Sessile organisms are reliant on the use of refugia or chemical and physical defences to avoid or reduce attacks from predators. Many solitary ascidians possess a thick leathery outer tunic, sometimes reinforced with inorganic inclusions, that generally acts to deter predators. The genus Herdmania is remarkable among ascidians in possessing an abundance of barbed needle-like calcareous spicules in both the external tunic and the internal mantle. Our focus was on a habitat-forming subtidal ascidian – Herdmania grandis. We questioned why a large ascidian possessing a leathery test would possess spicules within the mantle – an apparent defence within a defence. We first quantified variation in spicule size and density within animals. Second, we examined the size and density of spicules for individuals of a range of sizes. Third, we incorporated these spicules at natural concentrations into feeding discs and tested their ability to deter gastropod predators that specialise in consuming ascidians and an asteroid that is considered a generalist consumer. Finally, we examined survivorship of lab-reared ascidian recruits when exposed to the starfish predator. Spicules constituted up to 30% dry weight (DW) of the tunic and an astounding 57% DW of the body wall (mantle). Spicules were significantly smaller in the contractile regions of the animal (the siphon and branchial basket) and were also found in lower densities, though not significantly. Mantle spicules were effective feeding deterrents; control feeding discs, lacking spicules, were consumed at three times the rate of discs with spicules (p < 0.05) by specialist ascidian predators, the Ranellid gastropods Cabestana spengleri and Ranella australasia. In contrast, we did not detect a significant difference in the consumption of feeding discs by the seastar Meridiastra calcar. In addition, the seastar reduced survivorship of 90 day old lab-reared recruits by four-fold. We contend that the primary defence mechanism of this ascidian is the leathery test and that this is effective against generalist consumers. The second line of defence – spicules in the mantle –is to thwart gastropod predators that bypass the tunic either by boring through it or by inserting their proboscis directly into soft tissues of their prey via the siphons. The presence of both a leathery tunic and spicules in H. grandis probably contributes to the dominance of this species on shallow subtidal reefs in southeastern Australia, where it is an important habitat-forming species. Our findings confirm the importance of considering a range of consumer strategies and life history stages in assessing potential control of engineering species by predation.