Journal of Coastal Research Charlottesville, Virginia
Beach Cusps and Burrowing Activity of Crabs on a Fine Grained Sandy Beach, Southeastern Nigeria
Effiom Edem Antia
Department of Geological Oceanography Institute of Oceanography University of Calabar, P.M.B 1115 Calabar, Nigeria ABSTRACT _
ANTIA, E.E., 1989. Beach cusps and burrowing activity of crabs on a fine-grained sandy beach, southeastern Nigeria. Journal of Coastal Research, 5(2), 263-270. Charlottesville (Virginia), ,ttlllllll:. ISSN 0749-0208. ;.:;rr:r::y. The significance of morphological features have usually been under-emphasized in ecological studies of sandy beaches. One consequence of the latter is the widely-held view that organisms euus~ ~ 1r" inhabiting beach ecosystems depict an unpredictable longshore distribution pattern. The above ==; &-Ub is refuted in this present study ofhurrow density of crab species (Ocypoda africana and Ocypoda cursor) encountered on cusps along a fine-grained, mesotidal, sandy beach fringing the south eastern coast of Nigeria. Generally, higher burrow densities were observed on transects along cusp horns compared with those of the adjacent bays. In contrast with the cusped foreshore segments, burrow densities on adjacent non-cusped segments were devoid of any predictable pattern. Averages of the mean values of burrow diameter and depth were relatively higher on cusp horn compared with cusp bay. Elevated values of depth-diameter ratio were also evident on the former. The lower burrow depth on bay is related to the moisture needs of the organisms and is a function of the water table. Cusp topography dictates that the latter would be closer to the surface on bays than on horns. The above results elucidate the essence for in-depth eva] uation of morphologic characteristics in the course of ecologic studies of beaches.
ADDITIONAL INDEX WORDS: Beach cusps, crab burrows, mesotidal, fine-grained sandy beach, ecologic studies, Nigerian coast.
INTRODUCTION and FRICKE (1983), McLACHAN (1983), BOW MAN and DOLAN (1985), and VILAS (1986). A multi-disciplinary approach to coastal The present study was mainly motivated by research has, in recent times, increased in the observations of BALLY (1983) and FLEM dimension. The subject of organism-sediment MING and FRICKE (1983). The studies of the relationship has consistently engaged the former, like most others previously reported, attention of biologists and geologists studying indicated the distribution of a number of macro the beach, tidal flat and nearshore environ fauna and meiofauna on sandy beaches to be ments. This report focuses on the above subject patchy and that the patchiness was random in from a geological perspective. occurrence. FLEMMING and FRICKE (1983) The literature on organism-sediment inter have reported on the important role which the action including those on the distribution of morphodynamic beach-states (detailed in beachfauna in time and space is enormous. Per WRIGHT and SHORT, 1983) plays in determin tinent contributions from both geological and ing the nature of the internal geometry and sed biological perspectives are contained in the fol imentary structure of a sandy beach. Conse lowing reports: e.g. JANSSON (1967), CUBIT quently, the state of a beach (reflective, (1969), HOWARD and DORJES (1972), HILL intermediate or dissipative) is considered a and HUNTER (1973, 1976), REINECK and critical factor in the study of meiofaunal and SINGH (1973), ELEFTHERIOU and NICHOL microfaunal ecology. The peculiar hydrody SON (1975), ECKMAN (1979), CRAIG (1980), namic signatures and scale of beach mobility HOLANOV and HENDRICKSON (1980), associated with each of the morphodynamic GRANT (1981), BALLY (1983), FLEMMING beach-states elucidate the importance of their evaluation in all aspects of ecologic studies on 87021 received 29 May 1987: accepted in revision 30 June 1988. sandy beaches. 264 Antia
The possible effect of morphological beach characteristics on the distribution of organisms inhabiting the sandy beach ecosystem has not N received sufficient attention in previous studies reported in the literature. Concerted efforts in this respect are, however, evident from studies I of GRANT (1981) and FLEMMING and FRICKE (1983). With the exception of the reports of CUBIT (1969) and BOWMAN and DOLAN (1985), no other study is known to this author in which faunal distribution has been examined in relation to beach cusps. However, in the above-mentioned studies attention was J SKm focussed on faunal distribution at specific levels on cusps. Furthermore, both reports were unin formative on either the dimensional or textural characteristics of beach cusps investigated. The aforementioned attributes uf cusps are Study area 0 very essential for pruper assessment of the Figure l. Study area. influence exerted by this and other similar rhythmic shoreline features on the distribution pattern of beach fauna. The preponderance of trast of 1°_2° in steepness between bays and cusps (at some beach sectors) and their coloni horns was usually evident. zation by crabs (of which Ocypoda africana and STUDY AREA AND PROCEDURE Ocypoda cursor were the major species) at the study area further motivated this initial report The study area (Figure 1), is locally referred aimed at evaluating the roles of physico-mor to as Ibeno Beach. It is located on the south phologic beach attributes on faunal distribu eastern segment of the Nigerian coast. The tion. beach is composed mostly of fine-grained sands; grain sizes however vary between 0.18 and 0.34 Beach Cusp mm. The steepest portion of the foreshore is 8°· 10°; a large proportion of the 50-100 m wide Beach cusps are sinusoidal morphologic fea intertidal zone has slopes less than 4°. tures of beach foreshores. They consist of a sea Tides are semi-diurnal and mesotidal in ward projection or hurn and a seaward-concave range (2-4 m). The predominant winds are from depression or bay. As indicated in studies of the southwest and these generate waves whose ANTIA (1987b), the literature on cusps is vast. breaker heights are usually 30-200 cm. Veloe Reports dealing on cusp origin and formation ities of wind and longshore current ranged are replete with contradictions. On the con between 5-10 mls and 0.15-0.8 mis, respec trary, remarkable consensus exists on cusp tively. More detailed information regarding characteristics such as grain-size statistics, variations in the physico-morphologic charac permeability and variations in steepness teristics uf Ibeno Beach and, most especially, between horns and bays. the ecologic consequences of such variations The sets of beach cusps investigated were on have been recently documented in ANTIA the average, 15 m long, 0.4 ill in relief and with (1986, 1987a,bl. On the contrary, there is a a spacing of 31 ill. The horns of cusps, in com dearth of information on the taxonomy and parison with the adjacent bays, depicted coarser other biological aspects of fauna in the study grain sizes, better sorting, higher permeability area. rates and steeper slopes. Specifically, the vari The study procedure entailed the establish. ations in the aforementioned between the horns ment of 2 m wide transects of equal length and bays were: 22%, 19% and 16% for mean, along the axes of horns and adjacent bays of sorting and permeability, respectively: a con- cusps. The transects extended seaward fromthe
Journal of Coast.a! Research, Vol. 5, No.2, 1989 Beach Cusps and Burrowing Activity 265
berm crest along the entire cusp length. A total gated cusps may be a consequence of equally of16 other transects were also established on small disparity in textural and related charac foreshore segments devoid of beach cusps. The teristics between the cusp components of this transects are designated A-P in the eastward beach. Increased textural inhomogeneity direction and cover 6.2 km of beach length. between the cusp components, as reported from Surf-scaling or reflectivity parameter values of comparative studies on the Nigerian coast the transect localities given in ANTIA (1987a), (ANTIA, 1987b), should result in higher con these being the basis of the morphodynamic trast of factors on which crab distribution are beach classification of WRIGHT and SHORT likely to depend. Textural differentiation (1983), indicate a fluctuating tendency between between horns and bays resulting in much intermediate and dissipative beach-states. higher permeability, wave swash control and Burrowing activities of the reported crab spe ease of burrowing on the horns would naturally ciesin relation to the variations in the morpho enhance crab colonization to the detriment of dynamic beach-states are receiving detailed the adjacent bays and, thus, give rise to a attention following initial studies of ANTIA higher statistically significant partitioning of (1986). However, for the present study, burrow burrow densities between cusp components diameter, vertical depth and density at each of shown in Figure 2. the transects were examined at low tide. The While the above result is not considered con diameter of the burrows was obtained with a clusive, primarily because of the limited time rule while vertical depth entailed lowering scale of observation and restricted environmen erect rods of appropriate size into the burrows tal condition (mesotidal , moderate-high until the bottom was felt. The length of rod energy), it is instructive in a number of ways. inserted into each burrow was determined with Of particular importance is the amplification of the corresponding burrow diameter. the observations of GRANT (1981). His studies Density of burrows at each transect was showed that morphologic bedforms such as rip obtained in a manner reported by MYERS ples had a modifying effect on faunal distribu (1979). This entailed counting the total number tion. The pattern whereby a higher number of of burrows occurring within a transect and burrowing crustaceans occurred on ripple crests dividing the obtained values with the area of as against the troughs was considered to be a the transect. In all, detailed assessment of bur consequence of active preference by the orga row density was made on 12 successive beach nisms rather than a passive hydrodynamic sort cusps in March of 1986. In addition, 66 pairs of ing. diameter-depth data of burrows were each The studies of GRANT (1981) and the present obtained from horns and bays of cusps at var observations illustrate the fact that ecologic ious times between April and June of 1986. sampling on beaches should always be made with due consideration on possible inhomogene RESULTS AND DISCUSSION ities in bedform morphology. Thus, the com monly reported unpredictable nature of the Longshore partitioning of the burrow densi longshore distribution pattern of macrofauna ties of the studied crab species between the on sandy beaches may be, to a large extent, horns and adjacent bays of cusps at the study attributable to non-assessment of the inhomo site is given in Figure 2. A first and general geneities in beach morphology. To account for impression evident from the latter is elevated the modifying effect of beach cusps on the dis burrow density values on horns relative to their tribution of crabs studied, and by implication adjacent bays. The burrow density pattern for burrowing beach fauna, one has to assess cusp both cusp components was similar. This evident characteristics and associated hydrodynamic trend is statistically supported by a correlation conditions. coefficient value, r , of 0.8. The regression equa It is obvious that the loosely packed grains on tion for this relationship is Y = 0.87X + 0.17, cusp horns will facilitate burrowing when com in which the variables Y and X denote burrow pared with the tight packing of grains typical density of horn and bay respectively. of cusp bays. Similarly, high rates of oxygen It is suggested that the low variation between and nutrient cycling most conducive for the sur burrow density of horn and bay of the investi- vival of burrowing organisms are likely to per-
Journal of Coastal Research. Vol. 5. No.2, 1989 266 Antia
N 1-0 ~horn ~ o__ ~ bay 0 -=- f c-, f 0+- f III 0·6 I c I QJ LJ I I :J f 0 I '- ) '- ~ ::::J ------0/ .0 e- 2
12 no of cusps
Figure 2. Variations in burrow density on cusps. sist beneath horns. This is due to the better bays gives rise to turbulent, channelled or rip sorting and higher permeability of sediments like seaward flow. Interaction of the latter flow comprising the horns than those evident on the with succeeding wave swash especially at high bays (ANTIA, 1987b). The relative rates of tide may aggravate turbulence leading to large exchange of oxygen and nutrients with depth at scale sediment re-working. The sediment reo horns and bays are not likely to reverse because working process will be enhanced by the meso sediments at sub-surface of horns and bays both tidal and low-lying characteristics of the stud showed similar upward coarsening trend ied beach. On the contrary, the coarser and bet (ANTIA, 1987b). The latter trend in sediment ter sorted sediments on the horns enhance texture has however been shown by CHAFETZ infiltration of swash thus promoting milder and KOCUREK (1981) to be typical of accu hydrodynamic conditions. mulational cusps. On the whole, horns of cusps are sites of slow With respect to hydrodynamics and related but steady sedimentation whereas bays are factor of sedimentation, there might be a ten zones of intense sediment re-working and ero dency to initially suppose that crabs would be sion. The high steepness of horns relative to bay more abundant within cusp bays than on horns slope supports the above disparity in sediment because of the less intense wave activity typical dynamics over cusps having spacing in the 30· of embayments. The latter condition is due to 60 m range or lower. In effect, micro- and meso divergence of wave energy from embayments scale cuspate bedforms are incapable of struc and subsequent convergence at horns or similar turing wave energy in a manner that promotes seaward projecting shoreline features. Al quieter or depositional conditions within cusp though faunal density studies of BOWMAN and bays relative to horns. It is, however, recog DOLAN (1985) showed the aforementioned ten nized that on very large cusps and other macro dency, it is important to state that their obser rhythmic shoreline features having spacing vations were conducted on macro-scale (spacing over 60 m, their embayments may indeed con > 60 m) cusps. In contrast to the latter, the stitute zones of mild hydrodynamic conditions meso-scale varieties at Ibeno Beach showed tur in contrast with the seaward projections bulent conditions to be associated with cusp (ridges, promontories, etc.). bays as against relatively quiet conditions at The aforementioned pattern of hydrodynamic the horns. condition is due to the fact that swinging As illustrated in Figure 3, after wave break swashes from adjacent horns or promontories ing, resulting swash after initial run-up on are not well re-inforced as backwash flow horns, swings onto adjacent bays by virtue of within the bays. Moreover, the refraction of the disparity in their relief and steepness. waves away from embayments becomes more Thus, convergence of adjacent swashes at the pronounced. Consequently, backwash flow
Journal of Coastal Research. Vol. 5. No.2, [989 ... Beach Cusps and Burrowing Activity 267
meso-( usp
1---->-< 60m-i
note
1.- cusp spacing l - length
macro- CUSp I v ev» II I. ~ 60m ------
\ ba(k vash , I \ +I , I \ \, \ \ Figure 3. Hydrodynamic patterns on cusps.
within embayments (Figure 3) is smoother and rows on cusps and diameter-depth ratio calcu broader in comparison with similar flows on lated from these were analyzed and are pre meso-scale cusps. sented in Table 1. Correlation coefficient, r, In contrast to the definite pattern of long between burrow diameter and depth obtained shore variation in burrow density over cusped from data on which Table 1 was made indicates foreshore segments, the non-cusped counter generally higher values on the horns (Figure 5) part given in Figure 4 showed no predictable compared with corresponding bays. Corre trend in accord with reports in the literature. sponding or adjacent bays as used in this report As previously stated, efforts are underway to refer to those on the left of the horns when fac evaluate to what extent the observed irregular ing the sea (that is, the bay eastward of a given longshore distribution of burrow density can be horn). accounted for by the changing values of surf Some of the interpretations afforded by Table scaling parameter. The latter incorporates vir 1 include the fact that the overall average of the tually all the physical factors often asserted to means of burrow diameter and depth was control faunal distribution on beaches. higher on the horn compared with bay. The con Measured diameter and depth data of bur- trasts were, however, not very remarkable,
Journal of Coastal Research. Vol. 5, No.2, 1989 268 Antia
April 11, '86 26, Hay 13, 23. ~ (}o .• o June 7, E -,. ~ 0 0
=-c-, 30 V1 c OJ "0 2- 0 ::J 2 c, ::J 1 0 .0
/ <, --_>#'" B H N o beach transects
Figure 4. Longshore temporal variations in foreshore burrow density.
Table 1. Mean (x) and standard deviation I(J") of burrow In contrast with burrow diameter, burrow dimensions on cusp horn and bay, lbeno Beach. depth is less reliable as an indicator of animal size. Studies documented by HOLANOV and diameter depth diameter HENDRICKSON (1980) and WILLIAMS (1983) cusp no. (J" X
Journal of Coastal Research, Vol. 5. No.2, 1989 Beach Cusps and Burrowing Activity 269
....-----. h0 r n ~--- bay
. 1 -~ ..... \ ,,\ :;: \ (lJ \ , 05 \ \ ~ \ \ , \ \ , c \ \ , 0 \,/ I ~ -c (lJ \ ~ \ ," 10 ~ \ , 0 ~ u no of cusps - 05
Figure 5. Diameter-depth correlation of burrows on cusps.
CONCLUSIONS parameters was calculated and found to be gen erally higher on the horns than bays. Further The information documented in this report assessment of this ratio on other morphologic corroborated the observations ofearlier authors features, different levels of beach and for bur such as GRANT (1981), FLEMMING and rows constructed by different beach organisms FRICKE (1983) and BOWMAN and DOLAN or species of the same organism would be quite (1985), all of whom have asserted the strong rewarding to the geologists and biologists. dependence of me io- and macro-fauna distri These share the belief that burrows can serve bution on sandy beaches on the morpho-struc as useful indicators of past and present states tural attributes of the beach. Macrofaunal den of an ecosystem. sity distribution in relation to cusped and non cusped foreshores herein presented as well as ACKNOWLEDGEMENTS over rippled bedforms (GRANT, 1981) provides abasis for refuting previous opinion that the UNESCO's fellowship at the Netherlands longshore distribution of fauna within a sandy Institute for Sea Research (NIOZ), Texel and beach ecosystem is unpredictable. The cusped the DAAD (FRG) study award at the University foreshore segments revealed elevated densities of Bremen and Senckenberg Institute for of burrows on the horns than the adjacent bays Marine Geology and Marine Biology, Wilhelms of cusps. A previous report (ANTrA, 1986) and haven, are contributory to the completion of the current assessment do indicate that the this study. The kind and continued help unpredictable longshore burrow density pat received from staff of these institutes, but most terns on the no n-cusped foreshores may be especially from Dr. Tj. van Weering, Prof. Dr. defined given data on the surf-scaling parame G. Wever and Dr. B.W. Flemming, are acknowl ter. edged. U.I. Enin and L. Etim at the Institute of Difficult to explain at the moment is the gen Oceanography (lOC), University of Calabar, erally better correlation of burrow diameter provided taxonomic information while U .A. and depth on horns than the adjacent bays. On Umana assisted in data collection. Funding was the other hand, the reason for the elevated bur provided by Mobil Producing Nigeria through a row diameter values on horns than bays is not contract on ecologic studies awarded to IOC. also known. However the deeper depth of bur rowing on horns is predictable given their LITERATURE CITED higher elevation from the beach water table and higher permeability rate than their adja ANTrA, E.E., 1986. Appraisal of crab burrows as pos sible bio-indicator of beach morphodynamics along cent bays. the Cross River state coastline. Paper presented at Considering the general variability of values 1 st National Conference on Oceanography in of burrow diameter and depth, the ratio of these Nigeria, University of Calabar, Calabar.
Journal of Coastal Research, Vol. 5, No.2, 1989 270 Antia
ANTIA, E.E., 1987a. A geomorphic model of beach Holmes and Gay. Journal of Experimental Marine changes on the Nigerian coast. In: N.C. KRAUS, Biology and Ecology, 46, 81-88. (ed.J Coastal Sediments 87, ASCE Publication, 2, HILL, G.H. and HUNTER, RE., 1973. Burrows of the 1793-1808. ghost crab Ocypode quadrata (Fabricius) on the bar ANTIA, E.E., 1987b. Preliminary field observations rier islands, south-central Texas coast. Journal of on cusp formation and characteristics on tidally Sedimentary Petrology, 43, 24-40. and morphodynamically - distinct beaches on the HILL, G.H. and HUNTER, R.E., 1976. Interaction of Nigerian coast. Marine Geology, 78, 23-33. biological and geological process in the beach and BALLY, R., 1983. Factors affecting the distribution of nearshore environments, northern Padre Island, organisms in the intertidal zones of sandy beaches. Texas. In: RA. DAVIS and P. ECHINGTON, (eds.), In: A. McLACHLAN and T. ERASMUS, (ed s.}, Beach and Nearshore Sedimentation. Tulsa: Society Sandy Beaches as Ecosystems. The Hague: Junk of Economic Paleontologists and Mineralogists, pp. Publishers, pp. 391-403. 169-187. BOWMAN, M.L. and DOLAN, R, 1985. The relation HOWARD, J.D. and DORJES, J., 1972. Animal-sedi ship of Emerita talpoida to beach characteristics. ment relationships in two beach-related tidal flats, Journal ofCoastal Research, 1,151-163. Sapelo Island, Georgia. Journal of Sedimentary CHAFETZ, H.S. and KOCUREK, G., 1981. Coarsen Petrology, 42, 608-623. ing-upward sequences in beach cusp accumulations. JANSSON, B.O., 1967. The significance of grain size Journal ofSedimentary Petrology, 51, 1157-1161. and pore water content for the interstitial fauna of CRAIG, F., 1980. Diel distribution difference in the sandy beaches. Oikos, 18, 311-322. sand crab, Emerita analoga (Stimpson) (Decapoda, MYERS, A.C., 1979. Summer and winter burrows ofa Hippidea). Crustaceana, 39, 287-300. mantis shrimp, Squilla empusa, in Narragansett CUBIT, J., 1969. Behaviour and physical factors caus Bay, Rhode Island (U.S.A.). Estuarine and Coastal ing migration and aggregation of the sand crab Marine Science, 8, 87-98. Emerita analoga (Stimpson). Ecology, 50, 118-123. McLACHLAN, A., 1983. Sandy beach ecology - a ECKMAN, J.E.. 1979. Small-scale patterns and pro review. In: A. MCLACHLAN and T. ERASMUS, cesses in a soft substratum, intertidal community. (Eds.), Sandy Beaches as Ecosystems. The Hague: Journal of Marine Research, 37, 437-459. ELEFTHERIOU, A. and NICHOLSON, M.D., 1975. Junk Publishers, pp. 321-380. The effects of exposure on beach fauna. Cahiers de REINECK, H.-E., and SINGH, LB., 1973. Deposi Biologie Marine, Tome XVII, 695-710. tional Sedimentary Environments. Berlin: Springer FLEMMING, B.W. and FRICKE, A.H., 1983. Beach Verlag, 439p. and nearshore habitats as a function of internal VILAS, F., 1986. Activity of amphipods on beach sed geometry, primary sedimentary structures and iments and nearshore environments; Playa Ladeira, grain sizes. In: A. McLACHLAN, and T. ERASMUS, N.W. Spain. Journal of Coastal Research, 2, 285 (eds.I, Sandy Beaches as Ecosystems. The Hague: 295. Junk Publishers, pp. 115-132. WILLIAMS, J.A., 1983. Environmental regulation of GRANT, J., 1981. Sand transport and disturbance on burrow depth distribution of the sand-beach arnphi an intertidal sand flat: infaunal distribution and pod Talitrus saltator. Estuarine, coastal and shelf recolonization. Marine Ecology - Progress Series, 6, science, 16,291-298. 249-255. WRIGHT, L.D. and SHORT, A.D., 1983. Morphody HOLANOV, S.H. and HENDRICKSON, J.R, 1980. namics of beaches and surf zones in Australia. In: The relationship of sand moisture to burrowing P.O. KOMAR, (ed.), CRC Hand-book ofCoastal Pro depth of the sand-beach isopod Tylos punctatus cesses and Erosion. Florida: CRC Press, pp. 35-64.
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