Stimulation of metamorphosis in Hydractinia echinata involves generation of lysophosphatidylcholine

Summary Whilst the significance of the phosphoinositide cycle in the activation of developmental events by extra-cellular signals is well established, the involvement of the phosphatidylcholine (PC) cycle is a matter just emerging. In the present study, the metabolism of phosphatidylcholine in early metamorphosis of Hydractinia echinata (Coelenterata; Hydrozoa) was investigated by incubation of planula larvae with ³H-choline, extraction of the metabolites and isolation of the metabolites by thin-layer chromatography (TLC). Phosphatidylcholine (PC), lysophosphatidylcholine (LPC), acetylcholine and glycerophosphocholine were the labelled metabolites. Induction of metamorphosis did not stimulate an increased incorporation of choline into PC. In larvae preincubated with ³H-choline to a steady state level of incorporation, a significant transient elevation of the radioactive label in LPC was observed 90 min after addition of metamorphosis stimulating agents. LPC probably derived from PC by the action of a phospholipase A₂ (PLA₂). LPCs from bovine and soybean origin as well as isolated larval LPC did not influence metamorphosis. PLA₂ from bee venom promoted Cs⁺-induced metamorphosis but did not influence phorbol ester-induced metamorphosis. The data suggest that a PLA₂ is activated during metamorphosis. This PLA₂ activation does not occur in those putative receptor cells which receive the primary external inducing stimulus but in the many larval cells which resume proliferation or differentiation in response to a second, internally propagated signal. Offprint requests to: T. Leitz     

Endogenous photoproteins,calcium channels and calcium transients during metamorphosis in hydrozoans

Summary There are species of hydrozoans, Eutonina victoria, Mitrocomella polydiademata, and Phialidium gregarium whose eggs contain calcium-specific photoproteins. These cytoplasmic photoproteins are synthesized during oogenesis. During the cleavage stages of embryogenesis they are distributed to all of the cells of the developing planula larva. The amount of photoprotein slowly declines during the development of the planula larva, and markedly declines when the planula undergoes metamorphosis to become a polyp.Oocytes, unfertilized eggs, and fertilized eggs prior to the first cleavage do not produce light when treated with KCl. The ability to respond to KCl appears about the time of first cleavage, and is correlated with the appearance of active membrane responses. Both the KCl response and the action potentials will occur in sodium-free sea water, and both are inhibited by calcium channel blockers. These and other experiments suggest that voltage sensitive calcium channels first become active at about the time of first cleavage. These channels also appear on the same schedule in both unfertilized eggs and in enucleated egg fragments, which have been artificially activated with A23187.Developing planulae produce few or no spontaneous light responses before gastrulation. Later the frequency and magnitude of spontaneous light production increases presumably due to an increasing frequency and magnitude of calcium transients. Both the natural trigger of metamorphosis (bacteria) and an artificial trigger (CsCl) cause a conspicuous series of calcium transients. When these transients are inhibited by calcium channel blockers, metamorphosis is also inhibited.     

Metamorphosis of <Emphasis Type="Italic">Hydractinia echinata</Emphasis>—natural versus artificial induction and developmental plasticity

Many marine invertebrates reproduce through a larval stage. The settlement and metamorphosis of most of the species are synchronised and induced by environmental organisms, mainly bacteria. The hydrozoan Hydractinia echinata has become a model organism for metamorphosis of marine invertebrates. In this species, bacteria, e.g. Pseudoalteromonas espejiana, are the natural inducers of metamorphosis. Like in other species of marine invertebrates, metamorphosis can be induced artificially by monovalent cations, e.g. Cs⁺. In this study, we present systematic data that metamorphosis—with both inducing compounds, the natural one from bacteria and the artificial one Cs⁺—are indeed similar with respect to (a) the morphological progression, (b) the localisation of the primary induction signal in the larva, (c) the pattern of apoptotic cells occurring during the initial 10 h of metamorphosis and (d) the disappearance of RFamide-dependent immunocytochemical signals in sensory neurons during this process. However, a difference occurs during the development of the anterior end, insofar as apoptotic cells and settlement appear earlier in planulae induced with bacteria. Thus, basically, Cs⁺ may be used as an artificial inducer, mimicking the natural process. However, differences in the appearance of apoptotic cells and in settlement raise the question of how enormous developmental plasticity in hydrozoans actually can be, and how this is related to the absence of malignant devolution in hydrozoans.     

Metamorphosis inHydractinia: Studies with activators and inhibitors aiming at protein kinase C and potassium channels

Summary Metamorphosis of planula larvae involves an activation of morphogenetically quiescent cells. The present work extends a previous study [Leitz T and Müller WA (1987) Dev Biol 121:82–89] on the participation of the phosphatidylinositol/diacylglycerol/protein kinase C system. Metamorphosis is stereospecifically induced by diacylglycerols, 1,2,-sn-dioctanoylglycerol (diC₈) being by far the most effective substance. K-252a and sphingosine, inhibitors of mammalian protein kinases C, profoundly inhibited metamorphosis. Phorbolester-binding studies and the corresponding Scatchard plots revealed a specific and saturable binding of [³H]phorbol 12,13-dibutyrate to a single site of particulate fractions ofHydractinia with a specific binding affinityK _d = 50 nM. K⁺ ionophores stimulated Cs⁺ — but inhibited diC₈-induced metamorphoses, K⁺-channel blockers enhanced the inducing action of Cs⁺ or diC₈. On the basis of these data and observations of others we propose that the activation ofHydractinia larvae takes place in some cells at the anterior end as a result of activation of a kinase-C-like enzyme, which directly or indirectly leads to the closure of K⁺ channels. Closure of these channels then causes depolarisation and, thus, release of an internal signal. This hypothesis unifies notions about the role of K⁺ channels and of the phosphatidylinositol system in initiation of metamorphosis inHydractinia.     

Evidence for the involvement of PI-signaling and diacylglycerol second messengers in the initiation of metamorphosis in the hydroid Hydractinia echinata Fleming

Metamorphosis of the planula larvae into polyps does not occur spontaneously but depends on the reception of external trigger stimuli. Artificially, metamorphosis can be initiated by a pulse-type application of Cs⁺ or tumor-promoting phorbol esters (W. A. Müller (1985) Differentiation 29, 216–222). In the present study we examined the putative involvement of the phosphatidylinositol system in signal transduction. Planulae of Hydractinia echinata were preincubated with [³H]-inositol. Upon exposure of the larvae to Cs⁺ the label in inositol trisphosphate (InsP₃) increased twofold as early as 15 sec after addition of Cs⁺. Within the first 60 sec the levels of inositol monophosphate (InsP₁) and inositol bisphosphate (InsP₂) were also elevated compared to the values in nonstimulated larvae. After 1 and 3 hr, respectively, of incubation with Cs⁺, only the label in InsP₂ was increased. When applied to saponin-permeabilized larvae, InsP₃ did not induce metamorphosis. But 1,2-dioctanoyl-rac-glycerol (diC₈) was effective in inducing metamorphosis with a half-maximal effective concentration of 9 μM. The percentage of metamorphosed animals after the application of 5 μM diC₈ (30 mM Cs⁺) was increased by the simultaneous application of 1 μM (0.1 μM) of the diacylglycerol kinase inhibitor R 59022. The results are interpreted as evidence for the involvement of the PI-signaling/diacylglycerol transduction system in the initiation of metamorphosis of planula larvae of H. echinata.     

Pattern of serotonin‐like immunoreactive cells in scyphozoan and hydrozoan planulae and their relation to settlement

The planulae of almost all investigated cnidarian species possess neuron‐like cells. The distribution of these cells is usually uneven throughout the long axis of the planula. The majority of these cells are located in the anterior half of the planula body. Scyphozoan planulae, as well as anthozoan planulae, have a sensory structure at the anterior pole called an apical organ, which is believed to take part in metamorphosis induction. Hydrozoan planulae also possess sensory cells. It has been previously shown in several cnidarian larvae that their neuronal cells contain the neurotransmitter, serotonin. The present study describes the peculiarities of serotonin‐like immunoreactive cells in Aurelia aurita (Scyphozoa) and Gonothyraea loveni (Hydrozoa) planulae. We show that several cells in the presumptive apical organ of A. aurita are immunoreactive to antibodies against serotonin, while G. loveni planulae have an accumulation of serotonin‐positive cells near the anterior pole. Additional serotonin‐like immunoreactive cells are found in the lateral ectoderm of both planulae. Treatment of A. aurita and G. loveni planulae with serotonin or its blockers show that serotonin is likely involved in the initiation of planula settlement.     

Metamorphosis ofHydractinia echinata Insights into pattern formation in Hydroids

Summary Hydractinia echinata is a marine, colony-forming coelenterate. Fertilized eggs develop into freely swimming planula larvae, which undergo metamorphosis to a sessile (primary) polyp. Metamorphosis can be triggered by means of certain marine bacteria and by Cs⁺. Half a day after this treatment a larva will have developed into a polyp. The induction of metamorphosis can be prevented by addition of inhibitor I, a substance partially purified from tissue ofHydra. The larvae ofH. echinata also appear to contain this substance. Inhibitor I appliedafter the onset of metamorphosis blocks its continuation as long as it remains in the culture medium. Cs⁺ applied within the same period of time also blocks the continuation of metamorphosis. However, these two agents have opposite effects on the body pattern of the resultant polyps. The experiments indicate that application of Cs⁺ triggers the generation of the pre-pattern. Inhibitor I appears to be a factor of this prepattern. A model is proposed which describes the basic features of head and foot/stolon formation not only forHydractinia but also for other related hydroids.     

The Embryonic Origin of Neurosensory Cells and the Role of Nerve Cells in Metamorphosis in Phialidium gregarium (Cnidaria,Hydrozoa)

Summary

The embryonic origin of the nervous system in Phialidium gregarium was investigated. Entoderm-free planulae, surgically produced by bisection at mid-gastrulation, and normal planulae were examined by light and electron microscopy to determine their cellular composition. The cell types that occur in the epidermis of the normal planula were described. The entoderm-free planulae were found to be devoid of interstitial cells and their derivatives, the nematocytes and ganglion cells. Neurosensory cells were present, however, indicating that they are derivatives of the ectodermal epithelium.

The role of nerve elements in the initiation of metamorphosis was also examined. Normal and entoderm-free planulae treated for four hours with 0.4% colchicine at two, three, or four days of development fail to undergo cesium-induced metamorphosis. Since such treatment in other hydrozoans eliminates interstitial cells and their derivatives [1-3], it might be argued that ganglion cells are necessary to initiate metamorphosis. The observation that entoderm-free planulae, devoid of interstitial cell derivatives, are capable of responding to induction by bacteria or cesium, however, indicates that in Phialidium the colchicine effect is on other cell types. The results are compared with findings for other Cnidaria.     

Embryonic development and metamorphosis of the scyphozoan <Emphasis Type="Italic">Aurelia</Emphasis>

We investigated the development of Aurelia (Cnidaria, Scyphozoa) during embryogenesis and metamorphosis into a polyp, using antibody markers combined with confocal and transmission electron microscopy. Early embryos form actively proliferating coeloblastulae. Invagination is observed during gastrulation. In the planula, (1) the ectoderm is pseudostratified with densely packed nuclei arranged in a superficial and a deep stratum, (2) the aboral pole consists of elongated ectodermal cells with basally located nuclei forming an apical organ, which is previously only known from anthozoan planulae, (3) endodermal cells are large and highly vacuolated, and (4) FMRFamide-immunoreactive nerve cells are found exclusively in the ectoderm of the aboral region. During metamorphosis into a polyp, cells in the planula endoderm, but not in the ectoderm, become strongly caspase 3 immunoreactive, suggesting that the planula endoderm, in part or in its entirety, undergoes apoptosis during metamorphosis. The polyp endoderm seems to be derived from the planula ectoderm in Aurelia, implicating the occurrence of “secondary” gastrulation during early metamorphosis.     

海蜇胚胎发育和变态过程超微观察

采用扫描电镜、透射电镜和蛋白银染色等方法研究了海蜇胚胎发育和变态过程中细胞超微结构变化。结果显示: (1)海蜇自受精卵至原肠期阶段细胞均等分裂, 细胞间存在大量连接, 细胞形态相近, 未出现显著分化; (2)海蜇自早期浮浪游虫阶段, 其外胚层细胞开始出现空泡化, 至4触手螅状体阶段外胚层细胞空泡体积逐渐增大, 而内胚层细胞仅在4触手螅状体阶段才出现空泡化。伴随着外胚层细胞空泡化比例的增大, 杯状体和4触手螅状体阶段出现疑似凋亡小体结构; (3)刺细胞分化于早期浮浪游虫期的外胚层近中胶层区域, 而后逐渐向外转移, 至4触手螅状体阶段发育成熟并转移至表面; (4)纤毛形成于早期浮浪幼虫, 在杯状体阶段逐渐退化, 并于4触手螅状体阶段完全消失; (5)在海蜇早期发育各个阶段, 其内部均发现大量着色较深的卵黄体, 且在浮浪游虫阶段首次发现了海蜇外层细胞主动吞噬细菌现象, 表明海蜇早期发育营养来自内源性和外源性两部分。研究结果可为阐明刺胞动物早期发育模式提供依据。     

Post-embryonic larval development and metamorphosis of the hydroidEudendrium racemosum (Cavolini) (Hydrozoa,Cnidaria)

The morphology and histology of the planula larva ofEudendrium racemosum (Cavolini) and its metamorphosis into the primary polyp are described from light microscopic observations. The planula hatches as a differentiated gastrula. During the lecithotrophic larval period, large ectodermal mucous cells, embedded between epitheliomuscular cells, secrete a sticky slime. Two granulated cell types occur in the ectoderm that are interpreted as secretory and sensorynervous cells, but might also be representatives of only one cell type with a multiple function. The entoderm consists of yolk-storing gastrodermal cells, digestive gland cells, interstitial cells, cnidoblasts, and premature cnidocytes. The larva starts metamorphosis by affixing its blunt aboral pole to a substratum. While the planula flattens down, the mucous cells penetrate the mesolamella and migrate through the entoderm into the gastral cavity where they are lysed. Subsequently, interstitial cells, cnidoblasts, and premature cnidocytes migrate in the opposite direction, i.e. from entoderm to ectoderm. Then, the polypoid body organization, comprising head (hydranth), stem and foot, all covered by peridermal secretion, becomes recognisable. An oral constriction divides the hypostomal portion of the gastral cavity from the stomachic portion. Within the hypostomal entoderm, cells containing secretory granules differentiate. Following growth and the multiplication of tentacles, the head periderm disappears. A ring of gland cells differentiates at the hydranth's base. The positioning of cnidae in the tentacle ectoderm, penetration of the mouth opening and the multiplication of digestive gland cells enable the polyp to change from lecithotrophic to planktotrophic nutrition.     

The effect of larval age on developmental changes in the polyp prepattern of a hydrozoan planula

The larvae of many marine organisms including hydrozoans are lecithotrophic and will not feed until after metamorphosis. In hydrozoans the aboral region of the planula becomes the holdfast and stolon, while the oral region becomes the stalk and hydranth that grows out of the holdfast following metamorphosis. If metamorphosis is delayed, the portion of the planula allocated to form holdfast and stolon shrinks and the region that forms the hydranth increases in size. Planulae also have the ability to regenerate their polyp prepattern. When the aboral region of the planula that does not normally form a hydranth is isolated and metamorphosis is delayed, it acquires the capacity to form a hydranth from the holdfast. A relatively high proportion of entodermal cells of young planulae engage in DNA synthesis (BrdU labeling index); as planulae age, the labeling index falls close to zero. When the polyp prepattern is modified during planula regeneration, entodermal cells are induced to engage in DNA synthesis. If DNA synthesis is inhibited in planulae, the polyp prepattern changes during regeneration and age-related developmental changes in planula are inhibited, suggesting that DNA synthesis is a necessary part of the pattern respecification process.     

Comparative fine structural studies of planulae and primary polyps of identical age of the sea pen,Ptilosarcus gurneyl

Electron microscopic study of an 18-day-old planulae and primary polyps of the sea pen, Ptilosarcus gurneyi, reveals 14 cell types: sustentacular cell A, sustentacular cell B, nerve cell, sensory cell, cnidoblast, interstitial cell, five types of gland cell (A, B, C, D and E), amoebocyte, style cell and endodermal cell. Of these, 9 are found in the planula, 12 in polyps and 7 are common to both stages. The fine structure of all cell types is described. Since the planulae and polyps in this study were identical in age of development, the gaining and losing of certain types of cells in the polyp are attributed to changes associated with settlement and metamorphosis. Modifications of the seven common cell types during metamorphosis can also be attributed to the change of life style from pelagic to benthic.     

Localization of sensory mechanisms utilized by coral planulae to detect settlement cues

Coral planulae are induced to settle and metamorphose by contact with either crustose coralline algae or marine bacterial biofilms. Larvae of two coral species, Pocillopora damicornis and Montipora capitata, which respond to different metamorphic cues, were utilized to investigate the sensory mechanisms used to detect metamorphic cues. Because the aboral pole of the coral planula is the point of attachment to the substratum, we predicted that it is also the point of detection for cues. To determine where sensory cells for cues are localized along the body, individual larvae were transversely cut into oral and aboral portions at various levels along the oral–aboral axis, and exposed to settlement‐inducing substrata. Aboral ends of M. capitata metamorphosed, while oral ends continued to swim. However, in larvae of P. damicornis, ¾ oral ends (i.e., lacking the aboral pole) were also able to metamorphose, indicating that the cells that detect cues may be distributed along the sides of the body. These cells do not correspond to FMRFamide‐immunoreactive cells that are present throughout the body. Cesium ions induced both aboral and oral ends of larvae of both species to settle, suggesting that oral ends have not lost their capacity to metamorphose, despite lacking sensory cells to detect natural cues. To determine whether sensory cells in larvae of P. damicornis are restricted to one side of the body, swimming behavior over substrata was observed in larvae labeled with diI, a red fluorescent lipophilic membrane stain. The larvae were found to rotate around the oral–aboral axis, with their surface against the substratum, not favoring a particular side for detecting cues. While clarifying the regions of the larval body important for settlement and metamorphosis in coral planulae, we conclude that significant differences between coral species may be due to differences in the distribution of sensory structures in relation to different planular sizes.     

Inductive activity of the posterior tip of planula in the marine hydroid <Emphasis Type="Italic">Dynamena pumila</Emphasis>

Activity of organizer regions is required for body plan formation in the developing organism. Transplanting a fragment of such a region to a host organism leads to the formation of a secondary body axis that consists of both the donor’s and the host’s tissues (Gerhart, 2001). The subject of this study, the White Sea hydroid cnidarian Dynamena pumila L. (Thecaphora, Sertulariidae), forms morphologically advanced colonies in the course of complex metamorphosis of the planula larva. To reveal an organizer region, a series of experiments has been performed in which small fragments of donor planula tissues were transplanted to embryos at the early and late gastrula stage, as well as to planulae. Only transplantations of a posterior tip fragment of a donor planula to a host planula of the same age led, in the course of metamorphosis, to the formation of a secondary shoot, which involved up to 50% of the host’s tissues. After transplantations of tissue fragments of the anterior tip and the middle of the planula body, the formation of any ectopic structures was never observed. It was concluded that the posterior tip of the planula has organizer properties in Dynamena.     

On some features of embryonic development and metamorphosis of Aurelia aurita (Cnidaria, Scyphozoa)

Aurelia aurita is a cosmopolite species of scyphomedusae. Its anatomy and life cycle are well investigated. This work provides a detailed study on development and structure of A. aurita planula before and during its metamorphosis. Intravital observations and histology study during the settlement and metamorphosis of the planulae demonstrated that the inner manubrium lining of primary polyp (gastroderm) develops from the ectoderm of the planula posterior end. The spatial and temporal dynamics of serotonergic cells from the early embryonic stages until the formation of the primary polyp were studied for the first time. In addition, the distribution of tyrosinated tubulin and neuropeptide RF-amide at different stages of A. aurita development was traced.     

Neural system reorganization during metamorphosis in the planula larva of Clava multicornis (Hydrozoa, Cnidaria)

The planula larva of the hydroid Clava multicornis (Forskål, 1775) has a complex nervous system, characterized by the presence of distinct, anteriorly concentrated peptidergic populations of amidated neurons, presumably involved in the detection of environmental stimuli and metamorphic signals. Differently from other hydrozoan larvae in C. multicornis planulae GLW-positive cells with putative sensory role have a peculiar dome-shaped forefront organization, followed by a belt of RF-positive nerve cells. By immunohistochemistry, we investigated the transformation of the peptidergic (GLW-amide and RF-amide) larval neuroanatomy at different stages of metamorphosis and the subsequent development of the primary polyp nervous system. By terminal transferase-mediated dUTP nick end-labeling assay, apoptotic nuclei were first identified in the anterior pole of the settled larva, in the same region occupied by GLW-amide positive putative sensory cells. In primary polyps, GLW-amide positive signals first encircled the hypostome area, later extending downwards along the polyp column or upwards over the hypostome dome, whereas RF-amide positive sensory cells initially appeared at the tentacles base to later extend in the tentacles and the polyp column. In spite of the possession of distinct neuroanatomies, different cnidarian planulae may share common developmental mechanisms underlying metamorphosis, including apoptosis and de novo differentiation. Our data confirm the hypothesis that the developmental dynamics of tissue rearrangements may be not uniform across different taxa.     

Ammonia,tetraethylammonium, barium and amiloride induce metamorphosis in the marine hydroid Hydractinia

Summary In Hydractinia metamorphosis from the swimming larval stage to the sessile polyp stage has been found to be inducible by several agents, including Li⁺, K⁺, Cs⁺, Rb⁺, diacylglycerol (DG), tetradecanoyl-phorbol-acetate (TPA) and some other tumour-promoting phorbol esters. Induction is antagonized by ouabain and compounds which are able to increase the internal level of S-adenosylmethionine (SAM). Based on the finding that Hydractinia larvae contain such compounds in a stored form, including N-methylpicolinic acid, N-methylnicotinic acid and N-trimethylglycine, as well as on the results of experiments with antagonists of SAM production and transmethylation, it has been argued that regulation of the internal SAM level plays a key role in the control of metamorphosis. However, it remains to be clarified whether the inducing agents act by decreasing the SAM level or by via different pathways. In the present study, substances chemically related to the substances known to induce or inhibit metamorphosis were tested for their metamorphosis-inducing abilities. Some were found to be effective, including NH₄ ⁺, methylamine, tetraethylammonium ions (TEA⁺), ethanolamine, Ba²⁺, Sr²⁺ and the diuretic, amiloride. It is of particular interest that in many organisms TPA and DG increase cytoplasmic pH while amiloride prevents a rise in pH_i. Several of the substances known to trigger metamorphosis may increase the internal NH₄ ⁺ concentration by hindering the export of the constantly produced NH₄ ⁺ through K⁺ channels or through the Na⁺-H⁺ antiport. Treatment with Cs⁺ for 1 h increases the internal level of NH₄ ⁺. Produced and applied ammonia, as well as applied methylamine and ethanolamine, may act by accepting methyl groups, thus reducing the SAM level.     

Ultrastructure of the calcium-sequestering gastrodermal cell in the hydroid Hydractinia symbiolongicarpus (Cnidaria, Hydrozoa)

Large, free-floating crystals of calcium carbonate occur in vacuoles of gastrodermal cells of the hydroid Hydractinia symbiolongicarpus. Here, morphological details about the process by which these cells accumulate and sequester calcium are provided by a cytochemical method designed to demonstrate calcium at the ultrastructural level. Electron-dense material presumably indicative of the presence of calcium was EGTA-sensitive and was shown by parallel electron energy loss spectroscopy (EELS) and energy spectroscopic imaging (ESI) to contain calcium. Calcium occurred in only one cell type, the endodermally derived gastrodermal cell. In these cells, the electron-dense material appeared first as a fine precipitate in the cytosol and nucleus and later as larger deposits and aggregates in the vacuole. During the life cycle, gastrodermal cells of the uninduced planula and the planula during metamorphic induction sequestered calcium. In primary polyps and polyps from established colonies, gastrodermal cells sequestered calcium, but the endodermal secretory cells did not. Our observations support the hypothesis that gastrodermal cells function as a physiological sink for calcium that enters the organism in conjunction with calcium-requiring processes such as motility, secretion, and metamorphosis.     

Settlement induction of Acropora palmata planulae by a GLW-amide neuropeptide

Complex environmental cues dictate the settlement of coral planulae in situ; however, simple artificial cues may be all that is required to induce settlement of ex situ larval cultures for reef re-seeding and restoration projects. Neuropeptides that transmit settlement signals and initiate the metamorphic cascade have been isolated from hydrozoan taxa and shown to induce metamorphosis of reef-building Acropora spp. in the Indo-Pacific, providing a reliable and efficient settlement cue. Here, the metamorphic activity of six GLW-amide cnidarian neuropeptides was tested on larvae of the Caribbean corals Acropora palmata, Montastraea faveolata and Favia fragum. A. palmata planulae were induced to settle by the exogenous application of the neuropeptide Hym-248 (concentrations ≥1 × 10⁻⁶ M), achieving 40–80% attachment and 100% metamorphosis of competent planulae (≥6 days post-fertilization) during two spawning seasons; the remaining neuropeptides exhibited no activity. Hym-248 exposure rapidly altered larval swimming behavior (<1 h) and resulted in >96% metamorphosis after 6 h. In contrast, M. faveolata and F. fragum planulae did not respond to any GLW-amides tested, suggesting a high specificity of neuropeptide activators on lower taxonomic scales in corals. Subsequent experiments for A. palmata revealed that (1) the presence of a biofilm did not enhance attachment efficiency when coupled with Hym-248 treatment, (2) neuropeptide-induced settlement had no negative effects on early life-history developmental processes: zooxanthellae acquisition and skeletal secretion occurred within 12 days, colonial growth occurred within 36 days, and (3) Hym-248 solutions maintained metamorphic activity following storage at room temperature (10 days), indicating its utility in remote field settings. These results corroborate previous studies on Indo-Pacific Acropora spp. and extend the known metamorphic activity of Hym-248 to Caribbean acroporids. Hym-248 allows for directed and reliable settlement of larval cultures and has broad applications to the study and rehabilitation of threatened Acropora populations in the Caribbean.