BASTERIA, 56: 99-103, 1992 A note onpteropod ink S. van der Spoel Institute forTaxonomic Zoology,University ofAmsterdam, P.O. Box 4766, 1009 ATAmsterdam, The Netherlands Inkejectionby the pteropodmollusc Cephalobrachiamacrochaeta isreported and comparedto thesamephenomenonearlier describedbyothersforHydromylesglobulosa. Histologicaldetails oftheintestinal systemare given. Key-words: Gastropoda, Pteropoda,Cephalobrachiamacrochaeta,behaviour,ink ejection. Inkejection in molluscsis well known inthe Cephalopoda, but ink production also characterizespteropods. Intheirexcellentbookonthebiology ofpelagic molluscsLalli & Gilmer (1989) described the observation made by Dr. G.R. Harbison that Hydromyles globulosa (Rang, 1825)[proposed asnomenconservandumby Tesch(1950) andPruvot-Fol(1942)] whendisturbed, ejects abrownishfluidthrough theanalopen- ing. That this behaviour is not completely unique among pteropods is shown by Cephalobrachia macrochaeta Bonnevie, 1913, a species that also ejects a fluid when disturbed. Cephalobrachia macrochaetawas collected at45°15.0'N29° 50.0'W, Sta. 14, haul5, Amsterdam MidNorthAtlanticPlanktonExpedition, between430 and1000m depth, meantemperature9°C, 18April1980.Intheaquarium, inwhichaspecimen washeld for some hours, production of an ink cloud at themoment it was photographed with flashlight was observed (figs. 1-2). The brownishto black ink was deliveredthrough themouth.SinceGymnosomata ingeneral are specialized predators onThecosomata, and as the most abundant representative of the latter at station 14 was Limacina retroversa (Fleminger, 1823), itmightbepossible thatthecolourofthestomach content is due to the dark purple L. retroversa specimens eaten. However, inthe stomach no fragments ofL. retroversa couldbe identified. To study the originoftheink, aspecimen was sectionedhistologically, 5 pm thick, and stained with Mallory-Heidenhain. The intestinal system in C. macrochaeta shows two typical structures, viz. a blind sac nearthe connectionstomach-hind-gut and a glandular sac in the middle of the hind-gut (fig. 3). The blindsacclose to thebeginning ofthe hind-gut isalso describedforHydromyles globulosa by Meisenheimer(1905); anepithelium almost similar to that ofthe liver- stomach complex was observed by Meisenheimer.Lalli & Gilmer (1989) mentioned this blindsac asthe possible source ofink. Inthe specimen ofC. macrochaetathe blind sac entrance is covered with ciliatedepithelium that is continuouswiththe hind-gut epithelium. Theinnerepithelium(fig. 4) is similartothat oftheliver-stomachcavity. The only remarkable structure ofthis blind sac is thatit has a circular closing muscle around the opening to the stomach. It is difficult to believe that the epithelia ofthis blindsac produce an'ink-likesubstance' asthey aresosimilartolivercells. Thisblind sac was hithertoconsidered unique inH. globulosa, which is no longer correct. 100 BASTERIA, Vol. 56, No. 4-6, 1992 Figs. 1-2.PhotographsofCephalobrachiamacrochaeta from dorsalduringactive‘ink’ejectioninanaquarium after disturbance. The middle ofthe hind-gut is slightly swollen in many Gymnosomata (Meisen- heimer, 1905; Tesch 1913), butintheliteraturenospecial epithelia are describedfrom this partofthe hind-gut. InC. macrochaetathereis alarge sacinthemiddleofthehind- gut, covered withacharacteristic glandular epithelium (fig. 5). It consists entirely of flat glandular epithelial cells with large secretion granules in the active cells. In Mallory-Heidenhain thesespherules stainorange. It isalsoremarkablethatthe open- ing ofthehind-gut tothe stomachandthe anus arebothprovided withcircular closing muscles. The delivery ofthecontents ofthe blindsac andofthe glandularsac, together with the hind-gut, iseffected by contractionofthe transversal body muscles that are very numerous in this area.Regulation ofthe ejection through the anus as well as through the mouthis possible as there are closing muscles at the openings (fig. 6). That H. globulosa takes a separate place among the Gymnosomata as proposed by Lalli& Gilmer, is still a validstatement, but it is doubtfulwhetherthe ink ejection in H. globulosa supportsthis. Both H. globulosa and C. macrochaetashow ink ejection, but the delivery oftheinkmass inthe two species is probably different; H. globulosabrings the ink through the anal opening intothe water, while C. macrochaeta ejects through Van derSpoel: A noteonpteropodink 101 Fig. 3. Cephalobrachiamacrochaeta, crosssectionatthelevel oftheanustoshow the mainintestinal structures. 1=epidermis ofbody wall, 2=longitudinal and transversal muscles ofbody, 3 =epithelium ofvisceral cavity, 4=epitheliumofliver, 5 =liver-stomach cavity, 6 =anus, 7 =osphradium, 8= circular muscle of anus, 9=glandularsac, 10=hind-gut, 11= circularmusclesathind-gutentrance, 12=kidney, 13= kidney aperture, 14=pericardial cavity, 15= circular muscle of blind sac, 16= blind sac, 17=blood vessel, 18= reno-pericardialduct, 19 =oesophagus, 20 =ciliatedepitheliumofhind-gut,21=outerlayer ofhind- gut, 22 =active secretory cells ofglandularsac,23 = youngmucuscell ofglandularsac. Fig. 4. Cephalobrachia macrochaeta, epitheliumofblind sac(forexplanationsee fig. 3). 102 BASTERIA, Vol. 56, No. 4-6, 1992 Fig.5. Cephalobrachiamacrochaeta,epitheliumofglandularsacneartheconnection with hind-gut(forexplana- tion seefig. 3). themouth, whichdoes notseem tobe asufficiently large differenceto supportphylo- geneticconclusions. Theeffectofdarkinkejection asa protectivemechanism in deep- seaanimalsis not clear. Thepresently discussed species lives at depthsofbetween400 mandmorethan 1000m, whereconstant darknessis foundanddarkink willbeinvisi- ble. We do not know what the ink looks like in total darkness; it may be phosphorescent. For preparing the histological slidesMr. A.F. de Fluiter is kindly acknowledged. Van derSpoel: A note onpteropodink 103 Fig. 6.Cephalobrachiamacrochaeta,diagramofstomach,hind-gut,blind sac andglandularsacwith therenal and pericardialcavity (forexplanation see fig. 3). REFERENCES LALLIC.M.,& R.W.GILMER, 1989.Pelagicsnails. Thebiologyofholoplanktonic gastropodmolluscs: 1-259. Stanford. MEISENHEIMER,J., 1905. Pteropoda.— Deutsch. Tiefsee Exp. 1898-1899,9: 1-314. PRUVOT-FOL, A., 1942. Les gymnosomes. — DanaRep. 4(20): 1-54. SPOEL, S.VAN DER, 1976. Pseudothecosomata,Gymnosomataand Heteropoda:1-484. Utrecht. , 1981. List ofdiscrete depth samplesand open nethauls ofthe AmsterdamMid North Adantic Plankton Expedition 1980. —Bull. Zool. Mus. Univ. Amsterdam8: 1-10. TESCH,J.J., 1913.Mollusca,Pteropoda.—In:F.E.SCHULZE,Das Tierreich.EineZusammenstellung und Kennzeichnung der rezentenTierformen 36: i-xvi, 1-154. , 1950. The Gymnosomata2. — Dana Rep. 6(36): 1-55.