is marked with a Y *