Practical evaluation of sinkage and trim effects on the drag of a common generic freely floating monohull ship

A practical method to account for the influence of sinkage and trim on the drag of a freely floating (free to sink and trim) common monohull ship at a Froude number F ≤ 0.45 is considered. The sinkage and the trim are estimated via two alternative simple methods, considered previously. The drag is also estimated in a simple way, based on the classical Froude decomposition into viscous and wave components. Specifically, well-known semiempirical expressions for the friction drag, the viscous pressure drag and the drag due to hull roughness are used, and the wave drag is evaluated via a practical linear potential flow method. This simple approach can be used for ship models as well as full-scale ships with smooth or rough hull surfaces, and is well suited for early ship design and optimization. The method considered here to determine the sinkage and the trim, and their influence on the drag, yields theoretical predictions of the drag of the Wigley, S60 and DTMB5415 hulls that are much closer to experimental measurements than the corresponding predictions for the hull surfaces of the ships in equilibrium position at rest. These numerical results suggest that sinkage and trim effects, significant at Froude numbers 0.25 < F, on the drag of a typical freely floating monohull ship can be realistically accounted for in a practical manner that only requires simple potential flow computations without iterative computations for a sequence of hull positions.     

An experimental study on water entry of asymmetric wedges

The aim of the paper is to provide an experimental reference for investigation of asymmetric water entry of wedges. Parameters of the study include initial deadrise angle, inclination angle and impact speed. Initial deadrise angles of the wedges were 20° and 30° with inclination angles ranging from 0° to 15° in 5° increments. Wedges were freely fallen from three different heights. Time histories of impact pressure and body acceleration were recorded. Sampling rate of measurements were set to 25 KHz. Main configuration of each test including mass of the wedge and water level were kept unchanged during all experiments. Additionally, several calibration tests were conducted to assess the repeatability and accuracy of the recorded data. The experimental results are compared with different entry theories and other available experiments. The comparison shows a reasonable agreement and indicates that the inclination angle can dramatically affect the impact pressure experienced by the wedges. Finally, the results show that the traditional asymmetric theories are not appropriate for all inclination angles.     

Investigation of drag crisis phenomenon using CFD methods

When fluid flow passes a cylinder, the drag crisis phenomenon occurs between the sub-critical and the super-critical Reynolds numbers. The focus of the present studies was on the numerical prediction of the drag crisis based on CFD methods. In this work, block structured meshes with refined grids near the cylinder surface and in the downstream were employed. Both 2D and 3D simulations were performed using various turbulence models, including the SST k − ω model, the k − ϵ model, the SST with LCTM, the DES model, and the LES model. In the convergence studies, the effects of the grid size, the time step, the first grid size and the aspect ratio (for 3D simulations) on the solutions were examined. The errors due to spatial and time discretizations were quantified according to a V&V procedure. Validation studies were carried out for various Reynolds numbers between Re = 6.31 × 10⁴ and 7.57 × 10⁵. The averaged drag force, the RMS of lift force and the Strouhal number were compared with experimental data. The studies indicated that standard 2D and 3D RANS methods were inadequate to capture the drag crisis phenomenon. The LES method however has the potential to address the problem.     

Force and flow characteristics of a circular cylinder with uniform surface roughness at subcritical Reynolds numbers

The main purpose of this study is to establish a better understanding of the relationship between drag reduction and surface roughness. Experiments were conducted to measure the force and flow characteristics of a circular cylinder with different types of artificial surface roughness over the range 6 × 10³ < Re < 8 × 10⁴ (Re is based on the cylinder diameter D). The roughness cylinder was formed by covering the exterior surface of the cylinder with uniformly distributed (1) sandpaper, (2) netting, and (3) dimples. The roughness coefficient ranged from k/D = 0.0028 to 0.025 (k is the roughness height). A detailed quantitative measurement of the flow field around the cylinder using Particle Imaging Velocimetry (PIV) was carried out. The hydrodynamic force coefficients (drag and lift) of the rough cylinders are compared against those of a smooth cylinder measured under the same flow conditions. It is found that certain configuration of surface roughness significantly reduces the mean drag coefficient of the cylinder, particularly at large Reynolds numbers. In addition, the root-mean-square (r.m.s.) lift coefficient of the rough cylinders is considerably lower than that of a smooth cylinder.     

Total kinetic energy in four global eddying ocean circulation models and over 5000 current meter records

We compare the total kinetic energy (TKE) in four global eddying ocean circulation simulations with a global dataset of over 5000, quality controlled, moored current meter records. At individual mooring sites, there was considerable scatter between models and observations that was greater than estimated statistical uncertainty. Averaging over all current meter records in various depth ranges, all four models had mean TKE within a factor of two of observations above 3500 m, and within a factor of three below 3500 m. With the exception of observations between 20 and 100 m, the models tended to straddle the observations. However, individual models had clear biases. The free running (no data assimilation) model biases were largest below 2000 m. Idealized simulations revealed that the parameterized bottom boundary layer tidal currents were not likely the source of the problem, but that reducing quadratic bottom drag coefficient may improve the fit with deep observations. Data assimilation clearly improved the model-observation comparison, especially below 2000 m, despite assimilated data existing mostly above this depth and only south of 47 °N. Different diagnostics revealed different aspects of the comparison, though in general the models appeared to be in an eddying-regime with TKE that compared reasonably well with observations.     

The daily catch: Flight altitude and diving behavior of northern gannets feeding on Atlantic mackerel

Predators utilize a variety of behavioral techniques to capture elusive prey. Behavioral flexibility is essential among generalist predators that pursue a diversity of prey types, and capture efficiency is expected to be intense during the breeding season for parents that engage in self- and offspring-provisioning. We studied the foraging behavior of parental northern gannets in the northwestern Atlantic (Gulf of St. Lawrence) when they were feeding on Atlantic mackerel almost exclusively. Data-loggers recorded short (mean duration: 6.3 s), high speed (inferred vertical speeds of up to 54.0 m*s^− 1, equivalent to 194 km*h^− 1), and shallow dives (mean depth: 4.2 m; maximum: 9.2 m). Dives tended to occur in bouts, varying between 0.3 and 4.6 per hour (mean = 1.6). During foraging, overall flight heights ranged from 0 to 70 m, with no clear preferences for height. Most plunge-dives were initiated at flight altitudes of 11–60 m (mean ± SE = 37.1 ± 2.8 m; range 3–105 m except for 1 of 162 dives that was initiated at the sea surface). Dive depth and flight altitude at plunge-dive initiation were positively and significantly correlated, though it appears that low flight altitudes were sufficient to reach dive depths at which mackerel were present. Almost all dives were V-shaped indicating that a high acceleration attack is the most effective strategy for gannets feeding on large rapid-swimming prey such as mackerel that owing to thermal preferences does not occur below the thermocline and are thus well available and essentially trapped in the water depths exploited by northern gannets.     

Spectral distribution of wave energy dissipation by salt marsh vegetation

Spectral energy dissipation of random waves due to salt marsh vegetation (Spartina alterniflora) was analyzed using field data collected during a tropical storm. Wave data (significant wave heights up to 0.4 m in 0.8 m depth) were measured over a two-day period along a 28 m transect using 3 pressure transducers. The storm produced largely bimodal spectra on the wetland, consisting of low-frequency swell (7–10 s) and high-frequency (2–4.5 s) wind-sea. The energy dissipation varied across the frequency scales with the largest magnitude observed near the spectral peaks, above which the dissipation gradually decreased. The wind-sea energy dissipated largely in the leading section of the instrument array in the wetland, but the low-frequency swell propagated to the subsequent section with limited energy loss. Across a spectrum, dissipation did not linearly follow incident energy, and the degree of non-linearity varied with the dominant wave frequency. A rigid-type vegetation model was used to estimate the frequency-dependent bulk drag coefficient. For a given spectrum, this drag coefficient increased gradually up to the peak frequency and remained generally at a stable value at the higher frequencies. This spectral variation was parameterized by employing a frequency-dependent velocity attenuation parameter inside the canopy. This parameter had much less variability among incident wave conditions, compared to the variability of the bulk drag coefficient, allowing its standardization into a single, frequency-dependent curve for velocity attenuation inside a canopy. It is demonstrated that the spectral drag coefficient predicts the frequency-dependent energy dissipation with more accuracy than the integral coefficient.     

Investigation of a longfin inshore squid's swimming characteristics and an underwater locomotion during acceleration

In the present study, locomotion of a real longfin inshore squid (Doryteuthis pealeii) was numerically investigated. Geometry of a real squid was obtained using computed tomography (CT) images. In addition to a two-dimensional axisymmetric squid model, a modified squid model with no cavities around her head and an ellipse shaped model were generated with a fineness ratio (the ratio of body length to maximum body diameter) of 7.56. These numerical models were exposed to an acceleration with two different velocity programs. Added mass and basset forces on bodies were calculated during acceleration of the squid models starting from rest. Pressure and viscous drag forces were also calculated due to pressure variation along the squid models and friction on the surface of the models. The effect of a nozzle diameter on jet velocities and propulsive efficiency at all bodies were evaluated when time dependent velocity profiles (from 0 to 10 m/s in 0.5 and 1 s time durations) were set for the inlet of computational domain. The modified squid model required least thrust force during acceleration phase of time dependent velocity profile compared to the other models while the 0.02 m nozzle diameter provided largest propulsive efficiency for all models.     

Experimental investigation of water slamming loads on panels

Rigid-body slamming has become increasingly important as ships travel at higher speeds experiencing larger loads during hull impacts against surface water which can result in structural damage and crew injury. It is necessary to characterise the hydrodynamic loading during water impacts.We present a series of experiments conducted in order to study slamming force events seen by flat plates during free surface impacts. The experiments focus on the characterisation of the loads experienced by flat plates during the first phase of the slamming event, the water entry. They have been conducted in an especially designed test apparatus, the Slingshot Impact Testing System (SITS), which allows us launching objects against the free surface of an open channel, with the possibility of setting up different speeds and deadrise angles. We can study slamming with trapped air between the plate and the water free surface, at high impact speeds and small deadrise angles, allowing us to quantify the resulting cushioning effect. High velocity impacts up to 5 m/s were conducted at angles between 0.3° and 25°. It was found that the trapped air phenomenon significantly cushions flat plate impacts with angles less than 5° and impacts with larger angles adhere to Von Kármán's equations.     

Comparison of numerical solution and semi-empirical formulas to predict the effects of important design parameters on porpoising region of a planing vessel

Stability of the marine vessels in different conditions is one of the most important problems in the design of a planing vessel. In this research, the effects of some important design parameters (mass, longitudinal center of mass, deadrise angle, and length) of DTMB 62 model 4667-1 planing hull on the drag and also on the longitudinal dynamic stability (porpoising) are investigated in the velocity range of 2.12–8.486 m/s in calm water. In this paper, both numerical simulation of Reynolds Average Naiver Stokes (RANS) equations and semi-empirical formulas of Savitsky are used to analyze the motion of a 4667 planing vessel in calm water with two degrees of freedom (2DOF). For this purpose a finite volume, ANSYS-FLUENT, code is used to solve the Navier-Stokes equations for the simulation of the flow field around the vessel. In addition, an explicit VOF scheme and SST- Kω model is used with dynamic mesh scheme to capture the interface of a two-phase flow and to model the turbulence respectively, in 2DOF model (heave and pitch). Also, the results of both methods are compared with each other. According to the present results, changing the aspect ratio of the vessel and also the longitudinal center of gravity have the most effect on the porpoising region.     

Vertical changes in abundance,biomass and community structure of copepods down to 3000 m in the southern Bering Sea

Vertical changes in abundance, biomass and community structure of copepods down to 3000 m depth were studied at a single station of the Aleutian Basin of the Bering Sea (53°28′N, 177°00′W, depth 3779 m) on the 14th June 2006. Both abundance and biomass of copepods were greatest near the surface layer and decreased with increase in depth. Abundance and biomass of copepods integrated over 0–3000 m were 1,390,000 inds. m^?2 and 5056 mg C m^?2, respectively. Copepod carcasses occurred throughout the layer, and the carcass:living specimen ratio was the greatest in the oxygen minimum layer (750–100 m, the ratio was 2.3). A total of 72 calanoid copepod species belonging to 34 genera and 15 families occurred in the 0–3000 m water column (Cyclopoida, Harpacticoida and Poecilostomatoida were not identified to species level). Cluster analysis separated calanoid copepod communities into 5 groups (A–E). Each group was separated by depth, and the depth range of each group was at 0–75 m (A), 75–500 m (B), 500–750 m (C), 750–1500 m (D) and 1500–3000 m (E). Copepods were divided into four types based on the feeding pattern: suspension feeders, suspension feeders in diapause, detritivores and carnivores. In terms of abundance the most dominant group was suspension feeders (mainly Cyclopoida) in the epipelagic zone, and detritivores (mainly Poecilostomatoida) were dominant in the meso- and bathypelagic zones. In terms of biomass, suspension feeders in diapause (calanoid copepods Neocalanus spp. and Eucalanus bungii) were the major component (ca. 10–45%), especially in the 250–3000 m depth. These results are compared with the previous studies in the same region and that down to greater depths in the worldwide oceans.     

Determining the weights of two types of artificial reefs required to resist wave action in different water depths and bottom slopes

The interaction between waves and artificial reefs (ARs; a hollow cube weighing 8.24 kN (0.84 t) and a water pipe weighing 1.27 kN (0.13 t)) in shallow waters was investigated with respect to variations in design weight, orientation (for cube; 45° and 90° angles, for pipe; 0°, 90°, and 180° angles to flow), depth (1–20 m), and bottom slope (10^?1, 30^?1, and 50^?1). Physics equations and FLUENT software were used to estimate resisting and mobilising forces, and drag coefficients. Drag coefficients for the hollow cube were 0.76 and 0.85 at 45° and 90° angles to the current, respectively, and 0.97, 0.38, and 1.42 for the water pipe at 0°, 90°, and 180° angles to the current, respectively. Deepwater offshore wave conditions at six stations were transformed into shallow nearshore waters representative of the artificial reef site. Waters deeper than 12 and 16 m are safe to deploy blocks with angles of 45° and 90°, respectively. However, water pipes constructed at angles of 90° and 180° to the current were estimated as being unstable for 365 out of 720 cases at all stations (only one station was stable for all cases). Water pipes angled at 0° were found to be stable in all 360 cases. Slope had a significant effect on weight and depth. Results from this study provide an important reference for engineers performing projects aiming to increase the performance and service life of ARs.     

Pigments in sediments beneath recently collapsed ice shelves: The case of Larsen A and B shelves,Antarctic Peninsula

In March 2002, 3200 km² of the Larsen B ice shelf collapsed off the Eastern Antarctic Peninsula (EAP). In the austral summer of 2006, sea floor sediment was recovered beneath the extinct Larsen B ice shelf and in a region off the Northern Antarctic Peninsula (NAP), which has been free of ice shelves for more than 1000 yr. To assess changes in the chemical composition of the sediment after ice shelf collapses, chlorophylls and pheophytins were measured in sediment cores at six stations. This is the first time that chlorophyll pigments have been analysed in sediment samples from regions under recently collapsed ice shelves. Five years after the ice shelf collapse, Chla and Chlc concentrations were similar in the interfacial sediment (upper 1 cm) of NAP and EAP regions. However, in EAP Chla and Chlc concentrations decreased more rapidly with depth in the sediment column and were negligible below 2 cm depth. The high Chla to Pheoa ratios indicated that sedimentary pigments found in EAP had undergone limited degradation suggesting that they were locally produced rather than laterally advected. Complementary information from excess ²¹⁰Pb activity and diatom valve distributions provided further evidence that the pigment fluxes to the seabed in EAP took place only after the ice shelf collapse.     

Large eddy simulations of a circular cylinder at Reynolds numbers surrounding the drag crisis

Large eddy simulations of the flow around a circular cylinder at high Reynolds numbers are reported. Five Reynolds numbers were chosen, such that the drag crisis was captured. A total of 18 cases were computed to investigate the effect of gridding strategy, turbulence modelling, numerical schemes and domain width on the results. It was found that unstructured grids provide better resolution of key flow features, when a ‘reasonable’ grid size is to be maintained.When using coarse grids for large eddy simulation, the effect of turbulence models and numerical schemes becomes more pronounced. The dynamic mixed Smagorinsky model was found to be superior to the Smagorinsky model, since the model coefficient is allowed to dynamically adjust based on the local flow and grid size. A blended upwind-central convection scheme was also found to provide the best accuracy, since a fully central scheme exhibits artificial wiggles, due to dispersion errors, which pollute the solution.Mean drag, fluctuating lift Strouhal number and base pressure are compared to experiments and empirical estimates for Reynolds numbers ranging from 6.31 × 10⁴ to 5.06 × 10⁵. In terms of the drag coefficient, the drag crisis is well captured by the present simulations, although the other integral quantities (rms lift and Strouhal number) show larger discrepancies. For the lowest Reynolds number, the drag is seen to be more sensitive to the domain width than the spanwise grid spacing, while at the higher Reynolds numbers the grid resolution plays a more important role, due to the larger extent of the turbulent boundary layer.     

On the nature of oceanic eddies shed by the Island of Gran Canaria

We use hydrographic and buoy data to compare the initial temperature fields and Lagrangian evolution of water parcels in two vortices generated by the southward flowing Canary Current passing around the island of Gran Canaria Island. One vortex is anticyclonic, shed in June 1998 as the result of an incident current of about 0.05 m s⁻¹, and the second one is cyclonic, shed in June 2005 with the impinging current estimated as 0.03 m s⁻¹. The two vortices exhibit contrasting characteristics yet display some important similarities. The isopycnals are depressed in the core of the anticyclonic vortex, at least down to a depth of 700 m, whilst they dome up in the core of the cyclonic vortex but only down to 450 m. In the top 300 m the depression/doming of the isotherms is similar for both vortices, with a maximum vertical displacement of the isotherm of about 80 m, which correspond to temperature anomalies of some 2.5 °C at a given depth. A simple method is developed to obtain the initial orbital velocity field from the temperature data, from which we estimate peak values of 0.7 and 0.5 m s⁻¹ for the anticyclonic and cyclonic vortices, respectively. The buoys, three for the anticyclonic vortex and two for the cyclonic one, were drougued at 100 m depth, below the surface mixed layer, and their initial velocities are consistent with the above values. In both vortices, the buoys revolve either within a central core, where the rotation rate remains stable and large for several weeks, or in an outer ring, where the rotation rate is significantly smaller and displays large radial fluctuations. Within the inner core the anticyclonic vortex has significant inward radial velocity, while the cyclonic vortex has near-zero radial mean motions. The cyclonic vortex rotates more slowly than the anticyclonic, their initial periods being 4.5 and 2.5 days, respectively. A simple axisymmetric model with radial diffusion (coefficient K_h≅25 m² s⁻¹) and advection reproduces the observations reasonably well, the diffusive effect being more important than that resulting from the observed radial advection. The model also supports the hypothesis that the rotation rate of cyclonic vortices is less than that of anticyclonic vortices, as otherwise they would become inertially unstable. Both the buoys data and sea surface temperature images confirm that the vortices evolve from youth to maturity, as the cores shrink and the outer rings expands, and then to a decay stage, as the core rotation rates decrease, though frequent interactions with other mesoscale structures result in more accelerated aging. Despite these interaction they last many months as coherent structures south of the Canary Islands.     

Evidence of sediment gravity flows induced by trawling in the Palamós (Fonera) submarine canyon (northwestern Mediterranean)

Three mooring arrays were deployed in the Palamós Canyon axis with sediment traps, current meters and turbidimeters installed near the bottom and in intermediate waters. Frequent sharp and fast turbidity peaks along with current speed increases were recorded, particularly at 1200 m depth in spring and summer. During these events, near-bottom water turbidity increased by up to more than one order of magnitude, current velocity by two to four times and horizontal sediment fluxes by one to three orders of magnitude. When these events occurred, 9–11 days integrated downward particle fluxes collected by the near-bottom sediment trap increased by two to three times. These events were identified as sediment gravity flows triggered by trawling activities along the northern canyon wall. Sediment eroded by the trawling nets at 400–750 m depth on this wall seems to be channeled through a gully and transported downslope towards the canyon axis, where the 1200 m mooring was located. The sediment gravity flows recorded at the 1200 m site were not detected at deeper instrumented sites along the canyon axis, suggesting that they affect local areas of the canyon without traveling long distances downcanyon. These observations indicate that trawling can generate frequent sediment gravity flows and increase sediment fluxes locally in submarine canyons. Furthermore, in addition to the various natural processes currently causing sediment gravity flows and other sediment transport events, human activities such as trawling must be taken into account in modern submarine canyon sediment dynamics studies.     

Depth-related distribution and abundance of seastars (Echinodermata: Asteroidea) in the Porcupine Seabight and Porcupine Abyssal Plain,N.E. Atlantic

The depth-related distribution of seastar (Echinodermata: Asteroidea) species between 150 and 4950 m in the Porcupine Seabight and Porcupine Abyssal Plain is described. 47 species of asteroid were identified from ∼14,000 individuals collected. The bathymetric range of each species is recorded. What are considered quantitative data, from an acoustically monitored epibenthic sledge and supplementary data from otter trawls, are used to display the relative abundance of individuals within their bathymetric range. Asteroid species are found to have very narrow centres of distribution in which they are abundant, despite much wider total adult depth ranges. Centres of distribution may be skewed. This might result from competition for resources or be related to the occurrence of favourable habitats at particular depths. The bathymetric distributions of the juveniles of some species extend outside the adult depth ranges. There is a distinct pattern of zonation with two major regions of faunal change and six distinct zones. An upper slope zone ranges from 150 to ∼700 m depth, an upper bathyal zone between 700 and 1100 m, a mid-bathyal zone from 1100 to1700 m and a lower bathyal zone between 1700 and 2500 m. Below 2500 m the lower continental slope and continental rise have a characteristic asteroid fauna. The abyssal zone starts at about 2800 m. Regions of major faunal change are identified at the boundaries of both upper and mid-bathyal zones and at the transition of bathyal to abyssal fauna. Diversity is greatest at ∼1800 m, decreasing with depth to ∼2600 m before increasing again to high levels at ∼4700 m.     

Benthic response to shellfish farming in Thau lagoon: Pore water signature

Vertical distributions of dissolved species across the sediment–water interface (SWI), including major cations (sodium, potassium, magnesium, calcium), minor cations (lithium, strontium, barium), redox sensitive species (dissolved manganese, iron, sulfate, sulfide, ammonium) and other chemical parameters (pH, alkalinity, soluble reactive phosphorous, dissolved silica) were studied in a Mediterranean lagoon used for intensive shellfish farming. In order to quantify the impact of this activity on diagenetic processes and the influence of seasonal changes, two stations contrasted with respect to organic carbon fluxes were sampled in Thau lagoon from March 2001 to August 2002 during four field campaigns in winter, spring, summer and fall. Well-defined layers enriched with redox sensitive species were observed following the conventional sequence of early diagenetic reactions. However, differences were observed between both stations in depths and thickness layers. Concentration gradients extended down to more than 92 cm depth at the central position of the lagoon (station C4 – 8 m depth) and down to 40 cm depth inside shellfish farming zones (station C5 – 9 m depth). Station C4 showed an unusual diagenetic signature: sharp dissolved oxygen, iron, nitrate and manganese gradients existed at the SWI but gradients of dissolved sulfide and alkalinity as well as other parameters (dissolved silica, Ba, etc.) were recorded only from 25 to 30 cm depth downward. Seasonal changes were observed in pore water composition as deep as 30–50 cm in station C4 (only 15 cm in station C5). The center of the lagoon is not directly subjected to biodeposits deriving from shellfish activity. Isotopic and bioturbation data allowed to rule out a reworking of the sediment deeper than a few centimeters. In addition to organic content of the sediment, physical parameters were likely to induce the 10–20 cm gap between dissolved iron and sulfide profile as well as the higher vertical extent of diagenetic sequence observed at station C4. Conversely to station C5, station C4 underwent stronger currents and wave effect probably generating advective transport of water through the sediment, but no permeability data were available to confirm this hypothesis. During summer, climatic conditions generated vertical stratification of the water column and transient suboxic conditions at the bottom. Such conditions drove the upward shift of redox fronts, compacting the diagenetic sequence. These effects were reinforced at station C5 by shellfish and its farm structures (mainly attenuation of current and increased heat absorption).     

Bathymetric patterns of polychaete (Annelida) species richness in the continental shelf of the Gulf of California,Eastern Pacific

The mid-domain effect was tested to evaluate the bathymetric patterns of the polychaete species richness in the Upper and Lower Gulf of California as a possible hypothesis to explain the species richness gradient, exploring the overlapping of species depth ranges towards the middle continental shelf. The bathymetric gradient of the number of species was estimated with the depth ranges of 554 polychaete species, and the mid-domain effect was tested using a Monte Carlo simulation program at bands of 10 m depth. The Upper (251 species) and Lower (491 species) Gulf regions showed clear differences in their faunal composition (Jaccard similarity index = 0.34); the species richness pattern was characterized by a highly significant presence of polychaetes with short depth ranges (< 10 m). The richness distribution could be described as a cubic polynomial curve, but the maximum values in both Gulf regions (141 and 317 species, respectively for Upper and Lower Gulf regions) are strongly biased to shallow waters (40 m). This is not consistent with the peak of diversity at 60–70 m predicted by the model. The observed patterns cannot be reproduced by the mid-domain effect, suggesting the existence of non-random factors affecting the species richness gradients in the Gulf.