SIMoN
  Sanctuary Integrated Monitoring Network
Monitoring Project

Impacts associated with the recent range shift of the aeolid nudibranch Phidiana hiltoni (Mollusca, Opisthobranchia) in California

Principal Investigator(s)

  • Jeff Goddard
    University of California, Santa Barbara
  • John Pearse
    University of California, Santa Cruz
  • Terrence Gosliner
    California Academy of Sciences
Start Date: December 01, 2007
End Date: December 01, 2010

We observed a total of 20 taxa in the fecal material of Phidiana hiltoni. Eight taxa were hydroids, comprising 82% of the prey records, with Plumularia spp. the most frequent. Nudibranch cerata were observed in the feces of three individuals. Two cerata were from P. hiltoni, as indicated by the distinctive color of the digestive diverticulum, and the ceras in the feces of a P. hiltoni collected from Duxbury Reef was from a different aeolid nudibranch, likely either Cuthona fulgens or Hermissenda crassicornis, based on its orange and white color. One individual from Duxbury Reef had ingested an egg mass of a conspecific, and another individual had significant amounts of amorphous soft tissue in the feces, possibly the remains of a nudibranch.

In the laboratory feeding trials, Phidiana hiltoni attacked 15 of the 21 species of nudibranchs presented to it, ingesting individuals of all 15, whole or in part. Doto columbiana, Doto form A of Goddard (1996), Cuthona divae, Cuthona albocrusta, and Aeolidia papillosa were especially vulnerable to attack, and were not observed to repel P. hiltoni. In contrast, Cuthona lagunae, one Eubranchus rupium, and one 9 mm long Dirona picta always repelled P. hiltoni, or at most caused no visible reaction. The two dorids tested, Triopha maculata and Ancula gibbosa, tended to elicit neutral responses, and the remaining species elicited variable responses by P. hiltoni, ranging from aversion and withdrawal to attack and ingestion.



In the timed counts used in the comparison of abundance, before and after the arrival of P. hiltoni, we recorded 39 species of nudibranchs, plus P. hiltoni. Based on the criteria described previously, we considered 14 of these vulnerable to predation by P. hiltoni and the remaining 25 species not vulnerable. Changes in the abundance of P. hiltoni, one vulnerable species (Hermissenda crassicornis) and one not vulnerable (Triopha catalinae) are shown in Figure 3.


At Duxbury Reef the pooled abundance of the 14 vulnerable species declined an average of two-thirds following the arrival of P. hiltoni, which averaged 10.44 ± 5.71 individuals h-1 observer-1 (t 8.7 = -2.55, P = 0.02) (Fig. 4A). There was no significant difference in the pooled abundance of the non-vulnerable species before and after the arrival of P. hiltoni, nor was there any difference in the number of species in either group of nudibranchs between the two time periods (Fig. 4B).

Summary to Date

Based on these results, we conclude that Phidiana hiltoni likely caused the decline in abundance of vulnerable nudibranch species at Duxbury Reef, likely through a combination of direct predation and competition for prey.

Monitoring Trends

  • At both Pillar Point and Scott Creek, where P. hiltoni averaged 1.69 and 0.36 individuals h-1 observer-1, respectively, the vulnerable species tended to increase in abundance following the arrival of P. hiltoni (Fig. 5). However, these trends were not significant at either Pillar Point (t 4.5 = 1.35, P = 0.24, two-tailed test), or Scott Creek (t 8.3 = 1.59, P = 0.15, two-tailed test), and there was also no significant difference in the before/after change in abundance of the non-vulnerable species.

Study Parameters

  • Distribution
  • Abundance
  • Predation
  • Trophic association
  • Dispersal & Recruitment
  • Range/Biogeography

Study Methods

1. To estimate nudibranch abundance at Duxbury Reef prior to the arrival of P. hiltoni, we used timed counts of nudibranchs conducted by TG or JG from 1969 to 1975. To estimate abundance since the arrival of P. hiltoni, we used timed counts conducted roughly quarterly since December 2007 in the same area as the original counts.

2. Using fecal analysis, we examined the diet of 62 P. hiltoni freshly collected from three sites in central California and one site in southern California.


3. We conducted laboratory behavioral trials to test the propensity of P. hiltoni to attack and consume small nudibranchs. We used 21 species, including two dorids, seven dendronotids, one arminid, and 11 aeolids.

4. Based on taxonomic affinity and the results of the diet studies and feeding trials, we assigned nudibranch species recorded from Duxbury Reef as vulnerable or not to predation by P. hiltoni. We then used the historical and recent abundance data to compare the mean pooled abundance of each group before and after the arrival of P. hiltoni. We also did this using historical and recent data from two other sites (Pillar Point and Scott Creek) within the new range of P. hiltoni, but where P. hiltoni has been one to two orders of magnitude less abundant than at Duxbury Reef.

Figures and Images

Figure 1. (A) Phidiana hiltoni from Pillar Point, California. Image by Douglas Mason. (B) General view of study area at Duxbury Reef, Marin County, California, and location of Duxbury Reef on west coast of North America (inset).


Figure 2. Northern range shift of P. hiltoni in California. Open circles represent negative records; filled circles, positive records. Not shown are negative records, obtained since 1992, from north of Bodega Head, or negative records from Drake’s Estero, near Point Reyes. Site codes: A = Asilomar, AN = Año Nuevo Cove, BH = Bodega Head, BP = Bolinas Point, BR = Bird Rock, CR = Chimney Rock, DR = Duxbury Reef, FR = Fitzgerald Marine Reserve, MB = Muir Beach, PB = Pescadero State Beach & Bean Hollow State Beach, PP = Pillar Point, SC = Scott Creek, SCP = Santa Cruz Point East, SP = Soquel Point.


Figure 3. Changes in the abundance of Phidiana hiltoni, Triopha catalinae (not vulnerable to P. hiltoni), and Hermissenda crassicornis (vulnerable to P. hiltoni), at Duxbury Reef, 1969–75 and 2007–09.


Figure 4. Mean number (+ SE) of (A) individuals and (B) species of nudibranchs found at Duxbury Reef during 1969–75 (N = 7 counts) and 2007–09 (N = 5 counts), grouped by species vulnerability to predation by P. hiltoni as outlined in Methods. Key: grey, not vulnerable; black, vulnerable. Asterisks (*) indicate pairs of values for the different time periods significantly different at P < 0.05 by a one-tailed t-test assuming unequal variances.


Figure 5. Mean number (+ SE) of nudibranchs found at Pillar Point and Scott Creek, before and after arrival of P. hiltoni, grouped by species vulnerability to predation by P. hiltoni. Grey, not vulnerable; black, vulnerable.