Two polar lipid-soluble macrocycles containing a spiro-linked tricyclic ether ring system and an unusual seven-membered spiro-linked cyclic iminium moiety were isolated from the digestive glands of mussels (Mytilus edulis) and scallops (Placopecten magellanicus), named spirolides A-D. Herein, we report the structural elucidation of the two major components, spirolides B (1) and D (2). The compounds were purified from methanolic extracts of frozen shellfish digestive glands via bioassay-guided fractionation using silica gel, reversed-phase C18, gel permeation (Sephadex LH-20), and C18 HPLC. High-resolution LSIMS and DEI mass spectrometry determined the molecular formulae: C42H63NO7 (MH+ 694.4651) for 1 and C43H65NO7 (MH+ 708.4831, M+ 707.4975) for 2. IR spectra revealed hydroxy groups (3460 cm-1), a γ-lactone ring (1771 cm-1), and a carbon-oxygen/nitrogen double bond (1676 cm-1). 13C NMR identified 10 quaternary carbons, 10 CH, 16 CH2, and 7 methyl groups for 2. TOCSY, COSY, and HMBC experiments established carbon connectivity, functional group positions, and ring systems—both compounds contain 7 rings, including the characteristic spiro-linked tricyclic ether and seven-membered cyclic iminium moieties. MS/MS spectra confirmed structural features: 2 lost a pendant lactone ring to form m/z 608, and 1 showed analogous fragments 14 mass units lower, indicating 1 lacks a methyl group present in 2. Isotope-shift NMR experiments in CD3OD identified three exchangeable protons (secondary hydroxy at C10, tertiary hydroxy at C19, and a C27 proton from iminium tautomerization). The spiro-linked cyclic iminium group is rare, previously reported only in pinnatoxin A. Spirolides likely originate from microalgae, as they occur seasonally in shellfish digestive glands and were detected in plankton samples. They are potently toxic in mouse bioassays (LD100 250 μg kg-1 i.p.). In vitro assays showed no effects on NMDA, AMPA, or kainate receptors, no inhibition of PP1/PP2A phosphatases, no interaction with voltage-dependent Na channels, but weak activation of type L Ca channels (1.7 pmol dm-3).